FOUS11 KWBC 062030
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
330 PM EST Mon Jan 6 2025

Valid 00Z Tue Jan 07 2025 - 00Z Fri Jan 10 2025

...Mid-Atlantic...
Day 1...

Trailing mid-level shortwave behind the outgoing major winter=20
storm will pass quickly through the Mid-Atlantic early this=20
evening, spurring another inch or two of snow east of the=20
mountains. Over central WV, some upslope enhancement will help=20
squeeze out perhaps a few inches, with WPC probabilities for an=20
additional 4 inches of snow after 00Z around 30-50%. Snow will=20
taper off there by Tuesday afternoon.


...Great Lakes and Northeast...
Days 1-3...

A large, sprawling, and multi-pronged circulation over=20
southeastern Canada will send mid-level shortwaves into the eastern
Great Lakes and Northeast over the next few days, maintaining high
chances of at least light snow over the higher elevations (NY and=20
northern VT/NH) and also downwind of the Great Lakes in general=20
owing to the broad cyclonic flow.=20

For D1, a compact closed low near Montreal this evening will swing
through northern NY/VT overnight with some terrain enhancement=20
over the northern Adirondacks and into the Green Mountains. WPC=20 probabilities for at least 4 inches of snow tonight through Tuesday
are 50-80% there.

By D2, with already cold 850mb temps below -12C or so, a=20
reinforcing shot of cold Canadian air (via another short wave out=20
of Canada) will steepen lapse rates and reinvigorate lake effect=20
snow off the western Great Lakes, especially along the western=20
shore of Lower Michigan, and also off Lakes Erie/Ontario on NW=20
flow. WPC probabilities for at least 4 inches of snow D2 are=20
highest south of Muskegon right along the lake short, but will be=20
dependent on the fetch and trajectory of the band.=20

For D3, one last shortwave will push through the eastern Great=20
Lakes and New England, maintaining additional light snow off Lake=20
Ontario on NW flow and additional snow for the northern Adirondacks
and Green Mountains. WPC probabilities for at least an additional=20
4 inches of snow are moderate (40-70%) over these areas.


...Intermountain West, Rockies, and Southwest...
Days 1-3...

Broad height falls moving through the Great Basin tonight will=20
sharpen into a closed low over the lower CO River Valley late Tue.=20
With high pressure nosing down out of the northern Plains, upslope=20
snow will expand across the CO Front Range via an easterly low=20
level flow along with a relatively low DGZ. However, the snow=20
should not last too long as the upper low will likely move into=20
northwestern Mexico by the end of the period. This will help to=20
drag the snowfall southward as well through the Mogollon Rim,=20
Sangre de Cristos, Sacramento Mountains, etc. Models continue to=20
vary on QPF (and thus snow) over parts of the Southwest but at=20
least a large footprint of light snow is expected. For the D1-3=20
period, WPC probabilities for at least 4 inches of snow are >50%=20
over the Front Range into the CO Rockies, San Juans, and higher=20
elevations in AZ/NM above 8000ft or so.=20

Farther north, a shortwave moving out of Canada through Montana on
Wednesday will touch off some light to locally moderate snow over=20
much of the state that will push into Wyoming and the Black Hills=20
on Thursday. WPC probabilities for at least 4 inches of snow are=20
highest (>50%) over the Little Belts, Gallatin Range, and into the=20
Bighorns.


...Southern Plains...
Day 3...

As the aforementioned upper low closes off over the Southwest=20
Thursday and continues to dig into northwestern Mexico Friday it=20
will become anomalously deep (below the 0.5th climate percentile=20
per the NAEFS ESAT). Meanwhile, a separate shortwave rounding the=20
far eastern Pacific ridge is forecast to dive southward through=20
Utah and interact with this feature and spawn strong southwest flow
containing rich moisture and prominent upper divergence from an=20
increasingly buckled jet. Given the strong high situated to the=20
north over the Central Plains and a cold low-level airmass in=20
place, snow and mixed precipitation is expected over much of=20
western and north-central TX late Wed and through Thursday,=20
expanding eastward to the Ozarks by the end of the period. Thermal=20 uncertainty is high (as expected) due to the upstream players=20
(timing, depth, etc.) and the NBM ptype probabilities show a zone=20
of maximum uncertainty from near the Big Bend northeastward to=20
around TexArKana.=20

All ptypes are in play -- snow/sleet/freezing rain and plain rain=20
to the south -- and each will likely not be stagnant in location.=20
For now, the highest probabilities of snow lie on the northwest=20
side of the precipitation shield in the deeper colder air from the=20
Permian Basin across Northwest TX into North TX (near and northwest
of the D-FW Metroplex). There, WPC probabilities for at least 4=20
inches of snow are around 30-50%. To the southeast, within the zone
of maximum uncertainty, probabilitiesfor accumulating snowfall=20
decrease while probabilities for ice increase. Areas from near=20
Austin northeastwardthrough the Piney Woods into the Hill Parishes=20
of LA show the highest chance of at least 0.1 inches of ice=20
accumulation (30-60%).


This system has prompted the issuance of a new set of Key Messages
which are linked at the bottom of the discussion.

Fracasso/Snell


...Winter Storm Key Messages are in effect. Please see current Key
Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_2.png__;!!DZ3fjg!_p-d5kX8NzvPiea0SkbIvk3YnMjHKOh2q0s4iASxMHzCr= TEx8epFDhVUmnjbsdroCOxm3zPaNswlrn58foeDVnjrYAg$=20

$$

=3D =3D =3D
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FOUS11 KWBC 070855
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
355 AM EST Tue Jan 7 2025

Valid 12Z Tue Jan 07 2025 - 12Z Fri Jan 10 2025


...Great Lakes and Northeast...
Days 1-3...

A large, sprawling, and multi-pronged circulation over=20
southeastern Canada will send mid-level shortwaves into the eastern
Great Lakes and Northeast over the next few days before finally
sliding eastward south of Greenland by the end of D3. This will
maintain high chances of at least light snow over the higher=20
elevations (NY and northern VT/NH) and also downwind of the Great=20
Lakes in general owing to the broad cyclonic flow.=20

For D1, a compact closed low over northern VT early this morning will
swing through the Northeast today with some terrain enhancement=20
over the northern Adirondacks and into the Green Mountains.
Additionally, another trailing shortwave diving out of Canada=20
tonight over the Great Lakes and into the Northeast will contain a=20 reinforcing shot of arctic air. Even with already cold 850mb temps
below -12C or so this next surge of cold air will steepen lapse=20
rates and reinvigorate lake effect snow off the western Great=20
Lakes, especially along the western shore of Lower Michigan, and=20
also off Lakes Erie/Ontario on NW flow. WPC probabilities for at=20
least 4 inches of snow D1-D2 are 40-70% in the favorable northwest
flow snowbelts, as well as over the northern Adirondacks and=20
northern VT.

By the end of D2 and start of D3, one last shortwave will push=20
through the eastern Great Lakes and New England, maintaining=20
additional light snow off Lake Ontario on NW flow and additional=20
snow for the northern Adirondacks and Green Mountains. WPC=20
probabilities for at least an additional 4 inches of snow are=20
moderate (30-50%) over these areas.


...Rockies and Southwest...=20
Days 1-3...

Broad height falls moving through the Southwest today will sharpen
into a closed low over northern Baja California tonight. With high
pressure nosing down out of the northern Plains, upslope snow will
continue across the CO Front Range via an easterly low level flow=20
along with a relatively low DGZ on D1. However, the snow should=20
not last too long as the upper low sinks further south over central
Baja California. This will help to drag the snowfall southward as=20
well through the Mogollon Rim, Sangre de Cristos, Sacramento=20
Mountains, etc. Models continue to vary on the terrain influence on QPF
(and thus snow) over parts of the Southwest but at least a large=20
footprint of light snow is expected. For the D1-3 period, WPC=20
probabilities for at least an additional 4 inches of snow are >40%
over the Front Range into the CO Rockies, San Juans, and higher=20
elevations in AZ/NM above 8000ft or so.=20

Farther north, a shortwave moving out of Canada through Montana on
Wednesday will touch off some light to locally moderate snow over=20
much of the state that will push into Wyoming and the Black Hills=20
on Thursday. WPC probabilities for at least 4 inches of snow are=20
highest (>50%) over the Little Belts, Gallatin Range, and into the=20
Bighorns. As this shortwave sinks southward and interacts with the
upper low at the base of the trough, increasing snowfall is
possible across the southern NM ranges. WPC probabilities are low
(10-30%) across this region.


...Southern Plains...
Day 3...

As the aforementioned upper low over Baja California begins to
eject northeast on Thursday and merge with the diving shortwave
over the central Rockies, a strong 160-170kt southwest jet stretch
surges north and bulges over ridging in the Gulf of Mexico. This=20
will prompt rich moisture and prominent upper divergence from an=20 increasingly buckled jet. In the right entrance region of this jet,
surface cyclogenesis is expected along the western Gulf Coast by=20
late Thursday night with a precipitation shield blossoming to the=20
north. Given the strong high situated to the north over the Central
Plains and a cold low- level airmass in place, light snow and=20
mixed precipitation is expected over much of western and north-=20
central TX starting Thursday morning, expanding eastward to the=20
Ozarks by the end of the period. Thermal uncertainty is high (as=20
expected) due to the upstream players (timing, depth, etc.) with=20
recent trends suggesting a later start time and warmer low-level=20
air. Additionally, the northern precipitation gradient across OK=20
and northern AR will likely be sharp, with varying snowfall amounts
across short distances.

All ptypes are in play -- snow/sleet/freezing rain and plain rain=20
to the south -- and each will likely not be stagnant in location.=20
For now, the highest probabilities of snow lie on the northwest=20
side of the precipitation shield in the deeper colder air from
north-central TX (near and north of the D-FW Metroplex) through=20
southeast OK and into central AR. There, WPC probabilities for at=20
least 4 inches of snow through 12z Friday are around 50-70%. This
area was a slight trend to the northwest, with the Texas Big
Country seeing lower probs in response to a slower ejection of the
upper low. To the southeast, within the zone of maximum=20
uncertainty, probabilities for accumulating snowfall decrease while probabilities for ice increase. Areas from near Austin=20
northeastward through the Piney Woods into the Hill Parishes of LA=20
show the highest chance of at least 0.1 inches of ice accumulation=20
(30-60%). Given this is the first widespread winter storm for this=20
region, even light amounts of ice or snow could cause travel=20
disruptions.


This system has prompted the issuance of a new set of Key Messages
which are linked at the bottom of the discussion.

Fracasso/Snell



...Winter Storm Key Messages are in effect. Please see current=20
Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_2.png__;!!DZ3fjg!8nPd_Bff91Y4p4t7lT-CodU6YsoKrxbI2o3gyTzYGlF7A= xp6SMYgFloTTEkk4lPHnvJxe1bPTt95uG92sF4mv6Fj9pY$=20

$$

=3D =3D =3D
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FOUS11 KWBC 072054
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
354 PM EST Tue Jan 7 2025

Valid 00Z Wed Jan 08 2025 - 00Z Sat Jan 11 2025

...Great Lakes and Northeast...
Days 1-3...

Behind a departing low, Arctic air streaming over the Great Lakes will
cause additional lake-effect snows south and southeast of each of=20
the lakes in D1/Wednesday. Lake surface temperatures from GLERL for
Lake Michigan are around 45F/+6C, with the Arctic air moving over
the lake this evening expected to range between -12C and -15C at
850 mb. This will be ample instability to sustain cellular
convection within the lake-effect. WPC probabilities for at least 4
inches of snow are generally ranging between 30 to 70 percent, with
the highest probabilities occurring along much of the immediate
Lake Michigan shoreline in far western Michigan, as well as around
Erie, PA where a strong band originating off of Lake Huron is
reinforced by the brief time the band is over Lake Erie. Lesser
probabilities of 10 to 40 percent are off the southeast end of Lake
Ontario, extending into the Syracuse area. By D2/Thursday, high
pressure moving overhead of the Great Lakes will end the lake-
effect from northwest to southeast.

On D3/Friday, a potent upper level longwave trough will approach
the western Great Lakes. Ahead of the shortwave, weak surface
cyclogenesis will occur over Wisconsin and the U.P. of Michigan.
Moisture will stream north from the much stronger low to the south
(more on that below). This should cause widespread light snow over
the upper Lakes through Friday evening. Lake-enhancement due to
continued very cold temperatures could locally increase snow totals
in the form of bands embedded within the broader snowfall shield,
though those details have yet to be ironed out, depending on the
track of the low and how much Gulf moisture makes it to the region.

...Rockies and Southwest...
Days 1-3...

A broad vertically stacked cutoff low over the Southwest tonight
contrasting with an expansive area of surface high pressure over
the Northern Plains will create an E-SE wind across the Front=20
Range of Colorado tonight, as this will be the region between the
two weather features. A brief period of upslope flow may cause=20
some light snow across the region. As the polar high rapidly sinks=20 southward, so too will the light snow, moving into the mountains of
New Mexico and Arizona. Here too the duration of snow will be brief
as the flow turns to a much less favorable southerly direction.

The arrival of Arctic air into the Southwest and Southern Plains
will set the stage for a much more significant and widespread
snowfall event as the vertically stacked low merges with a strong
shortwave by the D2 period on Thursday. A plume of Gulf moisture
will overspread much of Texas, which will feed the surface low as
that moisture is hurled headlong into the Arctic air mass. WPC WSSI
values have an up to 80% chance of 2 inches of snow across much of
southern New Mexico and far west Texas and up to 50% for 4 inches
through the D2/Thursday period for the Sacramento Mountains, with=20
a 10 to 30% chance of 4 inches over the adjacent Plains and into=20
west and north-central Texas and far southern Oklahoma. The most=20
widespread snow will be into D3/Thursday night as a more portent=20
surface low develops and quickly moves east, taking much of the=20
Gulf moisture with it, as well as any widespread precipitation.

WPC probabilities have come down a bit from previous forecasts over
west Texas and New Mexico as the guidance struggles to resolve how
the cutoff low energy and the shortwave energy from the northern
stream interact with one another. Small changes in their
interaction would result in big changes in the snowfall pattern,
but for now the axis of heaviest snow has shifted east somewhat.=20

Farther north, a shortwave moving out of Canada through Montana on
the northern side of the aforementioned longwave trough on
Wednesday will touch off some light to locally moderate snow over
much of the state what will punch into Wyoming and the Black Hills
on Thursday. WPC probabilities for at least 4 inches of snow are=20
highest over the Little Belts, Gallatin Range, and into the=20
Bighorns, in the 30-50 percent range.


...Southern Plains and Southeast...
Days 2-3...

A potent positively tilted longwave trough containing plentiful=20
upper level energy from an aforementioned upper level low will=20
support a very strong upper level jet that stretches from West=20
Texas east through the Carolinas. Surface cyclogenesis will occur=20
Thursday night near the Gulf near the TX/LA border in the RER of=20
that jet, which will bed southward, thus maximizing the upper level
divergence over Louisiana. A plume of Gulf moisture will inject=20
into the low with maximum PWATs along the Texas Gulf Coast rising=20
to around 1.75 inches. While that level of moisture doesn't move=20
too far inland, a large fraction of it will, providing ample=20
moisture for the developing surface low. The associated low level=20
jet will advect much of that moisture up the Lower Mississippi=20
Valley.

Meanwhile, an Arctic air mass will be in place over the Central
Plains and Mid-South, resulting in a strong temperature gradient
which will both increase the forcing as well as lift that moisture
and effectively wring it out, resulting in widespread wintry
precipitation starting in TX/OK and spreading east as far as the
Appalachians by Friday evening. A tight temperature gradient on the
north and west side of the low will promote a similarly tight
gradient in precipitation amounts and in the areas where the cold=20
air at the surface is shallow, an area of freezing rain/sleet will
develop from east central Texas over northern Louisiana and into
central Mississippi, Alabama, and Georgia, with widespread snow
north of that from north-central Texas east through the Tennessee
Valley. There will be some overlap with the snowfall footprint=20
from this most recent winter storm, but much of the greatest=20
impacts will be south of there. Thus, for many this will be a very=20
impactful snowfall event and the first winter storm of the season=20
for areas such as Dallas-Fort Worth north and east through the=20
Ozarks and into the Memphis and Nashville metro areas.

WPC probabilities for Major to locally Extreme impacts extend from
the DFW Metroplex area north and east to the Ozarks, with a thin
stripe of extreme impacts from snow and ice along the Sabine River
from Tyler and Longview, TX east to north and east of Shreveport and
Monroe, LA. Just south of that stripe, moderate impacts from ice
are possible form central Texas north and west of Austin and San
Antonio to these same areas of east-central Texas and far northern
Louisiana. Expect considerable to substantial disruptions of daily
life. Travel is not advised.

The updated set of Key Messages are linked below.

Wegman

...Winter Storm Key Messages are in effect. Please see current=20
Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_2.png__;!!DZ3fjg!4gIUHv-LdivaJrm34s5qP1YkcDpwTyEMTxnhdNNJwgJbv= npigmJrDHsX7MD3ipL0Vr-PgkAYdaJlun4hJs5x1vtmk-k$=20

$$

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FOUS11 KWBC 080905
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
405 AM EST Wed Jan 8 2025

Valid 12Z Wed Jan 08 2025 - 12Z Sat Jan 11 2025

...Great Lakes and Northeast...
Day 1...

Broad upper low churning near Newfoundland and southeast Canada
will continue to help stream arctic air and shortwaves over the
Great Lakes through at least early Thursday. This will cause=20
additional lake- effect snows south and southeast of each of the=20
lakes in D1/Wednesday. Lake surface temperatures from GLERL for=20
Lake Michigan are around 45F/+6C, with the Arctic air moving over=20
the lake this evening expected to range between -12C and -15C at=20
850 mb. This will be ample instability to sustain=20
cellular convection within the lake-effect. WPC probabilities for=20
at least 4 inches of snow are generally ranging between 30 to 70=20
percent, with the highest probabilities occurring along much of the
immediate Lake Michigan shoreline in far western Michigan, around=20
Erie, PA where a strong band originating off of Lake Huron is=20
reinforced by the brief time the band is over Lake Erie, as well as
off the southeast end of Lake Ontario into the Syracuse area. By=20 D2/Thursday, high pressure moving overhead of the Great Lakes will=20
end the lake- effect from northwest to southeast until the next=20
round of light snow arrives from a central U.S. trough on D3.

...Rockies and West Texas...=20
Days 1-2...

The arrival of Arctic air into the Southwest and Southern Plains
will set the stage for a much more significant and widespread
snowfall event as the vertically stacked low merges with a strong
shortwave by the D2 period on Thursday. Strong isentropic ascent
will allow for a period of snow showers later today into tonight.
Additionally, increasing moisture will overspread much of Texas as
upper level southwesterly flow increases in response to the upper
trough sharpening and lifting north. WPC PWPF values have over 70%
chance of 2 inches of snow across much of southern New Mexico=20
peaking over the Sacramento Mts, with lower probs (10-30%) over far
west Texas and nearby southern High Plains northward into New=20
Mexico. and far west Texas and up to 50% for 4 inches through the=20 D2/Thursday period for the Sacramento Mountains, with a 10 to 30%=20
chance of 4 inches over the adjacent Plains and into west and=20
north-central Texas and far southern Oklahoma. The most widespread=20
snow will be into D3/Thursday night as a more portent surface low=20
develops and quickly moves east, taking much of the Gulf moisture=20
with it, as well as any widespread precipitation.=20

Farther north, a shortwave moving out of Canada through Montana on
the northern side of the aforementioned longwave trough on
Wednesday will touch off some light to locally moderate snow over
much of the state what will punch into Wyoming and the Black Hills
tonight into Thursday. WPC probabilities for at least 4 inches of=20
snow are highest over the Little Belts, Gallatin Range, and into=20
the Bighorns, in the 70-90 percent range.


...Southern Plains and Southeast...
Days 2-3...

...Major winter storm is forecast to span from north- central=20
Texas and southeast Oklahoma beginning on Thursday before crossing through
much of the Mid- South and into northern portions of the=20
Southeast by the end of the week...=20

A potent positively tilted longwave trough containing plentiful=20
upper level energy from an aforementioned upper level low will=20
support a very strong upper level jet that stretches from West=20
Texas east through the Carolinas. Surface cyclogenesis will occur=20
Thursday night near the Gulf near the TX/LA border in the RER of=20
that jet, which will bed southward, thus maximizing the upper level
divergence over Louisiana. A plume of Gulf moisture will inject=20
into the low with maximum PWATs along the Texas Gulf Coast rising=20
to around 1.75 inches. While that level of moisture doesn't move=20
too far inland, a large fraction of it will, providing ample=20
moisture for the developing surface low. The associated low level=20
jet will advect much of that moisture up the Lower Mississippi=20
Valley.

Meanwhile, an Arctic air mass will be in place due to a retreating
high pressure system over the Central Plains and Mid-South,=20
resulting in a strong temperature gradient which will both increase
the forcing as well as lift that moisture and effectively wring it
out, resulting in widespread wintry precipitation starting in=20
TX/OK and spreading east as far as the Mid-Atlantic Saturday=20
morning. The tight thermal gradient on the north and west side of=20
the low will promote a similarly tight gradient in snowfall amounts
and in the areas where the cold air at the surface is shallow, an=20
area of freezing rain/sleet will develop from east central Texas=20
over northern Louisiana and into central Mississippi, Alabama, and=20
Georgia, with snow to the north of that from north-central Texas=20
east through the Tennessee Valley. For many this will be a very=20
impactful snowfall event and the first winter storm of the season=20
for areas such as Dallas- Fort Worth north and east through the=20
Ozarks and into the Memphis and Nashville metro areas. Potential
forecasting challenges include banding potential on the northern
and northwest side of the low increased by strong mid-level fgen
and isentropic ascent through the DGZ. This may lead to mesoscale
banding that will become more notable once inside the full suite of
CAMs (most likely by the 12z cycle today). This is particularly a
concern for north-central TX and OK on D1 where overall guidance
had recently trended down in amounts. For the DFW metro region,
upper-end amounts have increased as some ensemble members
(particularly the ECENS) pick up on this banding potential which
allows for mixed precip to quickly change to heavy snow Thursday
night, with the grand ensembles 75th percentile up to 9 inches.
Given the mesoscale nature of these snowbands, conditions will
likely drastically change over the course of a tens of miles, with
the heaviest snow north-northwest and lesser amounts to the south.

Spanning farther east a WAA thump of snow is likely across the
Mid-South Friday morning extending toward the Southeast by the
afternoon hours. The southern track of this low pressure system
supports snow potential reaching pretty far south compared to
climatology, with ECMWF EFI values of 0.7-0.95 and an extreme=20
shift of tail of +8 stretching from northern MS to central GA.
Additionally, the deep cold air mass in place reinforced by a fresh
snowpack to the north will allow for low-level cold air to hold on
while warm air surges north in the mid-levels. This means that
areas along and just south of the I-20 corridor that begin as snow
will likely change over to sleet and freezing rain for a
potentially extended period of time.

WPC probabilities for moderate to major impacts extend from the=20
DFW Metroplex area north and east to the Ozarks, through Memphis
and northern MS before also spreading east to northern GA and the
southern Appalachians. For snowfall amounts, WPC probabilities for
at least 4 inches are high (70-80%) across northern MS, AL, and=20
GA, as well as much of the TN and the southern Appalachians. A=20
stripe of low (10-30%) > 0.1" ice probabilities stretch from=20
central TX through northern LA/southern AR, northern MS, central=20
AL, and into central/northern GA and the SC Midlands. However, it=20
can be expected for these chances to increase with time.

The updated set of Key Messages are at the bottom of the discussion.


...Pacific Northwest and Northern Rockies...
Day 3...
Shortwave entering the Pacific Northwest on Friday will spread high
elevation snow to the Cascades and Northern Rockies as it pushes
eastward and snow levels are expected to start around 4500ft and
fall to around 3000ft by the end of the forecast period. WPC
probabilities for at least 8 inches are between 40-70% for the WA
Cascades and northern ID mountains.

Snell


...Winter Storm Key Messages are in effect. Please see current=20
Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_2.png__;!!DZ3fjg!_xHO6Dt48d-Gpn23y4nQstU3Ez3g4UCSjM1B8fkHO8kWU= kAdC_Qq72gbyQbx2lza-9v0d6Qijjj9nvdQcp35HRAbUTo$=20

$$

=3D =3D =3D
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FOUS11 KWBC 130217
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
917 PM EST Sun Jan 12 2025

Valid 00Z Mon Jan 13 2025 - 00Z Thu Jan 16 2025

...Northern Rockies/Western High Plains...
Day 1...

Lingering upslope flow and the passage of a weak 500mb vort max
Monday morning will keep periods of mountain snow in the forecast
in parts from the Little Belt and Big Snowy in central Montana,as
well as the Absaroka, Big Horns, and Black Hills through midday
Monday. Snowfall will taper Monday afternoon once the 500mb vort
max passes to the south. WPC probabilities in the aforementioned
mountain ranges (particularly in the less populated, higher
elevation areas) sport high chances (>70%) for additional snowfall

4" through 06Z Tuesday.

...Great Lakes...
Days 1-3...

An occluding area of low pressure is responsible for a swath of
snow over the Upper Great Lakes this afternoon. Periods of snow
will continue and fall heavily at times over the Minnesota
Arrowhead this afternoon thanks to additional lake-enhancement from
onshore winds of Lake Superior. As the low moves over Lake
Superior tonight, CAA on the backside of the storm will
reinvigorate LES bands from far northern Wisconsin to the western
half of the Michigan U.P.. The system will make its way slowly east
over Lake Superior on Monday with the TROWAL on the backside of the
storm helping to sustain ongoing LES bands over the Michigan U.P.
Monday night and into Tuesday morning. Most of the northern coast
of the Michigan U.P. sports high chances (>70%) for snowfall
totals >8" through Tuesday night, but the highest totals are
likely to occur in the Huron Mountains and along the Keweenaw
Peninsula where WPC probabilities show moderate-to-high chances
(50-70%) for snowfall totals >12".

Farther east, the cold front tracking across the Great Lakes gives
rise to CAA withing WSW flow over Lakes Michigan, Erie and
Ontario. Expect multi-banded segments emerge off of Lake Michigan
tonight and linger through Tuesday with the heaviest totals over
northwest portions of Michigan's Mitten. WPC probabilities do show moderate-to-high chances (50-70%) from snowfall totals >8" along
the northern most tier of Michigan's Lake Michigan coast through
Tuesday evening. Farther east, dominant LES single-banded segments
will develop off Lakes Erie and Ontario beginning Monday and
persist into the middle of the week. WPC probabilities show high
chances (>70%) for snowfall amounts >18" in the Tug Hill with some
localized spots potentially approaching 30". Farther south, the
Chautauqua Ridge and areas from northwest PA to western NY
communities just south of Buffalo could see as much as 12-18" of
snowfall through Tuesday evening. Note snow is likely to continue
into Tuesday night, although the passage of a 500mb shortwave
trough may shift the LES single-bands farther south into central NY
and along the PA/NY east of Lake Erie. The WSSI already depicts
Major Impact potential for areas south of Buffalo (including a
subsection of I-90) and in the Tug Hill for Monday night and into
Tuesday.

...Northern Mid-Atlantic...
Days 2-3...

While actual snowfall amounts are expected to be minor, both Monday
and Tuesday have the chance to see snow squalls traverse the region
each afternoon and evening. Monday's snow squall potential comes
via a cold front that will push through the Southern Tier of New
York and all of Pennsylvania. Lapse rates will steepen but the
strength of the front and depth of the lapse rates should make for
hit-or-miss squall potential. By Tuesday, a positively tilted 500mb
trough will provide a better source of synoptic-scale lift aloft,
along with strong 700mb CAA. Latest guidance shows western PA and
along the NY/PA border posing the better chances for squalls
Tuesday afternoon, but these snow squalls may be able to traverse
the Keystone State Tuesday evening given the exceptional PVA aloft.
While any totals would generally be on the lighter side, the
sudden bursts of heavy snow could mean dramatic reductions in
visibility and quick accumulations on roadways.


For the Day 1-3 period, the probability of significant icing of at
least 0.10" is less than 10 percent across the CONUS.


Mullinax






$$

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FOUS11 KWBC 130719
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
219 AM EST Mon Jan 13 2025

Valid 12Z Mon Jan 13 2025 - 12Z Thu Jan 16 2025

...Northern Rockies/Western High Plains...
Day 1...

Weak shortwave diving through central/eastern MT this morning will
sink southward through WY atop a surface boundary over eastern MT
into the Black Hills, supporting generally light snow with some
terrain enhancement. Areas in the Black Hills have a high chance

70%) of at least 4 inches of snow today.

...Great Lakes...
Days 1-3...

Occluded area of low pressure over Lake Superior this morning will
only slowly move to the east, leaving a surface trough across the
region and NW flow into the U.P. of Michigan. Additional height
falls via a cold closed low moving out of Canada into northern MN
will maintain/invigorate cyclonic flow across all the Great Lakes
behind the cold front moving into New England. Lake effect snow
will pick up in earnest over the eastern Great Lakes and maintain
itself over Lake Superior into the U.P. and northwest Lower MI for
the next 2-3 days. Snow will gradually wind down from NW to SE late
D2 into D3 ahead of a Canadian system.

For the period, WPC probabilities of at least 8 inches of snow are

50% over the Keweenaw Peninsula and over the eastern U.P. on NW

flow, as well as into northwestern Lower MI. East of Lake Erie, WPC probabilities for at least 8 inches of snow are >50% from Erie, PA
through the Chautauqua Ridge to near the Buffalo southtowns with
some totals likely over a foot. East of Lake Ontario, snowfall will
maximize into the Tug Hill Plateau where WPC probabilities for at
least 12 inches of snow are high (>70%) and even have a small area
of >50% probs for at least two feet around Redfield.


...PA/NY...
Days 1-2...

Approaching cold front today may instigate some snow squalls
across the region given relatively steep low-level lapse rates. On
Tuesday, approaching vort max may again provide an atmosphere
conducive for some snow squalls, with the models generally showing
an area of >1 in the snow squall parameter. Amounts will generally
be light but these can be hazardous to drivers.


For the Day 1-3 period, the probability of significant icing of at
least 0.10" is less than 10 percent across the CONUS.


Fracasso

$$

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FOUS11 KWBC 131930
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
230 PM EST Mon Jan 13 2025

Valid 00Z Tue Jan 14 2025 - 00Z Fri Jan 17 2025

...Northern Rockies/Plains...
Day 1...

Deep northwesterly flow, with embedded energy aloft, will support
periods of snow and a stripe of mostly light accumulations from
the northern Rockies southeastward into the mid Missouri Valley.
Additional accumulations after 00Z are expected to be no more than
an inch or two for most locations. However, WPC probabilities
indicate that orographic enhancement may support some totals of at
least 4 inches in the north-central Wyoming and south-central
Montana mountains and in the Black Hills.


...Great Lakes...
Days 1-3...

An occluded surface low will move east of the Great Lakes through
the St. Lawrence Valley, leaving deep cyclonic flow in its wake.
Lake effect snows will continue to develop and intensify, with
locally heavy amounts beginning to accumulate in the typical
snowbelts this evening. Lake effect snows will continue into
Wednesday before an approaching shortwave diving southeast through
central Canada introduces some warm advection/synoptic-driven
snows into the region by late in the day.

Some of the heaviest lake effect snows are expected to fall east
of Lake Ontario where a single band is expected to drift slowly
south across the region over the next day. WPC probabilities
indicate that accumulations greater than a foot are likely from
this evening into late Tuesday across the region. Other areas
impacted will likely include the northwestern Pennsylvania and far
western New York counties, where localized two-day totals over a
foot can be expected. Lesser accumulations are expected elsewhere,
including the northern Michigan snowbelts.


...Pennsylvania...
Day 1...

Steep lapse rates in addition to an amplified shortwave moving
through the base of the broader scale trough may support snow
showers and potential squalls east of Lake Erie on Tuesday. The NAM
and GFS continue to show Snow Squall Parameter values greater than
2, especially across western to central Pennsylvania tomorrow afternoon/evening. Accumulations will generally be light, but a
brief period of intense, wind-driven snow may create hazardous
driving conditions.


For the Day 1-3 period, the probability of significant icing of at
least 0.10" is less than 10 percent across the CONUS.


Pereira




$$

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FOUS11 KWBC 140708
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
208 AM EST Tue Jan 14 2025

Valid 12Z Tue Jan 14 2025 - 12Z Fri Jan 17 2025

...Great Lakes...
Days 1-3...

Deep cyclonic flow is in place over the Great Lakes and will start
to erode from NW to SE starting later today. Light to moderate
snow over the western Great Lakes will wane later tonight with an
additional few inches of snow over the favored lake belts. East of
Lakes Erie/Ontario, locally heavy amounts will fall over the
typical snowbelts today then start to lessen in intensity and
transition from single banded to more multi-banded as the upper
trough swings through. Starting tomorrow evening, an approaching
shortwave and surface warm front will bring in some light WAA-
driven snow to the western Great Lakes that will progress through
the eastern Great Lakes.

For the lake effect snow, WPC probabilities of at least an
additional 6 inches of snow D1-1.5 are highest (>70%) over
northwest PA into southwestern NY where 10+" are likely in the
band. East of Lake Ontario, WPC probabilities for at least an
additional 12 inches of snow are highest in the Tug Hill Plateau.
By D3, as the weaker system moves through the area, WPC
probabilities for at least 4 inches of snow are low (10-40%)
downwind of Lakes Erie/Ontario and also over the central
Appalachians thanks to some modest upslope.


...Eastern OH/W PA...
Day 1...

Steep lapse rates amplified by an amplified shortwave moving
through the base of the broader scale trough may support snow
showers and potential squalls east and southeast of Lake Erie this
afternoon. The NAM and GFS continue to show Snow Squall Parameter
values greater than 2 as 850mb temps of -15C or so move across the
region. Accumulations will generally be light, but a brief period
of intense, wind-driven snow may create hazardous driving
conditions.


For the Day 1-3 period, the probability of significant icing of at
least 0.10" is less than 10 percent across the CONUS.


Fracasso



$$

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FOUS11 KWBC 142008
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
308 PM EST Tue Jan 14 2025

Valid 00Z Wed Jan 15 2025 - 00Z Sat Jan 18 2025

...Great Lakes & Central Appalachians...
Days 1-3...

Multiple days of lake-effect snow are expected through the end of
the workweek as Arctic high pressure over the Plains advects frigid
air over the lakes, resulting in heavy snow downwind. Clipper
disturbances crossing the lakes will shift the bands north and
south, as well as introduce variability into how strong the bands
are.

The first disturbance will be exiting the lower lakes at the start
of the D1/Wednesday period this evening. Ongoing single banded
lake-effect will shift southward into the disturbance. Once the
flow becomes more northerly tonight behind the disturbance, the
single bands should shift west and break into much weaker multi-
bands as the flow becomes more perpendicular to the lakes.

This southward shift will be short-lived however as the center of
the Arctic high builds into the Ohio Valley and the next
disturbance approaches the upper lakes from the heart of Canada.
Thus, expect a rapid northward shift in the bands from west to east
as southwesterly flow builds in behind the high.

The second disturbance will be the stronger of the two as it moves
into the upper lakes Wednesday into Wednesday night. It stays
plenty cold enough for all snow. For most areas away from the lakes
the snow should remain light...however where the lakes can locally
enhance the moisture associated with this disturbance, heavy snow
is possible. For the lower lakes, the flow will remain
northwesterly into Wednesday before the northward shift occurs
Wednesday night into Thursday. With this disturbance approaching
the lake-effect should remain rather disorganized as it shifts
north.

On Thursday, the light snow with the disturbance will have
overspread all the lakes, though behind it Lake Superior will
increasingly favor the lake-enhanced areas of the northern U.P.
Further, heavier snow lifted by the central Appalachians will begin
to impact much of West Virginia and western Pennsylvania Thursday
morning. The upslope snow will continue into Thursday night before
a new high pressure area building in increases subsidence and dry
air and ends the snow.

Warm advection ahead of a third disturbance will effectively end
much, if not all of the lake-effect during the day Friday. This
third disturbance should remain almost entirely north of the
lakes, though some light snow may spread into the Arrowhead late
Thursday night.

WPC Probabilities show a low (10-30%) chance of 6 inches of snow
or more for the D1/Wednesday period southeast of the lower lakes.
There's also a low (10-30%) chance of 4 inches of snow across the
central Appalachians, particularly along the Allegheny Front of
PA/MD/WV.

...Northern High Plains...
Day 3...

An Arctic front at the leading edge of some of the coldest air of
the season will race southward out of Canada into the northern High
Plains of MT/WY/ND on Friday. Supported by the RER of the jet, a
shearing shortwave, and the contrast of warmer air ahead of the
front, this "blue norther" scenario appears highly favorable for
snow squalls, and more persistent heavy snows into the Little Belt,
Big Snowy, Bighorns, and the Absarokas.

The front will enter the Montana Hi-Line Thursday evening, racing
south down the Plains through Montana and clearing Wyoming by
Friday evening. Along with the front and favorable forcing, expect
snow squalls to develop with the front with a first peak in
intensity (based on the NAM) in the predawn hours Friday morning
across central Montana. The Snow Squall Parameter nears the top of
the scale (a 5) at this time. A secondary peak follows it during
the day Friday in Montana as the initial wave tracks down the
Wyoming Plains. The snow squalls will be characterized by rapid
changes in visibility at the initial onset of the snow as clear
conditions can drop to near whiteout in a matter of seconds. Once
the squall clears visibility improves about as fast as it initially deteriorated. For most of the Plains, total accumulations are
expected to be minor, however since the snow will be heavy, brief
periods of hazardous driving conditions are likely. In the
aforementioned mountains, expect a more prolonged period of heavy
snow as upslope effectively wrings out the moisture, resulting in
multiple inches of accumulation.



For the Day 1-3 period, the probability of significant icing of at
least 0.10" is less than 10 percent across the CONUS.


Wegman

$$

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FOUS11 KWBC 142010
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
310 PM EST Tue Jan 14 2025

Valid 00Z Wed Jan 15 2025 - 00Z Sat Jan 18 2025

...Great Lakes & Central Appalachians...=20
Days 1-3...

Multiple days of lake-effect snow are expected through the end of
the workweek as Arctic high pressure over the Plains advects frigid
air over the lakes, resulting in heavy snow downwind. Clipper
disturbances crossing the lakes will shift the bands north and
south, as well as introduce variability into how strong the bands
are.

The first disturbance will be exiting the lower lakes at the start
of the D1/Wednesday period this evening. Ongoing single banded=20
lake-effect will shift southward into the disturbance. Once the
flow becomes more northerly tonight behind the disturbance, the
single bands should shift west and break into much weaker multi-
bands as the flow becomes more perpendicular to the lakes.

This southward shift will be short-lived however as the center of
the Arctic high builds into the Ohio Valley and the next
disturbance approaches the upper lakes from the heart of Canada.
Thus, expect a rapid northward shift in the bands from west to east
as southwesterly flow builds in behind the high.

The second disturbance will be the stronger of the two as it moves
into the upper lakes Wednesday into Wednesday night. It stays
plenty cold enough for all snow. For most areas away from the lakes
the snow should remain light...however where the lakes can locally
enhance the moisture associated with this disturbance, heavy snow
is possible. For the lower lakes, the flow will remain
northwesterly into Wednesday before the northward shift occurs
Wednesday night into Thursday. With this disturbance approaching
the lake-effect should remain rather disorganized as it shifts
north.

On Thursday, the light snow with the disturbance will have
overspread all the lakes, though behind it Lake Superior will
increasingly favor the lake-enhanced areas of the northern U.P.
Further, heavier snow lifted by the central Appalachians will begin
to impact much of West Virginia and western Pennsylvania Thursday
morning. The upslope snow will continue into Thursday night before
a new high pressure area building in increases subsidence and dry
air and ends the snow.

Warm advection ahead of a third disturbance will effectively end
much, if not all of the lake-effect during the day Friday. This
third disturbance should remain almost entirely north of the=20
lakes, though some light snow may spread into the Arrowhead late=20
Thursday night.

WPC Probabilities show a low (10-30%) chance of 6 inches of snow
or more for the D1/Wednesday period southeast of the lower lakes.
There's also a low (10-30%) chance of 4 inches of snow across the
central Appalachians, particularly along the Allegheny Front of
PA/MD/WV.

...Northern High Plains...
Day 3...

An Arctic front at the leading edge of some of the coldest air of
the season will race southward out of Canada into the northern High
Plains of MT/WY/ND on Friday. Supported by the RER of the jet, a
shearing shortwave, and the contrast of warmer air ahead of the=20
front, this "blue norther" scenario appears highly favorable for
snow squalls, and more persistent heavy snows into the Little Belt,
Big Snowy, Bighorns, and the Absarokas.=20

The front will enter the Montana Hi-Line Thursday evening, racing
south down the Plains through Montana and clearing Wyoming by
Friday evening. Along with the front and favorable forcing, expect
snow squalls to develop with the front with a first peak in
intensity (based on the NAM) in the predawn hours Friday morning
across central Montana. The Snow Squall Parameter nears the top of
the scale (a 5) at this time. A secondary peak follows it during
the day Friday in Montana as the initial wave tracks down the
Wyoming Plains. The snow squalls will be characterized by rapid
changes in visibility at the initial onset of the snow as clear
conditions can drop to near whiteout in a matter of seconds. Once
the squall clears visibility improves about as fast as it initially deteriorated. For most of the Plains, total accumulations are
expected to be minor, however since the snow will be heavy, brief
periods of hazardous driving conditions are likely. In the
aforementioned mountains, expect a more prolonged period of heavy
snow as upslope effectively wrings out the moisture, resulting in
multiple inches of accumulation.



For the Day 1-3 period, the probability of significant icing of at
least 0.10" is less than 10 percent across the CONUS.


Wegman


...Winter Storm Key Messages are in effect. Please see current=20
Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_1.png__;!!DZ3fjg!8mkUs5mLvapn18tIaBsy7s85v0KmqYDxDc591HTmIcouY= XD6WVAo1nzReAEnnKvCOHqzlfdF1-n2bn0hhPkZJicB22k$=20

$$

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FOUS11 KWBC 150731
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
231 AM EST Wed Jan 15 2025

Valid 12Z Wed Jan 15 2025 - 12Z Sat Jan 18 2025

...Great Lakes & Central Appalachians...=20
Days 1-2...

Ongoing lake-effect snow over the eastern Great Lakes will wind=20
down today as a new shortwave moves in from the northwest, bringing
light snow to the Upper Midwest/Western Great Lakes on WAA. As the
warm front passes, winds will switch to NW as the cold front makes
its approach but weakens across the region in response to height=20
rises from the west. Regardless, some lake enhancement or lake=20
effect snow is likely over much of the region but with overall=20
light amounts over the U.P. and into western Lower Michigan.=20

East of Lakes Erie and Ontario, system will bring in some light=20
snow followed by a period of lake enhanced/effect snow D2 before=20
ending D3. To the south, shortwave will swing right through the=20
central Appalachians, maximizing upslope into eastern WV where=20
several inches of snow are likely D2. WPC probabilities for at=20
least 6 inches are high (>70%) especially over 2000ft.=20


...Northern/central Rockies/High Plains...
Day 3...

As an upper ridge builds across the Northeastern Pacific Thursday=20
into Friday, downstream response will be digging troughing out of=20
western Canada nearly due south through the High Plains via a=20
strong cold front ("blue norther"). Though moisture will be=20
limited, strong northerly flow will support upslope enhancement=20
into some of the terrain over central/western Montana (esp the=20
Little Belts and Big Snowy Mountains) southward into the Bighorns,=20 Absarokas, and into the southeastern WY ranges.=20

As the front dives southward, steeper lapse rates will support=20
snow squalls along the front Friday in Montana progressing into=20
Wyoming. Snow squall parameter per the guidance still shows values=20

1 (and even >3) suggesting the possibility of bursts of snow with=20

sharply reduced visibility leading to near whiteout conditions. CAM
guidance should shed a little more light on the threat over the=20
next two days, but we have outlined this area in our Key Messages=20
(see below). WPC probabilities for at least 4 inches of snow over=20 Montana/Wyoming are moderate (40-70%) and mostly over the terrain=20
with light snow (1-2") elsewhere.=20

As the system races southward, cold front will slow a bit across=20
the Rockies but continue to plunge through the High Plains,=20
favoring upslope enhancement into the Front Range late Fri/early=20
Sat. Snow will expand through the I-25 corridor into the Denver=20
Metro area with higher amounts across the Front Range as=20
temperatures fall into the teens, helping to increase SLRs from=20
~12:1 up toward ~18:1. Additional snowfall is likely past 12Z Sat.=20
Through then, WPC probabilities for at least 4 inches of snow are=20
at least 50% across the Front Range.


For the Day 1-3 period, the probability of significant icing of at
least 0.10" is less than 10 percent across the CONUS.


Fracasso


...Winter Storm Key Messages are in effect. Please see current Key
Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_1.png__;!!DZ3fjg!-ldOWVByooDW7BLWs9uj_P_dU1lz4Xcjq6A0kq2xC_drX= XAWf_Odc8mo06IxKc1G0vi-hLX-Bnp1mtzxLsY2Lqe6Ygg$=20

$$

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FOUS11 KWBC 152045
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
345 PM EST Wed Jan 15 2025

Valid 00Z Thu Jan 16 2025 - 00Z Sun Jan 19 2025

...Great Lakes & Central Appalachians...=20
Days 1-2...

A trough extending from a low moving east over southern Hudson Bay
will push across the western Great Lakes tonight and the eastern on
Thursday. Areas of light snow can be expected ahead of the trough
with up to 24 hours of moderate lake effect snow behind on NWly=20
flow. Day 1 snow probabilities for >4" are 30-60% for preferred=20
U.P. and western MI snowbelts with 50-80% values on Day 1.5 east of
Lake Erie.=20
The broad NWly flow allows ideal upslope flow for snow=20
showers in the central Appalachians. Day 1.5 snow probabilities for

6" are 60-90% from the Laurels of PA through the Allegheny=20

Highlands in WV.

Day 3...

Strong surface low development shifting east across Ontario Friday
is on the leading edge of an Arctic plunge (see Key Messages linked
below) down the Great Plains. Broad cyclonic flow under this trough
sends notable moisture north from the Southeast and over the Great
Lakes and eventually Northeast. Precip onset in the central
Appalachians is Saturday morning. Sufficient cold air should linger
at the surface for a wintry mix including freezing rain to occur.
Probabilities for significant freezing rain are currently low, but
there is a threat particularly in the sheltered valleys in VA/WV/MD
into PA.


...Rockies and High Plains...=20
Days 2/3...

A rather amplified upper ridge builds across the Northeastern=20
Pacific and Alaska Thursday into Friday which opens polar air to
plunge south from the Canadian Prairies with a strong cold front=20
("blue norther") sweeping down the Great Plains Friday through
Saturday. Though moisture will be limited, strong Nly flow around=20
a 1055mb high that pushes into MT Saturday evening will support=20
upslope enhancement along the length of the eastern slopes of the=20 Rockies.=20

As the front dives southward, steeper lapse rates will support=20
snow squalls along the front early Friday in central Montana=20
progressing across Wyoming and possibly northern Colorado through=20
Friday evening. Snow squall parameter values in the 12Z NAM are 2
to 4 over these areas suggesting bursts of snow with sharply=20
reduced visibility leading to near whiteout conditions. CAM=20
guidance should shed a little more light on the threat over the=20
next two days, but we have outlined this area in our Key Messages=20
(see below). Day 2 WPC snow probabilities for >4" remain moderate
(40-70%) over terrain in central Montana/northern Wyoming.

As the front races southward, enhanced lift from a reinforcing
shortwave trough on the backside of the sprawling upper trough will
allow more moderate snow over the CO Front Range and the I-25=20
corridor through the Denver Metro area. Day 3 snow probabilities
for >4" are high 60-90% in terrain and around 50% in the foothills
to the I-25 corridor from southern WY through CO.

Jackson



...Winter Storm Key Messages are in effect. Please see current=20
Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_1.png__;!!DZ3fjg!7fFclx_AnPLodOb4JPeD6gm9AJQrac1NXsE8jjaGB1Y42= ZXjWJWwuDU-2GsJIUldzB0ef_IlMKhPPOSa7M_NfO0DjsA$=20

$$

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FOUS11 KWBC 160813
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
313 AM EST Thu Jan 16 2025

Valid 12Z Thu Jan 16 2025 - 12Z Sun Jan 19 2025

...Great Lakes & Central Appalachians...=20
Days 1...

A potent 500mb trough diving south through the Great Lakes this
morning will foster healthy upper level divergence over the
Northeast and Central Appalachians today while also directing=20
plume of 700-300mb layer-averaged moisture through these regions.=20
In addition, the brief shot of low-level CAA will trigger some=20
lake-effect snow showers today with the Chautauqua Ridge sporting=20 moderate-to-high chances (50-70%) for snowfall totals >4" through=20
Thursday evening. The Central Appalachians are most favored for=20
heavy snowfall totals given their favorable position beneath the=20
diffluent left-exit region of a 100kt 500mb jet streak and upwards=20
of 40kt westerly winds at 850mb aiding in healthy upslope snowfall.
Snow looks to fall heaviest beginning late morning and lasting=20
through the afternoon with rates topping 0.5"/hr in some cases. In=20
fact, given the steepening lapse rates Thursday afternoon over the=20 Mid-Atlantic, residents in the region should be on the lookout for=20
possible snow squalls given the favorable time of day and more than
adequate upper-level divergence aloft. Snow in the Central=20
Appalachians looks to continue Thursday night and finally tapers=20
off by Friday morning. WPC probabilities depict high chances=20

70%) for snowfall totals >6" from the Laurel Highlands of=20

southern Pennsylvania and near the MD/WV border to the Allegheny=20
Highlands in West Virginia. Some of the taller peaks of eastern=20
West Virginia could see totals around a foot of snow by Friday=20
morning.=20

Day 3...

By Friday night, a deepening area of low pressure will escort a
strong Arctic front south that will deliver a frigid air-mass that
is the coldest and most dangerous of the season this weekend and
into next week (see Key Messages linked below). The Arctic front
will turn on the LES machine over Lake Superior by early Friday
morning while periods of snow develop along and in wake of the
Arctic frontal passage over the Great Lakes Saturday afternoon,=20
then over the eastern Great Lakes and central Appalachians Saturday
evening. LES bands look to form over parts of central and western=20
New York Saturday night that likely persist through the remainder=20
of the weekend. Through 12Z Sunday, WPC probabilities show=20
moderate-to-high chances (50-70%) for snowfall >4" and low chances
(10-30%) for snowfall >6" in the Tug Hill Plateau, but expect=20
these probabilities and snowfall totals to rise as the LES bands=20
that linger in the medium range enter the short range over the next
24-48 hours.=20

...Rockies and High Plains...=20
Days 2-3...

The impending snowfall in these region, starting late Thursday
night in the northern Rockies/High Plains, is driven by a
combination of both upper level disturbances and the arrival of a
bitterly cold Arctic air-mass that is destined to be the coldest
air-mass of the season to infiltrate the Lower 48. This
exceptional cold front (a "blue norther") out ahead of will push=20
south Thursday night at the same time as 500mb PVA ahead of an=20
approaching upper trough occurs over the northern Rockies/High=20
Plains. Snow showers will breakout across Montana early Friday=20
morning and make its way south throughout the day into Wyoming,=20
then into Colorado by Friday night. Southern Montana and into=20
Wyoming, in particular, sport notably higher snow squall parameters
given the stronger surface- based heating that will cause steeper=20
low-level lapse rates. Snow squalls would cause rapid reductions in
visibility due to a combination of heavy snow rates and whipping=20
wind gusts, as well as rapid accumulations on roads as temperatures
plummet well below freezing. Motorists in these areas should keep=20
a close eye on potential snow squalls as it can cause dangerous=20
travel conditions in a matter of seconds.

As the front plunges south Friday evening, the snow potential
shifts south into Colorado with periods of snow getting started
near the start of the evening rush hour in the Denver/Boulder metro
area. While the snow squall potential may not be as high, the
easterly upslope-enhancement of snowfall rates will make for
occasional periods of heavy snow Friday evening. Snowfall rates
will weaken as the front works its way south of the Palmer Divide
Friday night and down the spine of the Sangre De Cristo early
Saturday morning with some snow along the Front Range lingering
into the day on Saturday as a dome of ~1050mb high pressure builds
in from the Canadian Prairies. WPC probabilities are keying in on
the Front Range as the focus for the heaviest snowfall with
moderate-to-high chances (50-70%) for snowfall >4" from the Laramie
Range on south to parts of the Sangre De Cristo. The I-25 corridor
from Fort Collins on south to the Colorado Springs area Friday=20
have similar probabilities for >4" of snowfall with even low=20
chances (10-30%) for greater than 6" between Friday evening and=20
Saturday morning in spots. Some of the higher peaks (>10,000ft)=20
have moderate chances (40-60%) for snowfall >8" through Saturday=20
morning. The WSSI is currently depicting Minor Impacts for just=20
about all of these referenced areas, suggesting the potential for=20
winter weather conditions that would require enhanced caution while
driving in these affected areas.


Mullinax


...Winter Storm Key Messages are in effect. Please see current=20
Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_1.png__;!!DZ3fjg!9K4pbM8Z7v2hp7d1Lh9n2vIQ0T1AkHWAjp6hGM6tfRADd= RzBz81dTmleQaRUY0mkeFCCwZQplmm3FzsoqM5HaT0_Tmw$=20

$$

=3D =3D =3D
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FOUS11 KWBC 162023
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
323 PM EST Thu Jan 16 2025

Valid 00Z Fri Jan 17 2025 - 00Z Mon Jan 20 2025

...Great Lakes & Central Appalachians...
Days 1 & 3...

Day 1...

Continued snowfall across the Buffalo South towns down
into the proxy of the Chautauqua Ridge will be ongoing for another
few hours with multi-hour accumulations approaching 4+" within the
most persistent bands downwind of Lake Erie. This setup will cease
with the loss of sufficient forcing, bringing an end to the threat
by the beginning of the 00z Friday time frame.

Over the Central Appalachians, the previously identified favorable
forecast for heavy snowfall remains steadfast as the overall
synoptic pattern maintained continuity within the 12z suite of NWP.
Heavy snowfall totals are anticipated given their favorable
position beneath the diffluent left- exit region of a 100kt 500mb
jet streak and upwards of 40kt westerly winds at 850mb aiding in
healthy upslope snowfall. This has been highlighted within the
latest hi- res window with the recent 18z RAP/HRRR output
indicating a healthy uptick in snowfall beginning ~20z, lingering
through portions of the overnight period before decaying prior to
sunrise as dry air advection and decreased mid-level forcing ushers
an end to the potential. Snow looks to fall heaviest between now
and 06z Friday with rates topping 0.5"/hr in some cases. Steepening
lapse rates have been analyzed via Mesoanalysis over the Mid-
Atlantic over the past few hours along and east of the Appalachian
front, likely providing residents in the region to be on the
lookout for possible snow squalls given the favorable time of day
and more than adequate upper-level divergence aloft. With the
heaviest snowfall still relegated to maintain the Central
Appalachians, WPC probabilities continue to depict locally elevated
chances (>40%) for additional snowfall totals >4" near the MD/WV
border down through the Highlands in West Virginia. Some of the
taller peaks of eastern West Virginia could see snow totals
between 8-12" by Friday morning from the cumulative nature of the
event, especially within Tucker, Randolph, and western Pocahontas
counties.


Day 3...

By Friday night, a deepening area of low pressure will escort a
strong Arctic front south that will deliver a frigid air-mass that
is the coldest and most dangerous of the season this weekend and
into next week (see Key Messages linked below). The Arctic front
will turn on the LES machine over Lake Superior by early Friday
morning while periods of snow develop along and in wake of the
Arctic frontal passage over the Great Lakes Saturday afternoon,
then over the eastern Great Lakes and central Appalachians Saturday
evening. LES bands look to form over parts of central and western
New York Saturday night that likely persist through the remainder
of the weekend. Through 12Z Sunday, WPC probabilities show
moderate- to- high chances (50-70%) for snowfall >4" and low
chances (10-30%) for snowfall >6" in the Tug Hill Plateau, but
expect these probabilities and snowfall totals to rise as the LES
bands that linger in the medium range enter the short range over
the next 24-48 hours.

Across the Central Appalachians, a secondary round of heavy snow
within the 72 hour period will take shape as the Arctic front
pushes across the terrain with prevailing westerly flow advecting
bitter cold air within the primary cold air advection regime post-
Arctic front. A period of upslope snowfall will begin Saturday
behind expected fropa, weakening as we step into early Sunday
laying the ground work for the next round of expected snowfall as
surface cyclogenesis is forecast across the Southeast thanks to an
amplifying trailing wave tracking towards the Mid Atlantic coast.
Ensemble spread has tightened as of the recent 12z suite, however
there is still some discrepancy on the magnitude of the snowfall
potential within the region. A lot is contingent on the
amplification regime of the trailing wave with most of the
ensembles now trending towards a more robust surface reflection and
regional ascent pattern across the entire Mid Atlantic extending
west into the Appalachian Front. Latest probabilities have
increased substantially run-to-run given the trends with the >4"
signal now within the moderate-high category (40-70%), even
carrying a small region of (40-60%) for >6" within the favored
areas of West Virginia, primarily the zones capable of both the
upslope component combined with the synoptic impacts anticipated
over the course of Sunday.


...Rockies and High Plains...
Days 2-3...

There has been little change with the impending snowfall in these
regions, starting late Thursday night in the northern Rockies/High
Plains, is driven by a combination of both upper level disturbances
and the arrival of a bitterly cold Arctic air-mass that is
destined to be the coldest air-mass of the season to infiltrate the
Lower 48. This exceptional cold front (a "blue norther") out ahead
of will push south Thursday night at the same time as 500mb PVA
ahead of an approaching upper trough occurs over the northern
Rockies/High Plains. Snow showers will breakout across Montana
early Friday morning and make its way south throughout the day into
Wyoming, then into Colorado by Friday night. Southern Montana and
into Wyoming, in particular, sport notably higher snow squall
parameters given the stronger surface- based heating that will
cause steeper low-level lapse rates. Snow squalls would cause rapid
reductions in visibility due to a combination of heavy snow rates
and whipping wind gusts, as well as rapid accumulations on roads as temperatures plummet well below freezing. Motorists in these areas
should keep a close eye on potential snow squalls as it can cause
dangerous travel conditions in a matter of seconds.

As the front plunges south Friday evening, the snow potential
shifts south into Colorado with periods of snow getting started
near the start of the evening rush hour in the Denver/Boulder metro
area. While the snow squall potential may not be as high, the
easterly upslope-enhancement of snowfall rates will make for
occasional periods of heavy snow Friday evening. Snowfall rates
will weaken as the front works its way south of the Palmer Divide
Friday night and down the spine of the Sangre De Cristo early
Saturday morning with some snow along the Front Range lingering
into the day on Saturday as a dome of ~1050mb high pressure builds
in from the Canadian Prairies. WPC probabilities are keying in on
the Front Range as the focus for the heaviest snowfall with
moderate-to-high chances (50-70+%) for snowfall >4" from the
Laramie Range on south to parts of the Sangre De Cristo. The I-25
corridor from Fort Collins on south to the Colorado Springs area
Friday have similar probabilities for >4" of snowfall with even low
chances (25-50%) for greater than 6" between Friday evening and
Saturday morning in spots. Some of the higher peaks (>10,000ft)
have moderate chances (40-60%) for snowfall >8" through Saturday
morning. The WSSI is currently depicting Minor Impacts for just
about all of these referenced areas, suggesting the potential for
winter weather conditions that would require enhanced caution while
driving in these affected areas.

...Mid Atlantic and Northeast...
Day 3...

A trailing wave along the southern periphery of the Arctic front
will exit out of the southeastern U.S with an expectation for the
wave to amplify as it approaches the Southern Mid Atlantic with
attendant surface cyclogenesis across the Deep South, moving
northeast. Recent ensemble and deterministic output via the 12z
suite of NWP has become increasingly aggressive with the signal for
a low end moderate event for heavy snow prospects, mainly from
southwestern VA up through the Central Mid Atlantic, eventually
bleeding into the periods following as the pattern shifts northeast
up through the Northern Mid Atlantic (PA/NY) and Southern New
England. There has been some discrepancy within the suite on the
exact placement of the heaviest precip potential with a split on
two camps favoring a more amplified setup (UKMET/CMC) and a less
amplified evolution (ECMWF) with the GFS now solidly in the middle
of the two camps. ECMWF AIFS ML output has been very steady over
the course of the last 3+ days of output with the latest run
showing great consistency when assessing run-to-run variability.
This lends credence to a slightly better confidence with regards to
snowfall potential across the area with the probabilities of >4"
going from close to 0% overnight towards (10-30%) within a span of
one run. It's a period to monitor as heavy snowfall threat would
impact the population centers of the DMV up through Philadelphia
and points northeast in the periods beyond.

Mullinax/Kleebauer


$$

= = =
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FOUS11 KWBC 170810
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
310 AM EST Fri Jan 17 2025

Valid 12Z Fri Jan 17 2025 - 12Z Mon Jan 20 2025

...Great Lakes...
Days 2-3...

By Friday night, a deepening area of low pressure will escort a
strong Arctic front south that delivers a frigid air-mass that is=20
the coldest and most dangerous of the season this weekend and into=20
next week (see Key Messages linked below). The Arctic front will=20
turn on the LES machine over Lake Superior by Friday night while=20
periods of snow develop along the northern and western coasts of
Michigan's Mitten by Saturday morning. Following an initial WAA
thump of snow Saturday afternoon, LES bands look to form over=20
parts of central and western New York Saturday night that likely=20
persist through the remainder of the weekend. Through 12Z Monday,=20
WPC probabilities show moderate-to-high chances (50-70%) for=20
snowfall >4" and low chances (~10%) for snowfall >8" in the Tug=20
Hill Plateau. The Michigan U.P. sports moderate-to-high chance
probabilities (50-70%) for >8" of snow in the Porcupine Mountains
and in parts of the eastern U.P.. Expect these probabilities and=20
snowfall totals to rise as LES bands increase in coverage an=20
intensity throughout the region as winds accelerate and the heart=20
of the bitterly cold temperatures aloft race over the Great Lakes=20
next week.


...Rockies and High Plains...=20
Days 1-2...

The combination of both upper level diffluence ahead of a deep-
layer trough and the arrival of a bitterly cold Arctic air-mass=20
that is destined to be the coldest air-mass of the season to=20
infiltrate the Lower 48 will bring about Snow squalls and mountain
snow to parts of these regions through Saturday. This exceptional=20
cold front (a "blue norther") out ahead of the upper trough will=20
push south this morning while at the same time, 500mb PVA ahead of=20
an approaching upper trough occurs over the northern Rockies/High=20
Plains. Snow showers will breakout across Montana this morning and=20
make its way south throughout the day into Wyoming, then into=20
northern Colorado by Friday afternoon. Southern Montana and into=20
Wyoming, in particular, sport notably higher snow squall parameters
given the stronger surface-based heating that will cause steeper=20
low-level lapse rates. Snow squalls are also possible today along=20
the cold front as it tracks east across the Dakotas Snow squalls=20
can cause rapid reductions in visibility due to a combination of=20
heavy snow rates and whipping wind gusts, as well as rapid=20
accumulations on roads as temperatures plummet well below freezing.
Motorists in these areas should keep a close eye on potential snow
squalls as it can cause dangerous travel conditions in a matter of
seconds.

As the front plunges south Friday evening, the snow potential
shifts south into Colorado with periods of snow beginning near the
start of the evening rush hour in the Denver/Boulder metro area.=20
While the snow squall potential may not be as high by this time,=20
the easterly upslope-enhancement of snowfall rates will make for=20
occasional periods of heavy snow Friday evening. Snowfall rates=20
will weaken as the front works its way south of the Palmer Divide=20
Friday night and down the spine of the Sangre De Cristo early=20
Saturday morning with some snow along the Front Range lingering=20
into the day on Saturday as a dome of ~1050mb high pressure builds=20
in from the Canadian Prairies. WPC probabilities are keying in on=20
the Front Range as the focus for the heaviest snowfall with=20
moderate-to-high chances (50-70+%) for snowfall >4" from the=20
Laramie Range on south to parts of the Sangre De Cristo. The I-25=20
corridor from Fort Collins on south to the Colorado Springs area=20
Friday have similar probabilities for >4" of snowfall with even
low chances (10-30%) for greater than 6" between Friday evening=20
and Saturday morning in spots. Some of the higher peaks (>10,000ft)
have moderate chances (40-60%) for snowfall >8" through Saturday=20
morning. The WSSI is currently depicting Minor Impacts for just=20
about all of these referenced areas, suggesting the potential for=20
winter weather conditions that would require enhanced caution while
driving in these affected areas. The one notable exception is the=20
Denver metro area as Moderate Impacts are now depicted, indicating=20
there are likely to be disruptions ot daily life with potential=20
closures and hazardous driving conditions.


...Mid Atlantic and Northeast...
Day 3...

...Confidence is increasing in a disruptive winter storm from the
Mid-Atlantic to the Northeast with locally heavy snowfall possible
along the I-95 corridor...

Model diagnostics: Over the past 24-36 hours, guidance has come=20
into better agreement on a deepening 500mb trough over the Ohio=20
Valley and higher height rises in the Atlantic south of Bermuda.=20
AI-based guidance (ECMWF-AIFS and GFS GraphCast) have been showing=20
this increasing potential over this time span while ensembles=20
(ECMWF-EPS, GEFS, GEPS) have all gradually trended snowier as=20
well. While this is the case, deterministic guidance still shows=20
the lingering uncertainties with the 500mb trough's amplitude and=20
position 12Z Sunday. The 00Z GFS is slightly deeper vs the 00Z=20
ECMWF. The 00Z CMC is similar to the ECMWF 500mb disturbance's=20
strength, but with less confluence over the Northeast, allowing=20
for a more storm track on the northern side of the track=20
distribution. The UKMET also has a similar look to the Canadian,=20
albeit with a colder boundary layer setup given stronger confluence
over the Northeast. Overall, the AI-based guidance and the trends=20
in the ensembles provide an increased confidence in a synoptic-=20
scale setup that is boosting confidence in a winter storm from the=20 Mid-Atlantic to the Northeast, but more specific details on storm=20
track still need to be ironed out over the next 24-36 hours.

Synoptic & mesoscale analysis: The approaching upper-level trough=20
Saturday night will amplify over the Ohio Valley and place a=20
strengthening 250-500mb layer averaged jet streak over the Mid-=20
Atlantic by Sunday morning. NAEFS shows 200mb and 500mb winds over
the Southeast U.S. are above the 90th climatological percentile=20
and foster healthy upper level divergence aloft on Sunday. At=20
lower levels, a cold front Saturday night will stall out along the=20 Mid-Atlantic coast and into southern Virginia. The front, residing
beneath the synoptically-favorable jet stream setup, will spawn an
area of low pressure Sunday morning in the Carolinas and gradually
strengthen as it heads for the Mid-Atlantic coast Sunday=20
afternoon. The heaviest banding of snow is likely to reside north=20
of 850mb frontogenesis, which will become more and more pronounced=20
as the 850mb low forms somewhere along the Jersey Coast and tracks=20
northeast towards Long Island and the MA Capes Sunday evening. The
storm is a progressive one, but given the sufficient synoptic and=20
mesoscale processes, the deformation zone of heavy snow would be=20
more than capable of generating ~1"/hr snowfall rates, especially=20
for areas from the Delaware Valley on north and east through the=20
Tri-State area and into southern New England where they are more=20
likely to have better mesoscale forcing as the 850mb low deepens=20
Sunday night.

Timing & snowfall/impact probabilities: As mentioned before, this=20
is a quick hitting system that is all but gone by midday Monday as=20
it races toward the Canadian Maritimes by Monday afternoon. Snow=20
is forecast to be at its heaviest Sunday morning and afternoon for
the DC/Baltimore metros, the Philly/NYC metros Sunday afternoon=20
and evening, then across southern New England Sunday night into=20
early Monday morning. Latest WPC probabilities show moderate-to-=20
high chances (50-70%) for snowfall totals >4" from the Delaware=20
Valley on north through the Lower Hudson Valley and across southern
New England. Low-to-moderate chances (30-50%) for >4" extend as=20
far north and east as Downeast Maine. WPC probabilities even show=20
some spotty low chance probabilities (10-20%) for snowfall totals=20

8" in parts of southern New England. The DC/Baltimore metro areas=20

sport low chance probabilities (10-30%) for >4" of snowfall at the
moment, while the Philly and NYC metros are in the moderate range=20
(40-60%) for >4" of snow. While exact amounts remain unclear, the=20
WSSI-P Minor probabilities are >60% for the full length of I-95=20
from DC to Boston, indicating the heightened potential for hazardous
travel conditions Sunday into Monday. WPC has initiated Key=20
Messages for this event and can be viewed at the bottom of this=20
discussion.


Mullinax



...Winter Storm Key Messages are in effect. Please see current=20
Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_1.png__;!!DZ3fjg!4dY4WsEIB1AHhiR9NCIDz0vRrw-ZiqvDYXbRMjxOAJEPJ= eZzAK0mqcDQjS5cH131sou9KCbLLMGDRnMBqkMXyPvRpLw$=20

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_2.png__;!!DZ3fjg!4dY4WsEIB1AHhiR9NCIDz0vRrw-ZiqvDYXbRMjxOAJEPJ= eZzAK0mqcDQjS5cH131sou9KCbLLMGDRnMBqkMXgKkDj9M$=20

$$

=3D =3D =3D
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FOUS11 KWBC 172046
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
346 PM EST Fri Jan 17 2025

Valid 00Z Sat Jan 18 2025 - 00Z Tue Jan 21 2025

...Great Lakes...
Days 1-3...

Low pressure over the north-central Ontario along with surface
ridge over western Canadian Prairies combine to push Arctic cold=20=20
south down the Great Plains and east across the Great Lakes through
Saturday night. The LES machine over Lake Superior begins this
evening for the U.P. on NWly flow, the western L.P. overnight.
Gulf-sourced moisture shifts in advance of the cold front will
surge over the eastern Great Lakes Saturday before tapering off
Saturday night. There's a bit of a lull Sunday over the eastern
lakes with LES kicking in there Monday. Day 1 snow probs for >4"
are 20-40% over the northern U.P. and 30-60% from the synoptic flow
south and east of Erie and east of Ontario. Day 2 snow probs for

4" are 40-80% in the U.P. and portions of the western L.P., then

higher on Wly flow for Day 3 with 40-70% over the same U.P./L.P.
zones and east of Erie/Ontario.


...Colorado Rockies and High Plains...=20
Day 1...

The combination of both upper level diffluence ahead of a deep-
layer trough and the arrival of a bitterly cold Arctic air-mass will
continue to produce some snow squalls into this evening along the
cold front from the north-central Rockies east across the Great
Plains.=20

Focused mountain snow is expected over the eastern slopes=20
of the CO Rockies and adjacent high plains including the I-25=20
corridor tonight into Saturday. The base of the main trough=20
descending from Canada has a vort lobe currently pushing south over
Idaho. This will shift SSE over the Four Corners through Saturday=20
which will aid lift over CO which has low level upslope flow post-=20
frontal with a 1050mb high moving into MT on Saturday. Snowfall rates
reach 1"/hr per the 12Z HREF on the eastern slopes of the Front
Range this evening before the bands work their way south across the
Raton Mesa early Saturday. Day 1 snow probs for >4" are high
(60-90% over the Front Range down to Pikes Peak and 30-50% for the
I-25 corridor through the length of CO.

Be cautious of hazardous driving conditions in any further snow
squalls and on the eastern slopes of CO terrain.


...Central Appalachians, Mid-Atlantic and Northeast...=20
Days 2-3...

...Confidence has further increased from a non-ECMWF deterministic
solution general model blend for a disruptive winter storm from=20
the central Appalachians, central Mid-Atlantic through the=20
Northeast with locally heavy snowfall possible along and west of the
I-95 corridor...

Guidance remains in good agreement on a two wave solution. The
initial wave on Saturday is a shortwave in advance of the main
polar sourced trough with the wave of concern the positively-tilted
trough axis that lifts from the Southeast Sunday through Sunday
night. AI-based guidance (ECMWF- AIFS and GFS GraphCast) have been
consistent with coastal low development off the Mid-Atlantic coast
Sunday lifting north past New England Sunday night. Ensembles=20
(ECMWF-EPS, GEFS, GEPS) continue to be snowier, and pretty much=20
all deterministics outside the ECMWF are as well. The 12Z CMC is
the farther west/most interior low track while the 12Z ECMWF is the
farthest east/offshore and has the lowest QPF of the=20
deterministics. The ECMWF-AIFS, however, now has five days of=20
consistent runs with a coastal low along the Mid- Atlantic coast.=20
This solution has drifted north and been more potent over the more
recent runs up through the 06Z. All to say that while most guidance
has considerable heavy snow (in the 6-10" range) from eastern PA up
through Maine, there remains some volatility and uncertainty in=20
the heavy snow swath location and intensity.

The heavier snow begins late Saturday night over eastern KY with
terrain and low level fgen aiding enhanced snow over the central
Appalachians through WV into PA and then northeast over the more
interior central Mid-Atlantic (generally NW from the I-95 corridor
of northern VA and north) with a max between eastern PA and
southern Maine. Day 2 snow probs for >6" are 40-70% over central
WV/far western MD into the Laurels of PA. Day 2.5 values for >6"
are 30-70% from eastern PA north of Philly through eastern Maine
with the highest probs from the Catskills through Mass and southern
NH. There is expected to be a tight rain/snow line around the DC
Metro and northeast east from I-95 through the NYC Metro and=20
Boston Metro. Only minor adjustments in this line makes for impact
changes to millions of people, so close attention should be taken
in the coming days.


...Gulf Coast...

Next week...
A reinforcing shortwave trough rounding the Four=20
Corners Monday brings about surface trough and eventual low=20
development off the lower TX Coast starting later Monday. Cold air=20
arrives into south Texas Sunday night, so overrunning flow may=20
cause an onset of wintry mix over south Texas prior to 00Z Tuesday.
Further info on this threat can be found in the medium range=20
discussion (PMDEPD) and in newly issue key messages linked below.


Jackson




...Winter Storm Key Messages are in effect and linked below...

Gulf Coast winter storm threat next week next week https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_1.png__;!!DZ3fjg!9_J7C6wqgz61rd07tBEl1b4LCkQu7IHF0S6ocUs0T6ad-= X3A1Hj7IkDRSh3-4UGXUCbHXK1vyD_wsPG76zlC8BUSDn4$=20

Northeast winter storm threat this weekend https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_2.png__;!!DZ3fjg!9_J7C6wqgz61rd07tBEl1b4LCkQu7IHF0S6ocUs0T6ad-= X3A1Hj7IkDRSh3-4UGXUCbHXK1vyD_wsPG76zlCdxl93ZE$=20

$$

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FOUS11 KWBC 180834
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
334 AM EST Sat Jan 18 2025

Valid 12Z Sat Jan 18 2025 - 12Z Tue Jan 21 2025

...Great Lakes...
Days 1-3...

Strong low pressure tracking through southeast Canada will escort=20
a strong Arctic front through the Great Lakes today that will
revitalize the LES machine throughout the region today. Strong
NWrly flow with exceptional CAA will trigger LES bands from the
U.P. of Michigan on south along the western-most portions of
Michigan's Mitten. Farther east, a slug of moisture aloft will run
parallel to the Arctic front as it races east today with the
Chautauqua Ridge and the Tug Hill the more favored locations for
locally heavy snow beginning this morning and concluding by
Saturday evening. WPC probabilities through 12Z Sunday show
moderate-to-high chances (50-70%) for snowfall totals >4" in the=20
Tug Hill while there are low-to-moderate chances (30-50%) for >4"=20
snowfall amounts from northeast Ohio to along the Chautauqua=20
Ridge. Portions of the eastern Michigan U.P. and the Porcupines=20
also sport moderate chances (40-60%) for snowfall amount >4". As=20
winds accelerate over the Great Lakes on Sunday and inverted=20
troughing ensue over Lake Superior, LES bands will increase in=20
both coverage and intensity in the usual snow belts of the=20
Michigan U.P. and the northern most portions of Michigan's Mitten.=20
Including Sunday and Monday, WPC probabilities depicted high=20
chances (>70%) for snowfall > than 8" along the eastern most areas=20
of Michigan's U.P. and just north of the Traverse City area.=20
Single-banded LES look to emerge off of Lakes Erie and Ontario=20
Monday evening and generate heavy snowfall rates of 1-2"/hr through
the day on Tuesday.


...Colorado Rockies and High Plains...=20
Day 1...

Mountain snow will continue over the eastern slopes and Front Range of
the Colorado and New Mexico Rockies, including the adjacent high=20
plains along the I-25 corridor today. The shortwave trough tracking
south through the Intermountain West this morning will generate=20 synoptic-scale lift over Colorado and northern New Mexico which=20
will also have low-level, post-frontal upslope NErly flow with a=20
1050mb dome of high pressure moving over Montana on Saturday.=20
Snowfall rates around 1"/hr are possible along the Raton Mesa early
Saturday and will work their way south along the spine of the=20
Sangre De Cristo Saturday afternoon. Saturday's snow probs for >4"=20
are high (60-80%) over the Front Range from Pikes Peak on south to=20
the Raton Mesa, while areas along I-25 have low-to-moderate=20
chances (20-40%) for >4" of snowfall.=20


...Central Appalachians, Mid-Atlantic and Northeast...=20
Days 1-3...

...Significant winter storm to produce a swath of heavy snow from
the Central Appalachians and northern Mid-Atlantic to the Northeast
Sunday and Monday...

As model guidance continues to come into better agreement on the
overall synoptic-scale setup, it is becoming clearer that a
significant winter storm is likely to produce heavy snow snow from
as far south and west as western Kentucky to as far north as
northern Maine. As the Arctic front approaches Saturday night, the
front will be draped over the Mid-Atlantic while at the same time,
strong upper-level divergence ensues aloft courtesy of a ~150kt=20
250mb jet streak's diffluent left-exit region is in place. As=20
850-700mb WAA increases over the Mid-Atlantic, it will clash with=20
the approaching Arctic air-mass to the north, resulting in a band=20
of heavy snow that initially starts out over the Laurel Highlands=20
and Allegheny Highlands early Sunday morning, then spreads over=20
northern MD and southern PA later on that morning. As the 850-700mb
FGEN increases throughout the day, snowfall rates will increase=20
across eastern PA, northern NJ, and the Lower Hudson Valley through
Sunday evening. 00Z HREF does suggest there are low-to-moderate=20
chances (20-50%) for 1"/hr snowfall rates during the day on Sunday.
By 00Z Monday, the 850mb low will track over Long Island and head=20
for the New England coast with the axis of heavy snow residing=20
along the western flank of the 850mb circulation. Heavy snow will=20
unfold across portions of coastal New England Sunday night where=20
snowfall rates >1"/hr are possible. The storm races northeast past=20
Nova Scotia by Monday morning with lingering periods of snow=20
throughout parts of Maine gradually tapering off by midday Monday.

Latest WPC guidance depicts a swatch of moderate-to-high chances
(50-70%) for snowfall >4" from central WV on north and east through
central PA. From the Poconos on north and east through the Lower
Hudson Valley, the Berkshires, and into both the Worcester Hills
and White Mountains, there are moderate-to-high odds (50-70%) for=20
snowfall >6", as well as moderate chances (4-60%) for snowfall >8".
Note that WPC probabilities for >12" are generally <10%,=20
indicating most areas are unlikely to receive a foot of snowfall,=20
although localized amounts in the Allegheny Highlands and parts of=20
interior New England cannot fully be ruled out. Latest WSSI shows a
healthy swath of Moderate Impacts from northern MD through the=20
Lower Susquehanna Valley, the Delaware Valley, the Tri-State area,=20
and into portions of southern New England. Hazardous driving=20
conditions are anticipated in these areas on Sunday. With bitterly=20
cold temperatures spilling in for the upcoming work-week, snow and=20
instances of re-freezing/black ice are likely to stick around for=20
much of the week in wake of this winter storm.=20

...Southern Plains & Gulf Coast...

...Disruptive winter storm growing in confidence across the
Southern U.S. next week...

Day 3...
For a winter storm to unfold from as south as the San Antonio area
on east to the central Gulf Coast, the first ingredient is for an
exceptionally cold air-mass to be locked in place. With the arrival
of the Arctic air-mass this weekend, bone-chilling temperatures
will be anchored in place by a dome of high pressure that NAEFS
shows is above the 97.5 climatological percentile as of 12Z Monday
over the Lower MS Valley. By 00Z Tuesday, 850mb temperatures remain
below the 2.5 climatological percentile and subfreezing 850mb temps
protruding as far south as the Gulf Coast. Meanwhile, a positively
tilted 500mb trough tracking through the Southwest will work in=20
tandem with the subtropical jet to increase upper-level divergence
over Texas Monday afternoon. In addition, SWrly 850mb flow will
deliver low-level moisture northward while at the same time, the
850mb WAA over South Texas will overrun the Arctic air-mass in
place over Texas. This will result in the beginning of an icy
wintry mix over south-central Texas and eventually spread east
towards the Upper Texas coast and Lower Mississippi Valley by early
Tuesday morning.=20

WPC probabilities show moderate chances (40-60%) for >0.01" of ice accumulations in south-central Texas through 12Z Tuesday, which=20
would result in slick and dangerous road conditions for the Tuesday
morning commute. Farther north, WPC probabilities show low-chance=20
odds (10-30%) for snowfall >2" in parts of eastern Texas and=20
western Louisiana. It is worth noting that the scope of this=20
discussion goes through 12Z Tuesday, so expect snowfall probabilities
to increase over the next 24 hours. Please be sure to visit WPC's=20
Medium Range discussion and our Key Messages for more details on=20
the potential impacts throughout the South later this week.

Mullinax




...Winter Storm Key Messages are in effect. Please see current=20
Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_1.png__;!!DZ3fjg!5PwFQD-qsJU9Ysmi489Esp-N2fQTM0x_Fu9glBej3DKqr= vAP7dye-s1pYXe9ZpFjRDu8cWSzqc14mQpuy0fhqWe23_E$=20

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_2.png__;!!DZ3fjg!5PwFQD-qsJU9Ysmi489Esp-N2fQTM0x_Fu9glBej3DKqr= vAP7dye-s1pYXe9ZpFjRDu8cWSzqc14mQpuy0fhQMMIutg$=20

$$

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FOUS11 KWBC 182107
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
407 PM EST Sat Jan 18 2025

Valid 00Z Sun Jan 19 2025 - 00Z Wed Jan 22 2025

...Great Lakes...
Days 1-3...

Deep low pressure centered over northern Ontario will maintain
cyclonic flow with Arctic cold over the Great Lakes through at
least Tuesday. NWly flow prevails through Sunday before backing
Wly Sunday night through Monday. 72hr snow probabilities for >12"
are over 50% in the western U.P., northern and southern western
shores of the L.P. and over 80% for the eastern U.P.
The shift to westerly flow over the eastern Great Lakes allows=20
single banding to develop by Monday afternoon with heavy snow=20
persisting into Tuesday night for Buffalo and the South Towns as=20
well as northern sections of the Tug Hill where Day 3 snow=20
probabilities for >8" are over 60% (80% for the Tug Hill).=20


...South-Central Plains...=20
Day 2...

The approaching longwave trough that will cause wintry weather
along the Gulf Coast (see section below) will cause snow banding=20
east from the southern CO/northern NM Rockies in a similar manner
to the banding that occurred today east of the Rockies (not the
heavy snow in the terrain last night) on Monday. Day 2 snow
probabilities for >2" are 20-60% over SW KS extending into western
OK.


...Central Appalachians, Mid-Atlantic and Northeast...=20
Days 1-2...

...Significant winter storm to produce a swath of heavy snow from
the Central Appalachians through the northern Mid-Atlantic and New
England Sunday through Sunday night...

The main guidance changes today on the Northeast winter storm are
with a slower timing. RAP based guidance (HRRR) has incrementally
slowed with each recent run since yesterday while the NAM based
models remain on the slower end. Meanwhile a slightly farther east
solution is seen for New England in the EC and 18Z HRRR models
which would allow heavier snow a bit farther east than before.
The Arctic cold front will push off/to along the Northeastern=20
Seaboard tonight while stalling over the Carolinas as the surface=20
wave tracks toward Cape Hatteras through Sunday morning. This wave=20
rapidly develops as it tracks north along the frontal zone through=20
Sunday night, reaching the Canadian Maritimes by 12Z Monday. Strong
upper- level divergence increases off the Mid-Atlantic Coast=20
through Sunday morning as the SWly 250mb jet exceeds 150kt with=20
diffluent left- exit region aiding the rapid surface/coastal=20
development. As 850-700mb WAA increases over the Mid- Atlantic, it=20
will clash with the approaching Arctic air- mass to the north,=20
resulting in a band of heavy snow that initially starts out over=20
the Allegheny Highlands early Sunday morning, then spreads over the
Laurels in PA across northern VA and MD through midday (which is a
later timing based on the slowed guidance). Eastern PA through=20
southern NY see great increases in 850-700mb FGEN through the=20
afternoon allowing snowfall rates to increase through this swath.=20
The 12Z HREF mean hourly snowfall hits 1"/hr over the northern half
of NJ by 00Z Monday with 1"/hr rates in a rather wide swath=20
tracking north through the night until it clears eastern Maine at=20
12Z. The storm races north through New Brunswick early Monday=20
morning with lingering periods of snow throughout parts of Maine=20
tapering off by midday Monday.

Day 1.5 WPC snow probabilities for >6" snow are 40-70% over central
WV terrain and over southern PA through Maine with 50% probs for

4" over far northern VA/northern MD, the Philly and NYC metros as

well as a stripe of southeast New England generally west from
Providence, RI. Hazardous driving conditions are anticipated in=20
these areas on Sunday. With bitterly cold temperatures spilling in=20
for the upcoming work-week, snow and instances of re-freezing/black
ice are likely to stick around for much of the week in wake of=20
this winter storm.=20


...Texas, Gulf Coast, and Deep South...

...Disruptive winter storm growing in confidence across the
Southern U.S. next week...

Days 2/3...=20
Exceptionally cold/dry air plunges through Texas tonight,=20
expanding east over the Gulf Coast through Sunday night. This sets=20
the stage for wintry precip to develop in South Texas Monday as an=20 approaching trough axis over the Four Corners allows a low level=20
inverted trough to develop over the western Gulf, causing an=20
onshore flow to develop and ride over the low level cold along and=20
inland of the lower TX coast. This trough continues to develop=20
through Monday night with precip expanding up through central and=20
into North Texas and southwest Louisiana by 12Z Tuesday. Snow can=20
be expected over central TX and north while an icy wintry mix can=20
be expected over south-central and interior lower TX then along the
TX and LA Coasts.=20

The trough axis reaches the TX Coast Tuesday evening with rapid
eastward expansion expected to the wintry mix through the day=20
Tuesday, likely reaching the FL Panhandle by Tuesday evening.
Moderate to locally heavy snow banding is expected from roughly
north of the Houston metro east across southern LA into southern MS
on Tuesday with notable snowfall for these areas. Day 3 snow
probabilities for >4" are 30-60% through this zone. The conceptual
model is for a stripe of sleet and freezing rain to be south of the
snow. As currently depicted those stripes look to both be on or
inland from the Gulf Coast. Day 3 ice probabilities for >0.1" are
currently around 10% in both southern LA and just south of I-10 in south-central TX, north of Corpus Christi. Details with p-ptype are
better done with higher-res models which offer output in the Days
1/2 timeframe, so expect greater detail to these wintry
accumulations at that time.

Please be sure to visit WPC's Medium Range discussion and our Key=20
Messages for more details on the impacts that will extend through=20
much of the Southeast through Wednesday.


Jackson




...Winter Storm Key Messages are in effect as linked below...

Southern/Gulf Coast Winter Storm... https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_1.png__;!!DZ3fjg!6x-Ue0uDj0cJ4rJL4sijZkv3DZJoOh0J5ABp9FJGa_qjN= _J_rE1bwGbYM6YEIU9u3a7iCQmYzoa4_5nV3qsyNgI5tA0$=20

Northeast Winter Storm... https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_2.png__;!!DZ3fjg!6x-Ue0uDj0cJ4rJL4sijZkv3DZJoOh0J5ABp9FJGa_qjN= _J_rE1bwGbYM6YEIU9u3a7iCQmYzoa4_5nV3qsyHxQifDw$=20

$$

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FOUS11 KWBC 190918
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
418 AM EST Sun Jan 19 2025

Valid 12Z Sun Jan 19 2025 - 12Z Wed Jan 22 2025

...Great Lakes...
Days 1-3...

Deep low pressure located over northern Ontario will maintain=20
cyclonic flow with Arctic cold over the Great Lakes through at=20
least Tuesday. NWly flow will be the prevailing wind direction=20
through Sunday before backing out of the west Sunday night through
Monday. 72hr snow probabilities for >12" are over 50% in the=20
western U.P., northern and southern western shores of the L.P. and=20
over 80% for the eastern U.P. The shift to westerly flow over the=20
eastern Great Lakes allows single banding to develop by Monday=20
afternoon with heavy snow continuing into Tuesday night for=20
Buffalo and the South Towns as well as northern sections of the Tug
Hill where Day 3 snow probabilities for >8" are over 60% (80% for=20
the Tug Hill). In fact, there are high chance probabilities (>70%)
for snowfall totals >24" along the eastern-most slopes of the Tug
Hill.


...South-Central Plains...=20
Day 2...

The incoming longwave trough that will produce wintry weather=20
along the Gulf Coast (see section below) will cause snow banding=20
east from the southern CO/northern NM Rockies in a similar manner=20
to the banding that occurred today east of the Rockies in the High
Plains on Monday. Weak 850mb FGEN and 700mb WAA aloft will be the
primary sources for this period of snow. Day 2 snow probabilities=20
for >2" are 30-50% over southwestern KS extending south into=20
northwestern OK.


...Central Appalachians, Mid-Atlantic and Northeast...=20
Days 1-2...

...Significant winter storm to produce a swath of heavy snow from
the Central Appalachians through the northern Mid-Atlantic and New
England today through Monday morning...

Guidance has generally come into a better consensus on where the=20
heaviest swatch of snowfall is expected the Lehigh valley and=20
Poconos on north and east through the Lower Hudson Valley, into the
Berkshires, Worcester Hills, and White Mountains. The overall=20
synoptic and mesoscale setup and progression also remains in good=20
shape.=20

The Arctic cold front will push off/along the East Coast tonight=20
while stalling over the Mid-Atlantic while a strengthening area of
low pressure tracks over eastern North Carolina. Upper-level=20
divergence increases over the Mid-Atlantic coast through Sunday=20
morning as a robust 250mb jet surpasses 150kt and its diffluent=20
left-exit region aids in rapid surface/coastal development. As=20
850-700mb WAA increases over the Mid-Atlantic, it will clash with=20
the approaching Arctic air-mass to the north, resulting in a band=20
of heavy snow that initially starts out over the Allegheny=20
Highlands early Sunday morning, then spreads over the Laurels in PA
across northern VA and MD through midday. Eastern PA through=20
southern NY will witness some of the best 850-700mb FGEN through=20
the afternoon, allowing snowfall rates to increase through this=20
swath. The 00Z HREF mean hourly snowfall hovers around 1"/hr over=20
the northern half of NJ by 00Z Monday with 1"/hr rates in a rather=20
wide swath tracking north through the night until it clears eastern
Maine at 12Z. The storm races north through New Brunswick early=20
Monday morning with lingering periods of snow throughout parts of=20
Maine tapering off by midday Monday.

WPC snow probabilities for >6" snow are 50-70% from central WV=20
terrain and the Laurel Highlands on north and east through eastern
PA, northern NJ, southern NY, and into much of central New=20
England. Eastern Maine has the best odds for >8" amounts with
50-70% chances for >8" through Monday morning. Moderate chance
probabilities (40-60%) for >4" of snowfall are present from the
western DC/Baltimore suburbs, into the Philly and NYC metros, and
along the southern New England coast. Hazardous driving conditions
are anticipated in these areas on Sunday and into Monday morning.=20
With bitterly cold temperatures spilling in for the upcoming work-
week, snow and instances of re-freezing/black ice are likely to=20
stick around for much of the week in wake of this winter storm.=20


...Texas, Gulf Coast, and Southeast...

...Significant winter storm growing in confidence across the=20
Southern U.S. this week..

Days 2-3... As a positively tilted trough over the southwestern=20
U.S. that promotes strong upper level ascent over Texas and the=20
Gulf of Mexico Monday night, low pressure forming along a=20
strengthening surface front off the Lower TX coast. Low level=20
easterly winds Monday evening will advect Gulf moisture into=20
southern Texas that will result in an icy wintry mix over the=20
region. Meanwhile, as the upper trough approaches Tuesday morning,=20
the an anomalous IVT (>500 kg/m/s, or above the 90th climatological
percentile per NAEFS) will overrun a strong 850mb FGEN area from=20
southeast TX to the central Gulf Coast. Moderate to locally heavy=20
snow banding is expected from the northern Houston metro suburbs on
east across southern LA into southern MS on Tuesday, with snow=20
moving into southern AL, the western FL Panhandle, and central GA=20
by Tuesday evening. The same swath of ice then moves over northern=20
FL and southern GA Tuesday night. Snow will increase in intensity=20
along the GA/Carolina coast Tuesday night as the divergent right-
entrance region of a roaring 200kt 250mb jet streak aligns=20
favorably with moist southerly flow. Snow will be heavy but=20
progressive with snow ending along the NC Outer Banks by midday=20 Wednesday.=20

Days 2-3 snow probabilities for >4" are 30-50% from the eastern
Houston suburbs to south-central MS. WPC probabilities for >4" of
snowfall rise to 40-60% from central SC on east to eastern NC.
There are ensemble members that bring the potential for >4" of
snow into the Norfolk/VA Beach area, but this remains low=20
confidence given the wide range of potential scenarios. In terms of
ice accumulations, WPC probabilities show low-to-moderate chances=20
(30-50%) for >0.1" of ice from the FL Panhandle and suburbs west of Jacksonville on north through southern GA and far southern SC.=20
Much of south Texas sports moderate-to-high chances (50-70%) for=20

0.01" of ice accumulation with low chances (<20%) for ice amounts=20
0.1". While exact amounts are unclear, there is no denying the=20

greater impact potential given the subfreezing surfaces throughout=20
the South. The WSSI shows Major Impact potential (considerable=20
disruptions and dangerous to impossible travel) in southern LA,=20
with a wide swath of Moderate Impacts from the northern Houston=20
suburbs to southern Alabama. Details with p-ptype are better done=20
with higher-res models which offer output in the Days 1/2=20
timeframe, so expect greater detail to these wintry accumulations=20
at that time.


Mullinax/Jackson



...Winter Storm Key Messages are in effect. Please see current=20
Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_1.png__;!!DZ3fjg!9AQ0cld342txxz0WyQR0cs1uyvcCw_ZYBnIOgIlcj0wo5= 9H8F45sFAlu5v1HoE21-E4KfjaUcTykdprxOcTiCiMxET8$=20

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_2.png__;!!DZ3fjg!9AQ0cld342txxz0WyQR0cs1uyvcCw_ZYBnIOgIlcj0wo5= 9H8F45sFAlu5v1HoE21-E4KfjaUcTykdprxOcTii3uhEBQ$=20

$$

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FOUS11 KWBC 192104
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
404 PM EST Sun Jan 19 2025

Valid 00Z Mon Jan 20 2025 - 00Z Thu Jan 23 2025

...Great Lakes...
Days 1-3...

Broad cyclonic flow encompassing much of the eastern CONUS will be
reinforced by dual shortwaves - one moving east of the Great Lakes
to start the period, and a second more amplified wave Tuesday
night. The overall flow pattern is reflective of a true mid-winter
airmass with cross-polar flow surging extremely cold air into the
eastern 2/3 of the CONUS, with 850mb temps progged to fall from
around -15C to as low as -30C (falling to below the 1st percentile
within the CFSR climatology). This will create persistent CAA
favorable for lake effect snow (LES).=20

Notably, the water temperatures across the Great Lakes have cooled
considerably thanks to a cold January, and ice is now evident in=20
satellite imagery and as analyzed by GLERL, which may somewhat=20
limit the intensity of LES. However, with such an extremely cold=20
airmass in place, and surges of enhanced ascent associated with the aforementioned shortwaves, inversion depths to 10,000 ft and lake-
induced instability of 500-1000 J/kg will still support periods of
intense LES, especially downstream of Lakes Erie and Ontario, with
2"/hr rates or more possible.

WPC probabilities D1 are high (70%+) for 4+ inches of snow across
the Western U.P, and northern L.P. of MI, as well as downstream of
Lakes Erie and Ontario. The heaviest LES commences D2 downstream of
Lakes Erie and Ontario where WPC probabilities for 8+ inches reach
above 50%, although upstream snowfall across MI may lessen in
coverage and intensity. Another surge of CAA will enhance LES
across the L.P. of MI once again D3, but the heaviest snow will
continue into the Tug Hill Plateau where event-total snowfall of=20
2-4 feet is forecast.


...South-Central Plains...=20
Day 1...

A shortwave digging through the broad cyclonic flow encompassing
much of the eastern 2/3 of the CONUS will dig E/SE from the Great
Basin as a southern stream impulse. This will eventually interact
with a northern stream shortwave to drive a full-latitude trough
development, and while it remains positively tilted, it will drive
downstream jet intensification, with the resulting overlap of PVA, jet
level diffluence, and mid-level isentropic ascent causing an
expansion of snowfall Monday afternoon through Monday night. Total
ascent is modest and the best ascent does not seem to overlap with
the DGZ, but an extremely cold column should still result in
efficient/fluffy SLR where snowfall occurs. This will cause some
modest snowfall accumulations for the TX/OK Panhandles and points
east into OK/KS, where WPC probabilities are as high as 30-50% for
2 inches of snow.


...Mid-Atlantic and Northeast...=20
Day 1...

Fast moving low pressure will bring significant snow to the Mid-
Atlantic and Northeast through Monday as it tracks northeast just
offshore the coast.

This low is being driven by an overlap of a negatively tilted
shortwave trough currently emerging from the TN VLY, with its track
likely to take it over the Washington, D.C. area before lifting
over Cape Cod and Downeast Maine overnight. A potent 150+ kt jet
streak arcing poleward will overlap with this shortwave, resulting
in a favorable environment for low pressure strengthening just
offshore. The resultant surface wave will deepen at a moderate
pace, but lift rapidly northeast, exiting into Canada by late
Monday morning. Despite the rapid pace of this system, a swath of
heavy snow is likely just inland from the coast, with the heaviest
snowfall accumulations likely just NW of I-95 from Washington,=20
D.C. to Boston, MA.=20

The primary driver of this heavy snow will be an overlap of
enhanced moisture rotating around the system on the accompanying
isentropic ascent, and a corridor of strengthening 700-600mb fgen.
While the forecast soundings do not scream intense snowfall due to
very dry air above the DGZ resulting in modest available PWs, the
most intense fgen does appear to overlap efficiently with the DGZ
suggesting potential for banded snow structures, within which the
WPC prototype snowband tool indicates the potential for 1-2"/hr
snowfall. The progressive nature of this system will limit overall accumulations, and precip may start as a rain/snow mix in many
areas before cooling dynamically to support all snow, but WPC
probabilities feature a high risk (>70%) for more than 4 inches of
snow in a continuous swath from far eastern Upstate NY through much
of southern and central New England from 00Z tonight through 18Z
Monday. Locally, more than 8 inches is possible (30-50% chance) in
Downeast Maine. With snowfall rates exceeding 1"/hr at times, this
will result in hazardous travel in many areas.

The progressive nature of this system will limit more substantial
impacts, but heavy snow and hazardous travel impacting much of the
northeast I-95 urban corridor has prompted the issuance of
collaborative Key Messages linked below (KeyMessage_2).


...Texas, Gulf Coast, and Southeast...
Days 2-3...

...Significant winter storm likely across portions of the Southern
U.S. this week...

A rare Gulf-Coast and Southeast Atlantic Coast winter storm is
becoming more likely beginning Tuesday as an anomalously cold
airmass settles across the eastern 2/3 of the CONUS and sets the
stage for the upcoming low pressure system.

Amplified troughing deepening across the east will spill cold air
all the way to the Gulf Coast as reflected by ECMWF EFI for
minimum temperatures falling below -0.9 on Tuesday, and
expanding/intensifying on Wednesday. This cold air will be
entrenched even before the low develops, and despite some
variability in model solutions among the various ensemble camps,
the colder ECMWF is favored at this time.

As this cold air floods across the region, an intensifying
shortwave will dig out of the Great Basin and then sharpen as it
pivots eastward from West Texas through the Gulf Coast Tuesday into
Wednesday. As this occurs, some interaction with a northern stream
impulse will yield a positively tilted full latitude trough
shifting east, with the resultant downstream jet streak
intensifying and shifting poleward to place the favorable RRQ atop
the Gulf Coast. The overlap of height falls and upper diffluence
will help spawn a wave of low pressure across the Gulf of Mexico,
and as 300K isentropic ascent maximizes to the north, precipitation
will begin to overspread eastern Texas Tuesday morning, before
becoming more expansive across the Gulf Coast and eventually
coastal Southeast Tuesday night into Wednesday.

The challenge with this event is where the snow/sleet/freezing rain
transition will occur, and where the northern cutoff of
precipitation will occur. Some very cold and dry air to the north
will likely cause a sharp precip gradient, but south of there will
be a swath of heavy snow driven by strong WAA, and places across
LA/MS/AL that rarely seen snow could experience 1"/hr snow rates
just north of the mixed transition zone. While there is still some
uncertainty into the placement of the mixed precip axis, especially
farther east into the Carolinas/Georgia/Florida, the guidance has
trended a bit colder and farther south, which is impacting the
recent WPC probabilities.

At this time, WPC probabilities indicate a high chance (50-90%)=20
for at least 2" of snow from far eastern TX through southern/central
LA and into southern MS. Locally, 4-6" of snow is possible,=20
especially where any banding sets up which could produce snowfall=20
rates above 1"/hr. The combination of the heavy snow, very cold=20 temperatures, and the simply rarity of this event could result in=20
major impacts and considerable disruptions to daily life. Farther=20
east, as the event translates downstream, WPC probabilities for 2+=20
inches of snow are as high as 30-50%, focused in the eastern=20
Carolinas, however uncertainty here is higher due to latitudinal=20 discrepancies in the axis of QPF from the available models.=20
Additionally, southeast of the heaviest snow, especially from the=20
Florida Panhandle east to the northern Peninsula and parts of=20
coastal GA/SC, freezing rain is likely, which has a 30-50% chance=20
of producing at least 0.1" of ice.

This event has prompted the issuance of collaborated Key Messages (KeyMessage_1) linked below.

Weiss



...Winter Storm Key Messages are in effect. Please see current=20
Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_1.png__;!!DZ3fjg!-nxEdZlTvQqjZY-_gv4eLYH3AbEq7u3ccz9oaX-qQu_AI= UPRGlwaoRUZz9Do_f317VHSQc-Mt4kkdYfSYzUFFCTAwc8$=20

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_2.png__;!!DZ3fjg!-nxEdZlTvQqjZY-_gv4eLYH3AbEq7u3ccz9oaX-qQu_AI= UPRGlwaoRUZz9Do_f317VHSQc-Mt4kkdYfSYzUFtVKDYtE$=20

$$

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FOUS11 KWBC 200824
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
324 AM EST Mon Jan 20 2025

Valid 12Z Mon Jan 20 2025 - 12Z Thu Jan 23 2025

...Great Lakes...
Days 1-3...

Broad cyclonic flow courtesy of a massive upper low over eastern=20
Canada will direct a series of shortwave troughs over the Great
Lakes and eastern U.S. the first half of the week. An exceptionally
strong shortwave trough on Tuesday (500mb heights as low as the 1st climatological percentile 12Z Tuesday) will deliver 1000-850mb=20
temps that are also below the 1st climatological percentile. While
the Great Lakes are gradually icing over with each passing day, the exceptionally colder air-mass traversing the Great Lakes paired
with the favorable upper-level synoptic-scale ascent will be more
than enough to support persistent and potent lake effect snow (LES)
bands. In fact, lake-induced instability of >500 J/kg would support
exceptional LES bands, particularly downstream of Lakes Erie and
Ontario where single-banded LES streamers are most likely. LES
bands should finally weaken by Wednesday morning as high pressure
moves over the Mid-Atlantic and a weak Alberta Clipper approaches
from the northwest. This will weaken the pressure gradient and
shift winds our of the southwest. Some light snow is possible in
the Great Lakes region on Wednesday as a result of the Clipper, but
snowfall rates will pale in comparison to the LES bands that will
be around through Tuesday afternoon.

WPC probabilities for Monday and Tuesday depict moderate chances
(40-60%) for snowfall >8" along the western-most portions of
Michigan's Mitten and along the northern coasts of the eastern
Michigan U.P., The heaviest LES bands take shape Monday night and
into Tuesday downstream of Lakes Erie and Ontario. By Wednesday
morning, WPC probabilities show high chances (>70%) for snowfall

18" in some of the South Towns near Buffalo and in the Tug Hill

Plateau. The event total snowfall in the Tug Hill is likely to
range between of 2-4 feet through Wednesday.


...Texas, Gulf Coast, and Southeast...
Days 1-3...

...Significant winter storm likely across portions of the Southern
U.S. this week...

A rare Gulf-Coast and Southeast Atlantic Coast winter storm is
becoming more likely beginning Tuesday as an anomalously cold
airmass settles across the eastern 2/3 of the CONUS and sets the
stage for the upcoming low pressure system.

As this cold air floods across the region, an intensifying
shortwave will dig out of the Great Basin and then sharpen as it
pivots eastward from West Texas through the Gulf Coast Tuesday into
Wednesday. As this occurs, some interaction with a northern stream
impulse will yield a positively tilted full latitude trough
shifting east, with the resultant downstream jet streak
intensifying and shifting poleward to place the favorable RRQ atop
the Gulf Coast. The overlap of height falls and upper diffluence
will help spawn a wave of low pressure across the Gulf of Mexico,
and as 300K isentropic ascent maximizes to the north, precipitation
will begin to overspread eastern Texas Tuesday morning, before
becoming more expansive across the Gulf Coast and eventually
coastal Southeast Tuesday night into Wednesday.

Guidance is keying in on the snow/sleet/freezing rain transition=20
from south-central TX on east to along and south of the I-10
corridor. The mechanisms for this vary, however, as in south-
central TX, it has more to do with the lack of moisture above 700mb
and an exceptional warm nose between 850-700mb, and from the Upper
Texas Coast to the the FL Panhandle, there is no shortage of
moisture but the warm nose is stronger. Some very cold and dry air
to the north will likely cause a sharp precip gradient as well,=20
but to the south of the precip gradient, a swath of heavy snow=20
driven by strong WAA and 850mb FGEN will result in eastern TX/LA/MS/AL witnessing up to 1"/hr snowfall rates just north of the mixed=20
transition zone, which is rare for the central Gulf Coast region.=20
Guidance has continued to suggest more suppression of the QPF axis
over the last 12 hours, and thus WSO and WSSI-P probabilities have
decreased in parts of northern GA and the central Carolinas=20
Tuesday night. That said, from the coastal Carolinas on south to=20
northern Florida, a combination of snow and ice will make for=20
treacherous travel conditions Tuesday night and likely into=20
Wednesday morning stubbornly remaining below freezing.=20

WPC probabilities indicate a moderate chances chance (40-60%) for=20
at least 4" of snow from far eastern TX through south-central LA=20
and into southern MS. Locally, 4-6" of snow is possible in south-
central Louisiana with locations such as Lake Charles and Baton
Rouge seeing such totals. These areas referenced above also ave the
better chances of witnessing up to 1"/hr snowfall rates. The=20
combination of the heavy snow, very cold temperatures, and the=20
rarity of this event could result in major impacts and=20
considerable disruptions to daily life. Elsewhere, WPC=20
probabilities for 2+ inches of snow are as high as 40-60% from the
northern Houston suburbs to as far east as just north of Mobile
Bay. Farther east, recent trends in guidance has reduced snowfall
probabilities in parts of GA and the central Carolinas with WPC=20 probabilities showing low chances (10-30%) for over 2" of snow.=20 Additionally, southeast of the heaviest snow, especially from the=20
Florida Panhandle east to the northern Peninsula and parts of=20
southern GA, freezing rain is expected with a 40-60% chance of=20
producing at least 0.1" of ice. Most icing totals in South-central
Texas should generally remain below 0.1", but these totals are
still highly unusual for these parts of the Lone Star State and are
likely to result in hazardous travel conditions Monday night and
Tuesday morning.

This event has prompted the issuance of collaborated Key Messages (KeyMessage_1) linked below.

Mullinax/Weiss



...Winter Storm Key Messages are in effect. Please see current=20
Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_1.png__;!!DZ3fjg!8WKOWRNHqJyy1sAB3dnJe0J2N2CUDn1l12XERyIjCAaRO= kU-D0A9X6u_09bWoIRYqh0gkEY5vVrmyYw_Lz6ANgUjg0s$=20


$$

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FOUS11 KWBC 201933
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
233 PM EST Mon Jan 20 2025

Valid 00Z Tue Jan 21 2025 - 00Z Fri Jan 24 2025

...Great Lakes...
Days 1-2...

A large cyclonic gyre encompassing much of the eastern 2/3 of the
CONUS and draped from a massive upper low centered over eastern
Canada will maintain CAA across the Great Lakes through Thursday,
with periodic shortwave impulses rotating through the flow and
overhead the region. The impressive upper low will maintain height
anomalies well below normal into Thursday, with persistent CAA
pushing W/NW flow atop the Great Lakes. The most impressive ascent
will accompany a sharp shortwave which will cross the region from
the west to east on Tuesday, driving enhanced ascent through height
falls and strong PVA. This will also result in one final subsequent
cold surge reflected by 850mb temps crashing to as low as -20C to
-30C over the Lakes, coldest across the U.P. of MI.

The continuous cyclonic flow aloft with embedded shortwaves will
result in a favorable environment for rounds of lake effect snow
(LES) in the favored W/NW snow belts downstream of all the lakes.
Despite lake surface temperatures that have cooled dramatically the
past few weeks (now generally 0C to +5C, with ice present in some
areas according to GLERL), the extreme cold air funneling through=20
will produce steep lapse rates, inversion depths towards 10,000=20
ft, and lake-induced instability of 500-1000 J/kg. This will=20
support heavy bands of LES, with single bands off of Lakes Erie and
Ontario supporting 2-3"/hr rates at times, and 1"/hr rates=20
elsewhere. The heaviest and most widespread snow is likely D1 into=20
D2 behind the potent shortwave and accompanying weakening front,=20
and coverage/intensity is likely to wane D2 into D3. However, at=20
least modest LES is progged to persist into the early part of D3=20
across the eastern lakes.

2-day snowfall probabilities from WPC ending 00Z/Thursday indicate
a high chance (>70%) for more than 8 inches for portions of the NW
L.P. of MI, as well as downstream of Lakes Erie and Ontario,=20
especially into the Chautauqua Ridge and Tug Hill Plateau. The
heaviest snow totals are likely in the Tug Hill, however, where WPC probabilities for more than 24 inches reach 30-50%, and locally 2-4
feet of snow is possible.

...Texas, Gulf Coast, and Southeast...
Days 1-3...

...Significant, potentially historic, winter storm to spread across
the Gulf Coast and portions of the Southeast through Wednesday...

A rare, potentially historic, Gulf-Coast and Southeast Atlantic
Coast winter storm begins tonight and spread across the region
through Wednesday. The anomalous to extreme cold plaguing the
eastern 2/3 of the CONUS will set the stage for this upcoming low
pressure system.

The primary mechanism driving this impressive event will be a
shortwave digging across the Great Basin and then
sharpening/intensifying as it pivots towards the Gulf Coast
Tuesday morning. This will interact with a northern stream impulse
to produce a positively tilted full-latitude trough surging
eastward through Wednesday. Downstream of this amplification, a SW
to NE oriented jet streak will amplify to above 150 kts, leaving
its favorable diffluent RRQ atop the Gulf Coast, leading to
pronounced deep layer ascent. At the same time, the aforementioned
Great Basin shortwave will continue to sharpen to produce
impressive height falls/PVA, overlapping with the upper diffluence
and the baroclinic zone positioned across the Gulf of Mexico to
spawn a wave of low pressure moving east across it.

Downstream of this developing wave, moisture advection will rapidly
intensify on 300K isentropic ascent and accompanying WAA, driving
PWs northward into the Gulf Coast and Southeast, with PW anomalies
approaching +1 sigma to fuel widespread and expanding
precipitation. While there remains some uncertainty as far as where
the mixed precip zone will occur, the guidance has continued to
trend a bit farther south/colder, suggesting that even the
immediate beaches from Houston to Panama City will experience
snowfall. Additionally, with extremely cold and dry air positioned
just to the north, and low-level northerly flow ageostrophically
enhancing the low-level baroclinic gradient, some increased fgen is
expected from eastern TX through the FL Panhandle. This will be
enhanced by the response to the upper jet aloft, and the WPC
prototype snowband tool indicates a high chance for 1+"/hr snow
rates from near Houston to Mobile Bay despite an elevated DGZ. This
could additionally cool the column causing the mixed precip zone to
push even farther south. These types of snow rates in areas that do
not typically receive them will create major to extreme impacts,
and historic snowfall is possible across this area. WPC
probabilities are moderate to high (50-70%) for more than 4 inches
on D1 from near Beaumont, TX through southern LA, with locally more
than 6 inches possible (30% chance) where the most intense banding
occurs. Surrounding this region, there exists a broad moderate
chance (30-70%) for more than 2 inches from east-central TX through
the NW Florida Panhandle.

Farther east into Georgia, the northern Florida Peninsula, and the
Carolinas, the guidance has been flip-flopping as far as the
northern extent of moisture. However, today's runs have pushed the
moisture a bit farther north despite the dry/cold airmass
positioned to the north, so even these areas may see rare
significant snowfall accumulations, despite more modest rates
overall. In this region, and generally on D2, WPC probabilities for
more than 2 inches are 30-50% from southern AL and south-central GA
eastward to Cape Fear and the Outer Banks of NC.

Finally, across portions of south Texas, and areas from the FL
Panhandle to the northern FL Peninsula, a mixed zone of snow,
sleet, and freezing rain is likely. There continues to be
uncertainty in the latitudinal placement of the heaviest freezing
rain/icing amounts, but current WPC probabilities for at least 0.1"
of ice are as high as 30% in south Texas, and 50-70% across the
northern FL Peninsula, where locally more than 0.25" of ice is
possible.

This event has prompted the issuance of collaborated Key Messages (KeyMessage_1) linked below.

Weiss


...Winter Storm Key Messages are in effect. Please see current=20
Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_1.png__;!!DZ3fjg!6e-O5Gpp8S2L_Q3YY_MWP1vXyey2AlBFNQSVwN2hFs3wn= ZFX2F8o-4XcBQFtJH5NMHtrg2BucxQJs7WuJ7JhCWxz_UA$=20

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_2.png__;!!DZ3fjg!6e-O5Gpp8S2L_Q3YY_MWP1vXyey2AlBFNQSVwN2hFs3wn= ZFX2F8o-4XcBQFtJH5NMHtrg2BucxQJs7WuJ7JhLauubDo$=20

$$

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FOUS11 KWBC 212004
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
304 PM EST Tue Jan 21 2025

Valid 00Z Wed Jan 22 2025 - 00Z Sat Jan 25 2025

...Great Lakes...
Days 1-3...

Upper troughing centered near the Davis Strait will send=20
shortwaves around its base across the Great Lakes for the next=20
couple of days, maintaining the lake-effect snow with some breaks=20
in between events and accentuated with a broader area of light=20
snow. Ongoing heavy snow east of Lake Ontario into the Tug Hill=20
will slowly weaken overnight as the trough axis passes through,=20
while the multi-band event over the Upper Great Lakes continues in=20
advance of a surface feature. That area of low pressure will=20
traverse Lake Superior and head eastward through southern Canada,=20
spreading light snow over the region tomorrow into Thursday before=20 weakening. 850mb temperatures are cold (-20C and lower) resulting=20
in more than enough lake-850 delta-T with fluffy snow given the=20
cold air mass.=20

The heaviest snow will be on D1 east of Lake Ontario and also over
northwestern Lower MI and the eastern U.P. on SSW winds. WPC=20
probabilities for at least 8 inches of snow over the next 72 hours=20
are highest in these locations (>40%) with a broader area of >4"=20
probs >50% over the northwestern U.P. and downwind of Lake Erie=20
just south of Buffalo.=20

=20
...Northern Rockies/High Plains...
Days 1-3...

Upper ridge axis over CA/OR will favor northerly flow across the=20
Northern Rockies onto the western High Plains with a few waves of=20
snowfall over the next three days. Though moisture is rather=20
limited, favorably-enhanced terrain will help wring out several=20
additional inches of the Little Belt and Big Snowy mountains into=20
the Bighorns and Black Hills. Lighter snow is expected for much of=20
the rest of the region days 1-2. The next system on D3 will focus=20
over western MT as the 120-kt jet moves through. WPC probabilities=20
for at least 8 inches of snow for the period are highest over the=20 aforementioned mountains.=20


...Texas, Gulf Coast, and Southeast...
Day 1...

...Significant and historic winter storm pushing through the=20 Southeast/southern Mid-Atlantic tonight...

The winter storm responsible for significant and record-breaking=20
snowfall over parts of the northwest Gulf Coast will continue=20
eastward and northeastward tonight. Cold air mass supports snow all
the way into the FL Panhandle with a mix of sleet/freezing rain=20
just to the south over the eastern portions of the I-10 corridor=20
and along the I-95 corridor from near Charleston southward to south
of Jacksonville. For the period after 00Z tonight, snow will=20
spread quickly into eastern NC (and even up into southeast MA/Cape=20 Cod/Islands) with the Southeast in the RRQ of a 200kt jet exiting=20
the East Coast into Atlantic Canada and a strong baroclinic zone=20
just offshore as the Arctic air has been entrenched to the coast.=20
Question remains how much mixing there will be on the=20
southern/southeastern side (southeast GA/coastal SC/northern FL)=20
and how quickly there may be ptype transition. So far for this=20
storm, things have been colder rather than warmer so have nudged in
this direction for the first few periods of this forecast=20
(starting 00Z tonight). With the long positively-tilted trough axis
approaching the East Coast 00-06Z tonight, broad SW flow will=20
continue to funnel moisture up and over the cold dome with enhanced
FGEN out of southeast GA northeastward to the NC Outer Banks.=20
There, WPC probabilities for at least 4 inches of snow are >30%=20
and even >50% over eastern NC.=20

Colder air will continue to push through the FL Panhandle this=20
evening with a mix changing to snow into the Tallahassee area where
WPC probabilities for at least an inch of snow are >50% (extending northeastward across southeastern GA and increasing to >70%. To=20
the south/southeast, a mix of sleet/freezing rain will likely=20
result in a zone of icing over northeastern GA into=20
southeastern/coastal GA/SC (roughly TLH-CHS-JAX-GNV) where enough=20
WAA above the surface may prevent a changeover to all snow until=20
perhaps the end of the event. WPC probabilities of at least a=20
hundredth of an inch of ice are >30% over northern FL into=20
southeastern GA and up to Charleston, SC. Within this region, WPC=20 probabilities for at least 0.10" icing are highest (>30%) between=20
TLH and JAX northward to around Brunswick, GA.=20

Travel will be severely impacted tonight in much of the region.=20
See our Key Messages for more information that cover this system as
well as the extreme cold over much of the eastern half of the=20
Lower 48.

Fracasso


...Winter Storm Key Messages are in effect. Please see current=20
Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_1.png__;!!DZ3fjg!8gpedAsCx7E8ZQR3vNfRrdKioWHQff0zoOjV8mlqPiLEm= lS7Posz1lg-uVKMbfRo5OLzpah_cukxPk9WGyMdseV7gx8$=20

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_2.png__;!!DZ3fjg!8gpedAsCx7E8ZQR3vNfRrdKioWHQff0zoOjV8mlqPiLEm= lS7Posz1lg-uVKMbfRo5OLzpah_cukxPk9WGyMdqebfQMk$=20

$$

=3D =3D =3D
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FOUS11 KWBC 220816
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
316 AM EST Wed Jan 22 2025

Valid 12Z Wed Jan 22 2025 - 12Z Sat Jan 25 2025

...Great Lakes...
Days 1-2...

The lake effect snow (LES) machine will continue for one more day
across the upper Great Lakes while the wind shifts out of the SW
over the eastern Great Lakes lead to the single bands off Lakes
Ontario and Erie tapering off. A clipper system tracking through
Lake Superior today will lead to modest WAA over Lakes Superior and
Michigan that, thanks in large part to the air-mass still remaining
plenty cold throughout the depth of the troposphere, will support
periods of snow through this evening. Cyclonic flow will persist in
wake of the weakening clipper system on Thursday which may trigger
a few LES bands off Lakes Superior and Michigan. This keeps snow
showers in the forecast on Thursday, but snow should taper off by
Friday as high pressure builds in throughout the region.

WPC 2-day probabilities showed moderate chances (40-60%) for
snowfall >8" along the western coast of Michigan's Mitten. There
are high chance probabilities (>70%) for snowfall totals >4" along
much of the northern tier communities of the Michigan U.P.. The
WSSI shows Minor Impacts in these areas through Friday morning.

=20
...Northern Rockies/High Plains...
Days 1-3...

Over the next few days, a series of upper level shortwave troughs
will track from NW to SE across the region, providing both upper-
level forcing and periodic rounds of Pacific moisture that foster
periods of snow. The progressive nature of these disturbances and
lack of deep moisture at mid-levels will help to keep most areas of
heavy snow limited to the mountain ranges of central Montana,
northern Wyoming, and the Black Hills. WPC probabilities show high
chances (>70%) for >8" of snow in the Little Belt, Big Snowy,
Absaroka, Big Horns, and Black Hills through Friday. The mountains
that are most likely to see 1-2 feet of snow are the Little Belt,=20
Big Snowy, and Big Horns, specifically in their taller/more remote
peaks.


...Southeast...
Day 1...

...Significant and historic winter storm concludes later this
morning...

The historic winter storm that produced heavy snowfall and
disruptive ice accumulations along the Gulf Coast yesterday is=20
producing heavy snow this morning in the Southeast from northern
Florida to the southern Mid-Atlantic coast. Snow and ice should
finally conclude around midday today, leaving frigid and near-=20
record cold in its wake for the remainder of the week. WPC=20
probabilities shows moderate-to-high chances (50-70%) for=20
additional ice accumulations >0.01" in northern FL, especially=20
west of the Jacksonville metro area. Additional snowfall=20
accumulations of 1-2" are expected along the Southeast coast with=20
the NC Outer Banks potentially receiving as much as 3" in some=20
locations.

In wake of this storm, travel will remain severely impacted=20
tonight throughout much of the South with lingering icy conditions.
See our Key Messages for more information that cover the final
stages of this system, as well as the extreme cold over much of=20
the eastern half of the Lower 48.


Mullinax



...Winter Storm Key Messages are in effect. Please see current=20
Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_1.png__;!!DZ3fjg!8SXu954Ppynob-NwireG6rZaEFrz03Xzu8NBak4Id9UqL= 3AG2a08kcUkdE5rOTGb4rDeb0861PQw4ho2i4BOiMIfgSo$=20

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_2.png__;!!DZ3fjg!8SXu954Ppynob-NwireG6rZaEFrz03Xzu8NBak4Id9UqL= 3AG2a08kcUkdE5rOTGb4rDeb0861PQw4ho2i4BOxTPf1CQ$=20

$$

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FOUS11 KWBC 221911
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
211 PM EST Wed Jan 22 2025

Valid 00Z Thu Jan 23 2025 - 00Z Sun Jan 26 2025

...Great Lakes...
Days 1-3...

A clipper system tracking through Lake Superior today will lead to
modest WAA-driven snow over the region from northwest to southeast
tonight into Thursday. With the in situ air mass still plenty=20
cold, periods of light snow are expected with the passage of the=20
system with some lake enhancement as well. Cyclonic flow will=20
persist in the wake of the weakening clipper system which may=20
trigger a few lake snow bands off Lakes Superior and Michigan=20
tomorrow then off Lakes Erie/Ontario tomorrow night before snow=20
winds down. Lastly, another fast-moving clipper should brush=20
through the region D3 (Sat) with another round of light snow but=20
perhaps some modest amounts over the Keweenaw Peninsula. WPC=20
probabilities for at least 8 inches of the 3-day period are highest

30%) over the Keweenaw, western Lower Michigan, and southeast of

Lake Ontario.

=20
...The West...
Days 1-3...

Upper ridge axis just off the West Coast will reorient itself=20
westward, allowing height falls to sink through the near-interior=20
West Fri/Sat. Lead/weaker height falls on the southwest side of a=20
trough axis out of central Canada tomorrow will bring some light=20
snow to the northern Rockies with amounts generally 1-4". For days=20
2-3, height falls combined with a modest 120-kt jet moving through=20
MT will promote broad lift over the northern Rockies (Fri),=20
especially western MT into northwestern WY, with some local=20 enhancement/upslope. By D3 (Sat), the jet will increase over the=20
central Plains, favoring additional lift over the central Rockies=20
(southern WY into CO) with continued local upslope on easterly flow
as high pressure noses down the western High Plains. Core of the=20
height falls will be farther west over NorCal/Great Basin but=20
moisture will be a bit lacking. Generally light snow is expected=20
over the northern/central Sierra and across northern NV/UT, though=20
this may increase toward the end of the period as the trough=20
sharpens and may close off into a closed low over NorCal late Sat.

WPC probabilities of at least 8 inches of snow over the next three
days are highest (>50%) over central Montana and northern Wyoming=20
(Little Belt, Big Snowy, Absaroka, and Big Horns) as well as into=20
the CO Rockies and southern WY (Medicine Bow and Park Range).


For the days 1-3 period, the probability of significant icing=20
greater than 0.10" is less than 10 percent.


Fracasso


...Winter Storm Key Messages are in effect. Please see current=20
Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_2.png__;!!DZ3fjg!8Ii1K-TB9siLZqPW2TdzuQALLYs9kRyu1tIITL9I7nMXg= y2R_BK66nRafQNgnOibq4tF4tQivh8bpDqVKV7J_t_nGs4$=20

$$

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FOUS11 KWBC 230718
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
218 AM EST Thu Jan 23 2025

Valid 12Z Thu Jan 23 2025 - 12Z Sun Jan 26 2025

...Great Lakes...
Days 1-3...

Cyclonic flow across the east will be amplified by a shortwave
moving through the flow which will push a cold front eastward
across the Great Lakes late Thursday into Friday, followed by
renewed CAA. This CAA will be somewhat short lived as a brief
period of shortwave ridging follows in its wake, primarily
resulting in subtle WAA D2, before a second, but weaker and
displaced farther north, shortwave digs across the region driving=20
another cold front eastward. This will result in two rounds of CAA
across the now cold lakes (GLERL total ice coverage up to 24%), so
despite steepening lapse rates the duration and intensity of any
subsequent lake effect snow (LES) will be modest.=20

This results in the heaviest snow likely occurring D3 as reflected
by WPC probabilities for more than 6 inches reaching 30-50% east of
Lake Ontario and across the Keweenaw Peninsula, but D1=20
probabilities for 4+ inches are also high (70%) in the eastern=20
U.P., along the eastern shore of Lake Michigan, and east of Lake=20
Ontario. 3-day total snowfall may eclipse 12 inches in the most=20
prolonged snow bands.

=20
...The West...
Days 1-3...

An upper ridge dominating the flow across much of the West will
quickly be replaced by an amplifying trough beginning the latter
half of D1. This will occur in response to a shortwave trough
digging across British Columbia/Alberta and connecting with
secondary energy from the Pacific moving into the Pacific
Northwest. Together, these will force a longwave trough to deepen,
with height falls rapidly beginning Friday across the Pacific and
Interior Northwest as secondary energy digs southward through the
trough. With time, this feature is progged to become even more
impressive, taking on a negative tilt near CA Saturday and then
potentially closing off into an amplified low as reflected by both
ECMWF and GFS deterministic 500mb fields, and supported by NAEFS
700-500mb height anomalies falling to below the 10th percentile
over CA and portions of the Great Basin.

This synoptic evolution will help push a cold front southward
through the Central Rockies and Great Basin, while the placement of
the upper low results in downstream divergence and pronounced SW
flow atop the sinking front. The result of this will be increasing
isentropic ascent and expanding precipitation, generally in the
form of snow as the swath of precip pivots south from the Northern
Rockies through the Great Basin, accompanied by snow levels falling
from 1500-3000 ft ahead of the front to less than 500 ft below it.
Most of the precipitation should be light to moderate as PW
anomalies are generally normal to below normal, but some heavier
snowfall is possible, especially D3 as a stripe of fgen develops in
the LFQ of a strengthening jet streak collocated with the
WAA/isentropic ascent from the Great Basin east to the Front Range
of CO. Some enhanced ascent will also occur in this area due to=20
increasing upslope flow on the NE flow around a high pressure to=20
the north.

This evolution will spread a swath of snowfall southward each day.
On D1, WPC probabilities for 4+ inches are moderate (50-70%) across
some of the higher terrain of central Montana. By D2 the coverage
of moderate probabilities increases and spread across the
Absarokas, NW WY ranges, and into the CO Rockies including the Park
Range. By D3, WPC probabilities for 4 inches or more are highest
across the Front Range and Park Range of CO, with some lower
probabilities as far east as the Sierra. Days 2-3 snowfall could
exceed 1 foot in parts of the Colorado Rockies.


For the days 1-3 period, the probability of significant icing=20
greater than 0.10" is less than 10 percent.


Weiss



...Winter Storm Key Messages are in effect. Please see current=20
Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_2.png__;!!DZ3fjg!50lahX5_yCKcjhnoIGMwfw8hINyLOuIiVeKkUO87i-5Of= o9ncFFPMjhQU-ApIxLUR0M0Nvu3iqZgSh_fb0IqDDeaAaY$=20

$$

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FOUS11 KWBC 231908
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
208 PM EST Thu Jan 23 2025

Valid 00Z Fri Jan 24 2025 - 00Z Mon Jan 27 2025

...Great Lakes...
Days 1-3...

Cyclonic flow across the east and three separate shortwaves
crossing the Great Lakes will promote continued CAA as well as
upslope flow and pulses of LES throughout the region. The initial
shortwave and period of CAA will be somewhat short lived tonight as
a brief period of shortwave ridging follows in its wake, primarily
resulting in subtle WAA on Friday, before a second, but weaker and
displaced farther north, shortwave digs across the region driving
another cold front eastward. Then, a third system will quickly
trail and enter the Upper Great Lakes by the end of D3, but with
better forcing and lake-enhancement north of the U.S. - Canadian
border. In total, this will result in two rounds of CAA across the cold
lakes (GLERL total ice coverage up to 24% with Lake Erie up to
80%), so despite steepening lapse rates the duration and intensity
of any subsequent lake effect snow (LES) will be modest.

This results in the heaviest snow likely occurring D2-D3 as
reflected by WPC probabilities for more than 6 inches reaching
70-90% east of Lake Ontario and across the Keweenaw Peninsula, but
D1 probabilities for 4+ inches are also high (70%) east of Lake
Ontario. 3-day total snowfall may eclipse 12 inches in the most
prolonged snow bands.


...The West...
Days 1-3...

An upper ridge dominating the flow across much of the West will
quickly be replaced by an amplifying trough beginning D1. This
will occur in response to a shortwave trough digging across British Columbia/Alberta and connecting with secondary energy from the
Pacific moving into the Pacific Northwest. Together, these will
force a longwave trough to deepen, with height falls rapidly
beginning Friday across the Pacific and Interior Northwest as
secondary energy digs southward through the trough. With time, this
feature is progged to become even more impressive, taking on a
negative tilt near CA Saturday and then potentially closing off
into an amplified low as reflected by both ECMWF and GFS
deterministic 500mb fields, and supported by NAEFS 700-500mb height
anomalies falling to below the 10th percentile over CA and
portions of the Great Basin.

This synoptic evolution will help push a cold front southward
through the Central Rockies and Great Basin, while the placement of
the upper low results in downstream divergence and pronounced SW
flow atop the sinking front. The result of this will be increasing
isentropic ascent and expanding precipitation, generally in the
form of snow as the swath of precip pivots south from the Northern
Rockies through the Great Basin, accompanied by snow levels falling
from 1500-3000 ft ahead of the front to less than 500 ft behind
it. Most of the precipitation should be light to moderate as PW
anomalies are generally normal to below normal, but some heavier
snowfall is possible, especially D2 as a stripe of fgen develops
in the LFQ of a strengthening jet streak collocated with the
WAA/isentropic ascent from the Great Basin east to the Front Range
of CO. Some enhanced ascent will also occur in this area due to
increasing upslope flow on the NE flow around a high pressure to
the north.

As the closed upper low lazily sinks southward on D3 along the
central CA coastline, periods of light to moderate snow are also
likely to develop across the southern Sierra Nevada and southern
California ranges. Precipitation will be mostly driven off of upper
divergence due to a strengthening southwesterly jet over the
Southwest and most importantly steep lapse rates (>7.5C 700-500 mb
per the ECMWF). This will allow for snowfall levels to fall to
around 4000ft in southern CA, with levels a bit higher in the San
Jacinto range though Sunday.

This evolution will spread a swath of snowfall southward each day.
On D1, WPC probabilities for 4+ inches are moderate (50-70%) across
some of the higher terrain of central/western Montana and western
WY. By D2 the coverage of moderate probabilities spreads southward
into the central Great Basin and the CO Rockies, including the
Park Range, Medicine Bow Range, and immediate Front Range. WPC
probabilities for 4 inches or more are highest from the Front
Range to the Park Range of CO, with some lower probabilities as
far east as the Sierra. By D3 these moderate probabilities then
center over the southern Sierra and southern CA ranges. Days 1-3
snowfall could exceed 1 foot in parts of the Colorado Rockies.


For the days 1-3 period, the probability of significant icing
greater than 0.10" is less than 10 percent.


Weiss/Snell






$$

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FOUS11 KWBC 240819
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
319 AM EST Fri Jan 24 2025

Valid 12Z Fri Jan 24 2025 - 12Z Mon Jan 27 2025

...Great Lakes...
Days 1-3...

Cyclonic flow will persist across the Great Lakes and Northeast
through the period, although modest amplitude of this troughing
will result in generally progressive flow. Within this flow several
shortwaves will traverse WNW to ESE atop the region, each one
driving renewed surges of CAA across the Great Lakes. There may be
as many as 4 weak shortwaves through the period, with subtle
thickness rises in between each feature, and this will result in
waves of lake effect snow (LES) with variable wind direction
driven by CAA behind each impulse. The Great Lakes have cooled
dramatically in the past 7-10 days, reflected by the warmest
waters now generally around +5 C, and regional ice coverage up to
24% (85% on Lake Erie now), which will somewhat limit the intensity
of any LES, and the heavy snow during this period will be more
driven by repeated rounds of moderate snow than very heavy rates.
The exception will most likely be east of Lake Ontario and across
the Keweenaw Peninsula. This is reflected by WPC probabilities for
4+ inches exceeding 70% both D2 and D3 east of Lake Ontario and
into the Tug Hill Plateau where 3-day snowfall of 1-2+ feet is
possible, and by 4+ inch probabilities exceeding 90% across the
Keweenaw on D2 where locally as much as 12 inches is possible.


...The West...
Days 1-3...

A shortwave digging south from British Columbia will begin to
amplify as its accompanying vorticity surges into WA/OR this aftn.
This feature will continue to dive southward while amplifying,
reaching CA by the start of D2, and most guidance now supports the
development of a closed low over central CA Saturday aftn/evening
where 500-700mb heights fall as low as the 1st percentile according
to the NAEFS climatology. This feature will likely then crawl
southward as it becomes cutoff, with multiple closed height
contours, over CA through D3, reaching potentially only as far
south as the Los Angeles area by the end of the forecast period.
This amplified closed low development and the accompanying
longwave trough will force downstream jet development, as the
subtropical jet arcs northeast from near Baja into the Central
Plains, reaching as high as 110 kts D2, and then as high as 150kts
D3 as secondary enhancement occurs over CA.

This evolution will have a two-pronged effect on the precipitation
and snowfall across the West. First, the shortwave digging south
will push a cold front southward beneath it, causing snow levels to
crash rapidly in its wake from 3000-5000 ft to below 500 ft,
although across the southern Great Basin and southern CA snowfall
levels will fall only to around 3000 ft. Most of the precipitation
D1-D2 associated with this front will be modest due to normal, to
below normal, PWs. However, the developing jet streak combined with
the frontal passage and post-frontal upslope flow will result in an
axis of stronger ascent through fgen (and the upslope), leading to
a swath of heavy snowfall from the Absarokas of MT southward
through WY, and most impressively into the Colorado Rockies,
including the Park Range, D1-2. The strongest fgen will likely
reside west-to-east from CO through the Sierra, providing
additional heavy snow accumulations for portions of UT and NV. WPC probabilities D1-2 are moderate to high (50-90%) for 4+ inches
across these areas, with the most substantial snowfall likely
across CO where 12-18 of total snowfall is possible.

D2-D3 snowfall begins to ramp up downstream of the slowly sinking
closed low, in response to increasing WAA/moist advection and
impressive deep layer ascent as mid-level divergence overlaps with
increasing LFQ jet-level diffluence. This will spread periods of
moderate to heavy precipitation northeast into the Sierra, as well
as the Transverse and Peninsular Ranges of CA, with some moisture
spilling into the Great Basin (including Mt. Charleston) as well.
The airmass across this region is likely to be characteristically
different from that farther to the north as the front stalls, so
snowfall in this area will be more elevated and with lower SLR.
Still, the favorable ascent and moisture overlap will likely
produce rounds of heavy snow, and WPC probabilities D2 are
moderate (50-70%) for 4+ inches in the Sierra, and expand D3 into
the southern CA ranges, reaching 70-90% for 4+ inches, and as high
as 10-30% for 8+ inches, with the highest accumulations expected
above 5000 ft.


For the days 1-3 period, the probability of significant icing
greater than 0.10" is less than 10 percent.


Weiss



$$

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FOUS11 KWBC 241837
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
137 PM EST Fri Jan 24 2025

Valid 00Z Sat Jan 25 2025 - 00Z Tue Jan 28 2025

...Great Lakes...
Days 1-3...

Cyclonic flow will persist across the Great Lakes and Northeast
through the period, although modest amplitude of this troughing
will result in generally progressive flow. Within this flow several
shortwaves will traverse WNW to ESE atop the region, each one
driving renewed surges of CAA across the Great Lakes. There may be
as many as 4 weak shortwaves through the period, with subtle
thickness rises in between each feature, and this will result in
waves of lake effect snow (LES) with variable wind direction
driven by CAA behind each impulse. The Great Lakes have cooled
dramatically in the past 7-10 days, reflected by the warmest
waters now generally around +5 C, and regional ice coverage up to
24% (85% on Lake Erie now), which will somewhat limit the intensity
of any LES, and the heavy snow during this period will be more
driven by repeated rounds of moderate snow than very heavy rates.

The exception will most likely be east of Lake Ontario and across
the Keweenaw Peninsula, where more persistent westerly fetch will
promote continued LES. For areas east of Lake Ontario (Tug Hill and
western Adirondacks) a heavier burst is likely on Sunday as a
potent short wave crosses over the region with uniform westerly
flow through the column and near the LFQ of a departing upper jet.
This will promote a single LES band with the potential of
containing 1-2"+ /hr rates. This is reflected by WPC probabilities
for 8+ inches exceeding 70% both D2 and D3 east of Lake Ontario and
into the Tug Hill Plateau where 3-day snowfall of 1-2+ feet is
possible. 4+ inch probabilities exceeding 90% also exist across
the Keweenaw on D1 where locally as much as 12 inches is possible.


...The West...
Days 1-3...

A shortwave digging south and amplifying as it exits the Pacific
Northwest tonight will continue to strengthen and eventually close
off over central CA late on Saturday. 500-700mb height falls are
forecast to dip as low as the 1st percentile according to the NAEFS climatology. This feature will likely then crawl southward as it
remains cutoff, with multiple closed height contours, over CA
through D3, reaching potentially as far south as the Los Angeles
area by the end of the forecast period. This amplified closed low
development and the accompanying longwave trough will force
downstream jet development, as the subtropical jet arcs northeast
from near Baja into the Central Plains, reaching as high as 110 kts
D2, and then as high as 150kts D3 as secondary enhancement occurs
over CA.

This evolution will have a two-pronged effect on the precipitation
and snowfall across the West. First, the shortwave digging south
will push a cold front southward beneath it, causing snow levels to
crash rapidly in its wake from 3000-5000 ft to below 500 ft,
although across the southern Great Basin and southern CA snowfall
levels will fall only to around 3000 ft. Most of the precipitation
D1-D2 associated with this front will be modest due to normal, to
below normal, PWs. However, the developing jet streak combined with
the frontal passage and post-frontal upslope flow will result in an
axis of stronger ascent through fgen (and the upslope), leading to
a swath of heavy snowfall from the Absarokas of MT southward
through WY, and most impressively into the Colorado Rockies,
including the Park Range, D1-2. The strongest fgen will likely
reside west-to-east from CO through the Sierra, providing
additional heavy snow accumulations for portions of UT and NV. WPC probabilities D1-2 are moderate to high (50-90%) for 4+ inches
across these areas, with the most substantial snowfall likely
across CO where 12-18 of total snowfall is possible.

D2-D3 snowfall begins to ramp up downstream of the slowly sinking
closed low, in response to increasing WAA/moist advection and
impressive deep layer ascent as mid-level divergence overlaps with
increasing LFQ jet-level diffluence. This will spread periods of
moderate to heavy precipitation northeast into the Sierra, as well
as the Transverse and Peninsular Ranges of CA, with some moisture
spilling into the Great Basin (including Mt. Charleston) as well.
The airmass across this region is likely to be characteristically
different from that farther to the north as the front stalls, so
snowfall in this area will be more elevated and with lower SLR.
Still, the favorable ascent and moisture overlap will likely
produce rounds of heavy snow, and WPC probabilities D2 are
moderate (50-70%) for 4+ inches in the Sierra, and expand D3 into
the southern CA ranges, reaching 70-90% for 4+ inches, and as high
as 20-50% for 8+ inches. The highest accumulations expected above
5000 ft, but snow levels could dip as low as 3500 ft underneath the
upper low.


For the days 1-3 period, the probability of significant icing
greater than 0.10" is less than 10 percent.


Weiss/Snell




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FOUS11 KWBC 250741
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
241 AM EST Sat Jan 25 2025

Valid 12Z Sat Jan 25 2025 - 12Z Tue Jan 28 2025

...Great Lakes and Interior Northeast...
Days 1-3...


Cyclonic flow will persist across the Great Lakes and Northeast
through the period, although modest amplitude of this troughing
will result in generally progressive flow. Within this flow several
shortwaves will traverse WNW to ESE atop the region, each one
driving renewed surges of CAA across the Great Lakes. Two
additional shortwaves are progged to move across the region, one
Saturday night, and another Sunday night, with subtle thickness
rises in between each feature, and this will result in waves of
lake effect snow (LES) with variable wind direction driven by CAA
behind each impulse. The Great Lakes have cooled dramatically in
the past 7-10 days, reflected by the warmest waters now generally
around +5 C, and regional ice coverage up to 24% (85% on Lake Erie
now), which will somewhat limit the intensity of any LES, and the
heavy snow during this period will be more driven by repeated
rounds of moderate snow than very heavy rates, but favorable fetch
across Lake Superior D1, and Ontario D2 will produce rounds of
heavy LES with rates 1"/hr or more possible. This is reflected by
WPC probabilities that are moderate (50-70%) for 6+ inches across
the Keweenaw Peninsula D1, and high (>90%) for 6+ inches D2 into
the Tug Hill Plateau.

Then on D3, the core of the cyclonic gyre begins to drop south
from the Hudson Bay, driving sharp height falls and extremely
confluent mid-level flow to its south. As this dives southward, it
will push a cold front draped west to east from Saskatchewan to
Upstate NY by the end of D3, causing enhanced ascent and strong
flow across the Lakes. This will result in additional moderate to
heavy snow, with expansion into Upstate NY, especially in the
upslope region of the Adirondacks, Monday. While moisture is
expected to be significant across due to both synoptic and lake
enhancement, there is some uncertainty as to how the snowfall will
accumulate due to SLRs that may be extremely fractured due to
strong winds within an otherwise favorable DGZ. The models have
trended upward with snowfall, however, and current WPC
probabilities for more than 4 inches of snow are as high as 70-90%
downstream of Lake Ontario and into the Adirondacks.


...The West...
Days 1-3...

A strung out lobe of vorticity emanating from a shortwave closing
off over Northern California this morning will help push a cold
front southward, but with only a lazy loss of latitude through the
period. This slow evolution will be in response to the
amplification of the mid-level pattern, as the shortwave deepens
into a closed low and then drifts over central CA through at least
Sunday night before finally dropping farther south towards northern
Baja late Monday /D3/. Height falls, although slow, will be
impressive as 500-700mb heights drop to around the 1st percentile
according to the NAEFS climatology, helping to drive deep layer
ascent through the region. This ascent will be additionally
enhanced by two distinct downstream jet streaks, one lifting away
into the Central Plains Sunday, with a more intense jet streak
developing over southern CA/northern Baja Monday. The overlap of
the LFQ of this jet streak, the mid-level height falls, and
impressive downstream mid-level divergence will result in
increasing lift across the Desert SW and Great Basin, leading to
expanding rounds of precipitation Sunday and Monday before
weakening and drying out late in the period. This will be in
addition to a round of heavy snow along the front and enhanced by
isentropic ascent, fgen, and upslope flow from the Sierra east to
the CO Rockies on D1.

WPC probabilities for heavy snow D1 are aligned west to east just
north of the surface front, from the central Sierra through the
northern CO Rockies, where they reach 70-90% for 6+ inches, highest
in the Park Range of CO, the Wasatch Front of UT, and the Sierra
in CA where locally as much as 10" of snow is possible on D1.
Although amounts are less otherwise, a fairly continues stripe of
2-4" of snow is possible along this axis today and tonight.

As the upper low consolidates, moisture and ascent become focused
in the southern half of CA, the Great Basin, reaching the Four
Corners by D3. Snow levels will hover around 3000-4000 ft during
this time, resulting in heavy snow in much of the higher terrain as
far south as the San Bernardino and San Jacinto Mountains. WPC
probabilities D2 are moderate (30-50%) for 6+ inches in portions of
the southern Sierra, as well as most of the Peninsular and
Transverse ranges, and also for Mt. Charleston in NV. By D3,
precipitation expands east, and WPC probabilities for 4+ inches
reach 10-30% across the Kaibab Plateau of AZ, with additional
significant snowfall again progged over Mt. Charleston and the San
Bernardino Mountains.


For the days 1-3 period, the probability of significant icing
greater than 0.10" is less than 10 percent.


Weiss



$$

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FOUS11 KWBC 260746
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
246 AM EST Sun Jan 26 2025

Valid 12Z Sun Jan 26 2025 - 12Z Wed Jan 29 2025

...Great Lakes and Interior Northeast...
Days 1-3...

A cold and snowy period is expected through Wednesday morning
across all of the Great Lakes. A persistent longwave trough will
remain in place through the period, with several disturbances
moving through. These shortwave disturbances will locally enhance
the coverage and intensity of snowfall over and downwind (south and
east) of the lakes. Cyclonic flow will keep a steady supply of cold
air moving over the lakes, which in turn will keep the lake-effect
snows going almost constantly through Tuesday, especially east of
Lake Ontario. This morning, the first of these disturbances will
cross over the lower lakes. The associated cold front will briefly
lower temperatures and increase forcing, resulting in heavier
lake-effect behind the front through the day today. Surface ridging
will quickly build in behind the front, but the second disturbance
quickly approaches behind the ridging. This back-and-forth between disturbance/trough and ridging will result in rapid sloshing of the
lake-effect over a larger area downwind of the lakes. However, the
bands won't stay in any one place very long, limiting the
accumulations for most. The disturbances themselves will largely
track well north of the Canadian border, so the souther/eastern
lakes should have the more persistent lake-effect since the
associated wind direction changes will be a bit slower to occur.

A third disturbance in the form of a clipper low will move across
all of the lakes Tuesday night into Wednesday, which will simply
maintain the lake-effect while shifting the bands with the wind.
The clipper itself will make for a general light snowfall over
areas outside of the lake-effect bands as well, though where the
bands are most persistent and where topography can uplift
additional moisture (such as on the Tug Hill Plateau of NY), is
where the greatest snowfall totals are expected.

GLERL analysis of the lakes shows the average lake temperature of
around 40 degrees for Lakes Michigan, Huron, and Ontario. Lake Erie
is over 80% ice covered, and Lake Superior is around 37 degrees.
This would correspond to 850 mb temperatures needing to be at or
colder than -9 to -10 degrees Celsius in order to maintain the
minimum instability for lake-effect. Through this period, only on
Monday ahead of the second disturbance's cold front will this
criterion not be met.

In addition to the lake-effect, the cold fronts associated with
each disturbance, especially the second one on Monday/Monday Night,
will be capable of causing snow squalls outside of the lake-effect
areas. The snow squall parameter will be over 5 at times as the
front moves over the U.P. late Monday afternoon, and the St.
Lawrence Valley Monday night. The parameter will lower a bit to
between 2 and 4 as it moves over the rest of New York State and New
England through Tuesday morning.

WPC probabilities for over 8 inches of snow through D3/Wednesday
morning are high (>80%) across the Tug Hill Plateau and western
Adirondacks of northern New York. Probabilities are moderate
(50-70%) across the Lake Superior shoreline of the eastern U.P.,
and low (10-30%) for northern Michigan and far western New York
south of Buffalo, due to Lake Erie being largely frozen over.

...Southwest, Great Basin, and Central Rockies...
Days 1-3...

An upper level cutoff low will begin the period this morning over
central California. This feature will be the focus for wintry
weather across the mountains of the Southwest through Wednesday
morning. The upper level low will drift south into southern
California through Monday, then drift eastward into Arizona through
Wednesday morning. Due to the slow movement of the low and the cold
air associated therewith, snow will be the dominant precipitation
type over many of the higher elevations. The low will also bring
much needed rainfall to the fire stricken areas in and around Los
Angeles.

While the cutoff low itself will be slow-moving, plentiful upper
level energy/shortwaves rotating around the low will locally
increase precipitation intensity, especially where flow off of the
Pacific lines up orthogonal to the terrain. This is most likely in
the southern Sierras and portions of the Transverse Ranges,
especially northeast/inland of Los Angeles. WPC probabilities of 8+
inches of snow are low (10-40%) for the southern Sierras and
portions of the Transverse Ranges through Wednesday.

By Tuesday, the cutoff low moving into Arizona will shift the
greatest lift and divergence east into the Four Corners region. A
lack of moisture generally over this area should confine the areas
of potential heavy snow to the higher elevations of Utah and
Colorado, but amounts are generally unlikely to exceed 8 inches.


For the Days 1-3 period, the probability of significant icing
greater than 0.10" is less than 10 percent.

Wegman



$$

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FOUS11 KWBC 261925
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
225 PM EST Sun Jan 26 2025

Valid 00Z Mon Jan 27 2025 - 00Z Thu Jan 30 2025

...Great Lakes and Interior Northeast...
Days 1-3...

A cold and snowy period is expected through Wednesday across the
Great Lakes and Interior Northeast as cyclonic flow and several
disturbances continue to influence a favorable pattern for bouts of light-to-moderate snowfall. These shortwave disturbances will
locally enhance the coverage and intensity of snowfall over and
downwind (south and east) of the lakes. The Great Lakes have cooled significantly over the last few weeks, with overall ice coverage
around 24% and Lake Erie alone up to 85%. The warmest lake
temperatures remain around 5 degrees C across lakes Ontario, Huron,
and Michigan, which is still favorable for LES as 850 mb temps drop
to as low as -20 degrees C. For the D1 period (00z Mon- 00z Tues)
brief ridging between systems will lower LES coverage due to more
southwesterly flow as a potent arctic cold front approaches the
Upper Great Lakes Monday evening. As this front crosses the region
and enters the Northeast Tuesday morning, a period of lighter
snowfall ahead of the front is likely across and just downwind of
the Great Lakes associated weak WAA, and with somewhat lower SLRs
given the very strong low- to- mid level flow fracturing dendrites.
Of somewhat more concern with the arctic boundary will be
associated with gusty winds and the potential for snow squalls as
the front progresses south and east. CAMs and other global guidance
continue to highlight elevated snow squall parameters as strong
fgen and relatively steep low-mid-level lapse rates promote a quick
burst of snow along the front through early Tuesday in the Interior
Northeast and New England. The greatest potential exists across
the Upper Great Lakes Monday evening and early Tuesday morning
across Upstate New York. These values and the snow squall threat
greatly decrease by mid- morning as the front loses it's punch once
it reaches the northern Mid- Atlantic.

A second disturbance in the form of a clipper low will move across
the Great Lakes Tuesday night into Wednesday, beginning in the
Upper Lakes and sliding eastward to New England on Wednesday. This
will maintain the lake- effect/enhanced snow while shifting the
bands with the wind. The clipper itself will make for a general
light snowfall over areas outside of the lake- effect bands as
well, though where the bands are most persistent and where
topography can uplift additional moisture (such as on the Tug Hill
Plateau of NY), is where the greatest snowfall totals are expected.

WPC probabilities for over 8 inches of snow on D2-D3 are highest
(50-70%) across the Tug Hill Plateau and western Adirondacks of
northern New York. Probabilities for at least 4 inches of snow are
medium (40-70%) across the Arrowhead of MN, U.P. and northern L.P.
of Michigan and high (>80%) across far western NY.

...Southwest and Central/Southern Rockies...
Days 1-3...

An upper level cutoff low will continue to impact the southern
Sierra and southern CA ranges through D1 until the system slides
inland and moves very slowly across the Southwest D2-D3. Due to
the slow movement of the low and the cold air aloft associated
therewith, snow will be the dominant precipitation type over many
of the higher elevations with snow levels D1 across southern CA
around and slipping just under 4000 ft.

While the cutoff low itself will be slow-moving, plentiful upper
level energy/shortwaves rotating around the low will locally
increase precipitation intensity, especially where flow off of the
Pacific lines up orthogonal to the terrain. A strong, but weakening
upper southwesterly oriented jet off the Pacific will also allow
for increased diffluence just to the north across the Southwest
through midweek. D1-D2 WPC probabilities for at least 4 inches are
high (>70%) across the southern Sierra, southern CA ranges, and
into the higher terrain of the Southwest, including parts of the
Mogollon Rim of AZ.

By Wednesday, the cutoff low moving into Arizona will meander east
with the greatest lift and divergence situated into the Four
Corners region and southern Rockies. A lack of moisture generally
over this area to begin with should confine the areas of
potentially heavy snow to the higher elevations of southern Utah
and Colorado, but by late D3 a resurging upper level jet streak
developing at the base of the upper low will strengthen and bring
another round of upper level divergence with enhanced moisture
advection entering the southern Plains (>90th climatological
percentile per the ECMWF). This leads to medium chances (40-60%)
for at least 4 inches of snowfall on D3 across the higher
elevations of the Four Corners, Mogollon Rim of AZ, and southern
Rockies of CO/NM. Greater coverage of snowfall is expected just
after the end of the D3 forecast period (00z Thurs) throughout the
southern Rockies. Snow levels are currently forecast to range from
around 4000 ft under the upper low over AZ and up to 5300 ft
across the Southern Rockies on Wednesday.


For the Days 1-3 period, the probability of significant icing
greater than 0.10" is less than 10 percent.

Snell






$$

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FOUS11 KWBC 270835
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
335 AM EST Mon Jan 27 2025

Valid 12Z Mon Jan 27 2025 - 12Z Thu Jan 30 2025

...Great Lakes, Northeast, Mid-Atlantic...
Days 1-3...

An active period of winter weather is expected through mid-week as
progressive flow drives multiple impulses with rounds of snow from
the Great Lakes through the Northeast.

The period begins with a lead shortwave exiting New England into
the Atlantic, leaving sharp NW CAA in its wake. THe most impressive
CAA is likely to linger primarily across Lake Ontario and into
Upstate NY, at least early in the period before a subtle surge of
WAA occurs ahead of the next front. This will result in a band of
heavy LES waning right at the start of the period, with minimal
additional LES accumulation.

However, the respite will be short as a potent shortwave,
basically demarcating the edge of the larger cyclonic gyre digging
south from Hudson Bay will drop south Monday night. This will
drive a potent cold front southward, first into the U.P and L.P. of
MI late Monday evening, and then continue to race east through the
interior Northeast and New England Monday night into Tuesday
morning. Impressive CAA along the front will drive an intense
baroclinic boundary, leading to a stripe of impressive low-level
fgen, especially in the 925-850mb layer. This will drive strong
mesoscale lift into a saturating column reflected by high low-level
RH, and as lapse rates steepen, some weak to modest instability
will develop. The high-res guidance has become more aggressive
showing a band/corridor of convective snow showers and snow
squalls, with HREF probabilities showing a 20-30% chance for 1"hr
snowfall rates within these elements. The accompanying steep lapse
rates will help mix down strong winds, encouraging the development
of low visibility during heavy snow showers. The limiting factor to
true snow squalls may be a relative dearth of 0-2km CAPE, and it
is beneficial that the most significant impacts will occur at
night, but still, convective snow showers and isolated snow squalls
are expected Monday night into Tuesday morning from the U.P. of
Michigan through much of the interior Northeast and New England,
and potentially as far southeast as the Mid-Atlantic.
Accumulations during this time will generally be minor, but some
LES and some upslope flow with the front across the Adirondacks has
resulted in WPC probabilities reaching 70-90% for 4 inches on D1,

Some modest LES will follow in the wake of this front, but more
significant and widespread snow will occur D2 into D3, as yet
another fast moving impulse dives through the broad cyclonic flow
across the east. This third impulse will track along the residual
baroclinic gradient left by the potent cold front, and will be
overlapped by a strengthening jet streak downstream of the trough
axis to produce more robust deep layer ascent. This will support
modest cyclogenesis as a clipper-type low swings southeast from
Minnesota Tuesday aftn, to Upstate New York Wednesday morning, and
then across New England, exiting into Atlantic Canada by Wednesday
evening. Brief but impressive WAA downstream of this low will
result in expanding precipitation falling as moderate to heavy snow
across many areas.

Then, in the wake of this system, additional LES will develop as
NW flow causes additional accumulations in the favored NW snow
belts, and with strong winds, some of this could push as far SE as
the coastal Mid-Atlantic states by the end of the forecast period.
WPC probabilities D2 are high (70-90%) for 4+ inches across the
U.P., the NW L.P. near Traverse City, and east of Lakes Erie and
Ontario, with a swath of moderate probabilities for 2+ inches
surrounding those areas for the more synoptically forced snowfall.
For D3, the greatest risk for 4+ inches of snow shifts into the
Adirondacks and Greens where WPC probabilities feature a high risk

70%) while additional modest LES occurs east of Lakes Erie and

Ontario. Locally 1-2 feet is possible in some areas through the
period.

Finally, a second round of convective snow showers or snow squalls
is becoming more likely on Wednesday, this time a little south of
the event Tuesday morning. These convective snow showers will occur
during a more favorable time of day to support better CAPE behind
the secondary front, and the SnSq parameter is suggesting a risk
across areas from eastern OH through southern Upstate NY and PA.
This secondary set of squalls, if they occur, will again be
accompanied by strong winds and heavy snow rates leading to the
potential for significant travel impacts despite modest snowfall
accumulations.

The two rounds of potential snow squalls have prompted the issuance
of Key Messages linked below.


...Southwest through the Four Corners...
Days 1-3...

The anomalous upper low pivoting slowly across CA will begin to
slip east this week, bringing snowfall across the Southwest US
through Thursday. This cutoff begins the period quite amplified,
with 500-700mb heights as low as -2 sigma over CA according to the
NAEFS ensemble tables. Two distinct spokes of vorticity will dance
around the core of this upper low, helping to elongate it with
time as it becomes stretched NE to SW into the Four Corners by
Tuesday night. The interaction of these vorticity impulses
rotating around the closed center will help keep the low amplified,
but also very slow moving, reaching NM/CO by the end of the
forecast period.

During this synoptic evolution, the upper pattern will alter
considerably as well. Initially, a strong 250mb jet streak will be
arced downstream of the upper low, lifting northeast into the
Southern Plains. However, this feature will weaken D1 as the upper
low elongates, leaving less substantial diffluent-caused ascent
into D2. However, a secondary surge of jet level energy will occur
Wednesday into Thursday as the jet streak re-amplifies and arcs
meridionally from the Gulf of California into the Southern Plains,
placing intense LFQ diffluent ascent over the Four Corners. This
jet evolution, combined with the mid-level closed low, and at least
subtle low-level baroclinicity in the vicinity of a weakening front
will cause waves of low pressure to move eastward from CA to the
Four Corners D1-D2. This will cause periods of snowfall from the
Transverse and Peninsular Ranges of CA eastward through the
Mogollon Rim and Kaibab Plateau where WPC probabilities are
moderate (30-50%) D1 for 4 inches of snow, and high (>70%, but only
in higher elevations of AZ) on D2.

However, the most significant snowfall is likely to begin D3 as a
low pressure develops near the TX Big Bend and moisture advection
surges on isentropic ascent across TX. The accompanying theta-e
ridge is progged to lift cyclonically into a TROWAL, with the most
intense theta-e advection being directed into the DGZ over NM/CO.
There is still quite a bit of uncertainty regarding the placement
and evolution of this system, but snowfall chances are increasing,
especially in the higher terrain above 5000 ft in the Sangre de
Cristos and San Juans. Here, WPC probabilities D3 are as high as
70% for 6+ inches, and impactful snow is becoming more likely for
parts of this area Wednesday into Thursday.


For the Days 1-3 period, the probability of significant icing
greater than 0.10" is less than 10 percent.


Weiss



$$

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FOUS11 KWBC 270856
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
356 AM EST Mon Jan 27 2025

Valid 12Z Mon Jan 27 2025 - 12Z Thu Jan 30 2025

...Great Lakes, Northeast, Mid-Atlantic...=20
Days 1-3...

An active period of winter weather is expected through mid-week as
progressive flow drives multiple impulses with rounds of snow from
the Great Lakes through the Northeast.

The period begins with a lead shortwave exiting New England into
the Atlantic, leaving sharp NW CAA in its wake. THe most impressive
CAA is likely to linger primarily across Lake Ontario and into
Upstate NY, at least early in the period before a subtle surge of
WAA occurs ahead of the next front. This will result in a band of
heavy LES waning right at the start of the period, with minimal
additional LES accumulation.=20

However, the respite will be short as a potent shortwave,
basically demarcating the edge of the larger cyclonic gyre digging
south from Hudson Bay will drop south Monday night. This will=20
drive a potent cold front southward, first into the U.P and L.P. of
MI late Monday evening, and then continue to race east through the
interior Northeast and New England Monday night into Tuesday=20
morning. Impressive CAA along the front will drive an intense=20
baroclinic boundary, leading to a stripe of impressive low-level=20
fgen, especially in the 925-850mb layer. This will drive strong=20
mesoscale lift into a saturating column reflected by high low-level
RH, and as lapse rates steepen, some weak to modest instability=20
will develop. The high-res guidance has become more aggressive=20
showing a band/corridor of convective snow showers and snow=20
squalls, with HREF probabilities showing a 20-30% chance for 1"hr=20
snowfall rates within these elements. The accompanying steep lapse=20
rates will help mix down strong winds, encouraging the development=20
of low visibility during heavy snow showers. The limiting factor to
true snow squalls may be a relative dearth of 0-2km CAPE, and it=20
is beneficial that the most significant impacts will occur at=20
night, but still, convective snow showers and isolated snow squalls
are expected Monday night into Tuesday morning from the U.P. of=20
Michigan through much of the interior Northeast and New England,=20
and potentially as far southeast as the Mid-Atlantic.=20
Accumulations during this time will generally be minor, but some=20
LES and some upslope flow with the front across the Adirondacks has
resulted in WPC probabilities reaching 70-90% for 4 inches on D1,

Some modest LES will follow in the wake of this front, but more
significant and widespread snow will occur D2 into D3, as yet
another fast moving impulse dives through the broad cyclonic flow
across the east. This third impulse will track along the residual
baroclinic gradient left by the potent cold front, and will be
overlapped by a strengthening jet streak downstream of the trough
axis to produce more robust deep layer ascent. This will support
modest cyclogenesis as a clipper-type low swings southeast from=20
Minnesota Tuesday aftn, to Upstate New York Wednesday morning, and=20
then across New England, exiting into Atlantic Canada by Wednesday=20
evening. Brief but impressive WAA downstream of this low will=20
result in expanding precipitation falling as moderate to heavy snow
across many areas.=20

Then, in the wake of this system, additional LES will develop as
NW flow causes additional accumulations in the favored NW snow=20
belts, and with strong winds, some of this could push as far SE as=20
the coastal Mid-Atlantic states by the end of the forecast period.
WPC probabilities D2 are high (70-90%) for 4+ inches across the
U.P., the NW L.P. near Traverse City, and east of Lakes Erie and
Ontario, with a swath of moderate probabilities for 2+ inches
surrounding those areas for the more synoptically forced snowfall.
For D3, the greatest risk for 4+ inches of snow shifts into the
Adirondacks and Greens where WPC probabilities feature a high risk

70%) while additional modest LES occurs east of Lakes Erie and

Ontario. Locally 1-2 feet is possible in some areas through the
period.

Finally, a second round of convective snow showers or snow squalls
is becoming more likely on Wednesday, this time a little south of=20
the event Tuesday morning. These convective snow showers will occur
during a more favorable time of day to support better CAPE behind=20
the secondary front, and the SnSq parameter is suggesting a risk=20
across areas from eastern OH through southern Upstate NY and PA.=20
This secondary set of squalls, if they occur, will again be=20
accompanied by strong winds and heavy snow rates leading to the=20
potential for significant travel impacts despite modest snowfall=20 accumulations.=20

The two rounds of potential snow squalls have prompted the issuance
of Key Messages linked below.


...Southwest through the Four Corners...=20
Days 1-3...

The anomalous upper low pivoting slowly across CA will begin to
slip east this week, bringing snowfall across the Southwest US
through Thursday. This cutoff begins the period quite amplified,
with 500-700mb heights as low as -2 sigma over CA according to the
NAEFS ensemble tables. Two distinct spokes of vorticity will dance
around the core of this upper low, helping to elongate it with=20
time as it becomes stretched NE to SW into the Four Corners by=20
Tuesday night. The interaction of these vorticity impulses=20
rotating around the closed center will help keep the low amplified,
but also very slow moving, reaching NM/CO by the end of the=20
forecast period.

During this synoptic evolution, the upper pattern will alter
considerably as well. Initially, a strong 250mb jet streak will be
arced downstream of the upper low, lifting northeast into the
Southern Plains. However, this feature will weaken D1 as the upper
low elongates, leaving less substantial diffluent-caused ascent
into D2. However, a secondary surge of jet level energy will occur
Wednesday into Thursday as the jet streak re-amplifies and arcs
meridionally from the Gulf of California into the Southern Plains,
placing intense LFQ diffluent ascent over the Four Corners. This
jet evolution, combined with the mid-level closed low, and at least
subtle low-level baroclinicity in the vicinity of a weakening front
will cause waves of low pressure to move eastward from CA to the
Four Corners D1-D2. This will cause periods of snowfall from the
Transverse and Peninsular Ranges of CA eastward through the
Mogollon Rim and Kaibab Plateau where WPC probabilities are
moderate (30-50%) D1 for 4 inches of snow, and high (>70%, but only
in higher elevations of AZ) on D2.

However, the most significant snowfall is likely to begin D3 as a
low pressure develops near the TX Big Bend and moisture advection
surges on isentropic ascent across TX. The accompanying theta-e
ridge is progged to lift cyclonically into a TROWAL, with the most
intense theta-e advection being directed into the DGZ over NM/CO.
There is still quite a bit of uncertainty regarding the placement
and evolution of this system, but snowfall chances are increasing,
especially in the higher terrain above 5000 ft in the Sangre de
Cristos and San Juans. Here, WPC probabilities D3 are as high as
70% for 6+ inches, and impactful snow is becoming more likely for
parts of this area Wednesday into Thursday.


For the Days 1-3 period, the probability of significant icing
greater than 0.10" is less than 10 percent.


Weiss




...Winter Storm Key Messages are in effect. Please see current=20
Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_1.png__;!!DZ3fjg!46-Xa_0_7L_F2cVP-1Gcpks77p8WJG_3BDNL_OZU18mwx= 1YGTcFck7IMmHGlCYuYOkhNHU9hMyKNM-e2GjD-1i4zQw4$=20

$$

=3D =3D =3D
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FOUS11 KWBC 271925
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
225 PM EST Mon Jan 27 2025

Valid 00Z Tue Jan 28 2025 - 00Z Fri Jan 31 2025

...Great Lakes, Northeast, Mid-Atlantic...=20
Days 1-3...

Current mid-level WV satellite imagery indicates a strong mid-
latitude gyre focused over the southern tip of the Hudson Bay with
a strong surface cold front analyzed just north of the Canadian=20
border with sights on the Upper Great Lakes and areas downstream.=20
The expectation is for this potent cold front to drive southward=20
through the U.P, pivoting south and southeastward into the L.P. of=20
MI this evening eventually racing eastward through the interior=20
Northeast and New England overnight into Tuesday morning.=20
Impressive CAA along the front will drive an intense baroclinic=20
boundary, leading to a stripe of impressive low- level fgen,=20
especially in the 925-850mb layer. This will drive strong mesoscale
lift into a saturating column reflected by high low-level RH, and=20
as lapse rates steepen, some weak to modest instability will=20
develop. High-res guidance has maintained an aggressive posture=20
with regards to a band/corridor of convective snow showers and snow
squalls with the strongest low-level FGEN indication within the=20
confines of the primary Arctic front. Time frame of interest is=20
between 06-12z Tuesday with a forward propagation from northwest to
southeast beginning in the NY state North Country with motions=20
downstream into the adjacent Adirondack through Western NY.=20

Recent HREF probabilities have increased from the previous forecast
with a more robust 30-50% chance for at least 1"hr snowfall rates=20
within strongest low-level forcing regime aligned with the Arctic
front. Other lower end probs exist behind the primary Arctic push
with an extension through parts of Upstate NY before the heavier
snow threat fades downstream as we lose the accompanying mid-level
forcing as it scoots off to the northeast. The accompanying steep
low to mid level lapse rates will help mix down strong winds,=20
encouraging the development of low visibility during any heavier=20
snow showers/squalls. The limiting factor to more robust snow=20
squalls may be a relative dearth of 0-2km CAPE, and it is=20
beneficial that the most significant impacts will occur at night,=20
but still, convective snow showers and isolated snow squalls are=20
expected Monday night into Tuesday morning from the U.P. of=20
Michigan through much of the interior Northeast and New England,=20
and potentially as far southeast as the Mid-Atlantic. Accumulations
during this time will generally be minor, but some LES and some=20
upslope flow with the front across the Adirondacks has resulted in=20
WPC probabilities reaching 70-90+% for 4 inches on D1 with some
mid-range probabilities (40-60%) located downwind of Lake Erie
within NY state.

Some modest LES will follow in the wake of this front, but more
significant and widespread snow will occur D2 into D3, as yet
another fast moving impulse dives through the broad cyclonic flow
across the east. This third impulse will track along the residual
baroclinic gradient left by the potent cold front, and will be
overlapped by a strengthening jet streak downstream of the trough
axis to produce more robust deep layer ascent. This will support
modest cyclogenesis as a clipper-type low swings southeast from=20
Minnesota Tuesday afternoon, to Upstate New York Wednesday=20
morning, and then across New England, exiting into Atlantic Canada=20
by Wednesday evening. Brief but impressive WAA downstream of this=20
low will result in expanding precipitation falling as moderate to=20
heavy snow across many areas.=20

Then, in the wake of this system, additional LES will develop as
NW flow causes additional accumulations in the favored NW snow=20
belts, and with strong winds, some of this could push as far SE as=20
the coastal Mid-Atlantic states by the end of the forecast period.
WPC probabilities D2 are high (70-90+%) for 4+ inches across the
U.P., the NW L.P. near Traverse City, and east of Lakes Erie and
Ontario, with a swath of moderate probabilities for 2+ inches
surrounding those areas for the more synoptically forced snowfall.
For D3, the greatest risk for 4+ inches of snow shifts into the
Adirondacks and Greens where WPC probabilities feature a high risk

70%) while additional modest LES occurs east of Lakes Erie and

Ontario. Locally 1-2 feet is possible in some areas through the
period.

Finally, a second round of convective snow showers or snow squalls
is becoming more likely on Wednesday, this time a little south of=20
the event Tuesday morning. These convective snow showers will occur
during a more favorable time of day to support better CAPE behind=20
the secondary front, and the Snow Squall parameter is suggesting a
risk across areas from eastern OH through southern Upstate NY and=20
PA. This secondary set of squalls, if they occur, will again be=20
accompanied by strong winds and heavy snow rates leading to the=20
potential for significant travel impacts despite modest snowfall=20 accumulations.=20

The two rounds of potential snow squalls continue the issuance of=20
Key Messages linked below.


...Southwest through the Four Corners...=20
Days 1-3...

Mid-level water vapor satellite imagery continues to highlight the
anomalous upper low pivoting slowly across CA with distinct
horizontal elongation within the confines of the disturbance.=20
Increased upper forcing directly under and downstream of the=20
primary circulation will bring increasing snowfall concerns across=20
the Southwest US through Thursday as the disturbance wanders=20
eastward, generally cut off from the mean upper flow. This=20
disturbance begins the period quite amplified, with 500-700mb=20
heights as low as -2 sigma over CA according to the NAEFS ensemble=20
tables. Two distinct spokes of vorticity will dance around the core
of this upper low, maintaining the elongated structure over time=20
as it stretches NE to SW into the Four Corners by Tuesday night.=20
The interaction of these vorticity impulses rotating around the=20
closed center will help keep the low amplified, but also very slow=20
moving, reaching NM/CO by the end of the forecast period.

During this synoptic evolution, the upper pattern will alter
considerably as well. Initially, a strong 250mb jet streak will be
arced downstream of the upper low, lifting northeast into the
Southern Plains. However, this feature will weaken D1 as the upper
low elongates, leaving less substantial diffluent-caused ascent
into D2. However, a secondary surge of jet level energy will occur
Wednesday into Thursday as the jet streak re-amplifies and arcs
meridionally from the Gulf of California into the Southern Plains,
placing intense LFQ diffluent ascent over the Four Corners. This
jet evolution, combined with the mid-level closed low, and at least
subtle low-level baroclinicity in the vicinity of a weakening front
will cause waves of low pressure to move eastward from CA to the
Four Corners D1-D2. This will cause periods of snowfall from the
Transverse and Peninsular Ranges of CA eastward through the
Mogollon Rim and Kaibab Plateau where WPC probabilities are
moderate to high (40-70%) D1 for 4 inches of snow.

However, the most significant snowfall is likely to begin late-D2
into D3 as a low pressure develops near the TX Big Bend and=20
moisture advection surges on isentropic ascent across TX. The=20
accompanying theta-e ridge is progged to lift cyclonically into a=20
TROWAL, with the most intense theta-e advection being directed into
the DGZ over NM/CO. There is still quite a bit of uncertainty=20
regarding the placement and evolution of this system, but snowfall=20
chances are increasing, especially in the higher terrain above 5000
ft in the Sangre de Cristos and San Juans. Here, WPC probabilities
for D2 and D3 are as high as 70% for 6+ inches, and impactful snow
is becoming more likely for parts of this area Wednesday into=20
Thursday.

For the Days 1-3 period, the probability of significant icing
greater than 0.10" is less than 10 percent.


Weiss/Kleebauer


...Winter Storm Key Messages are in effect. Please see current=20
Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_1.png__;!!DZ3fjg!5K-a8Bms6P3TGAFAYWXSVd6wyAz0ihHR8yW93jnKtuWMt= 6xtikKIfdX_alryZJeA0Sbe1xgcmd2bSEb7Y5P5qohAnz4$=20

$$

=3D =3D =3D
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FOUS11 KWBC 280709
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
209 AM EST Tue Jan 28 2025

Valid 12Z Tue Jan 28 2025 - 12Z Fri Jan 31 2025

...Great Lakes, Northeast, Mid-Atlantic...=20
Days 1-2...

Progressive pattern continues across the east as a cyclonic gyre
centered near the Hudson Bay drifts east, but vorticity lobes
shedding around it will maintain/re-amplify the upper trough into
Thursday.=20

The first of these shortwaves will be racing east across Upstate NY
and northern New England this morning, pushing an arctic front
southeast in tandem. Scattered convective snow showers and isolated
snow squalls will remain possible along this front through the
morning hours, with briefly heavy snow rates above 1"/hr and gusty
winds combining to produce scattered impacts as far east as I-95 to
NYC and Boston. However, the intensity of these features is likely
to be much less substantial than occurring overnight into Tuesday
morning.

Behind this front, a brief respite in snowfall will occur before a
second impulse digs rapidly from Alberta, Canada towards the Great
Lakes and Northeast. The interaction of this shortwave trough with
the leftover baroclinic boundary behind the cold front will yield
surface cyclogenesis in the form of an "Alberta Clipper" which is
progged to track quickly from the Arrowhead of MN this afternoon
through the Upper Great Lakes tonight, and then across New England
on Wednesday. Weak secondary development is possible east of Massachusetts/Maine Wednesday as well, but should have marginal
impact on the winter impacts. Ahead of this clipper, intensifying
SW flow will drive enhanced WAA and moist isentropic ascent,
especially along 285-290K surfaces, to produce an expanding swath
of moderate to heavy snow. The DGZ becomes exceptionally deep
during this time as reflected by SREF probabilities of 90% for
100+mb of depth during the period of most impressive WAA. This will
create a near iso-thermal layer within the -12C to -18C
temperatures, explaining this deep DGZ. PWs are progged to exceed
the 97th percentile over MN/MI according to NAEFS, which will help
additionally enhance snowfall, and despite PWs falling to the east
with time, a period of heavy snow is likely within this downstream
WAA.

Then, in the wake of this system and behind a subsequent cold front
Wednesday morning, lake effect snow (LES) will develop as NW flow=20
causes additional accumulations in the favored NW snow belts. With
strong winds progged in forecast soundings, some of this could=20
push as far SE as the coastal Mid-Atlantic states by Wednesday
night. WPC probabilities D1 and D2 for areas that receive both the
WAA snow and then the subsequent LES are moderate to high (50-90%)
for 4+ inches from the western U.P. southeast through Traverse City
area, east of Lakes Ontario and Erie, and into the western
Adirondacks, Greens of VT, and Whites of NH. Storm total snow of
12-18" is possible across the U.P. and the Tug Hill, with 6-12" in
lollipops elsewhere across this swath, especially in the higher
terrain.

Finally, a second round of convective snow showers or snow squalls
continues to look likely Wednesday. These convective snow showers=20
will occur during a more favorable time of day to support increased
CAPE forecast to reach 100-200 J/kg in the 0-2km layer, overlapping
some impressive low-level fgen from central New England, Upstate
NY, and into the northern Mid-Atlantic states. This secondary set=20
of squalls will again be accompanied by strong winds and heavy=20
snow rates leading to the potential for significant travel impacts=20
despite modest snowfall accumulations.=20

After this second front pushes east and snow squalls wind down
Wednesday evening/night, much of D3 will be quiet across the
region. A larger scale storm system is likely to approach late D3=20
from the south with increasing moisture and some light mixed=20
precipitation, but at this time any impacts from that event are=20
most likely into D4, just beyond this forecast period, and WPC
probabilities for 0.01" of ice peak around 50% across northern PA
before the end of the forecast period.

The two rounds of potential snow squalls continue the issuance of=20
Key Messages linked below.


...Southwest through the Four Corners...=20
Days 1-3...

Modestly anomalous upper cutoff low (NAEFS 700-500mb heights
falling to below the 10th percentile of the CFSR climatology) will
roll slowly eastward from southern CA this morning. This feature
will move slowly (such is the nature of cutoffs) across the Desert
Southwest and Four Corners Wednesday before pinwheeling into the
Central Plains on Thursday. The exact track and placement of this
upper low remains uncertain due to vorticity lobes which will
periodically dance around the central gyre, tugging it in subtly
different directions at different times, but the large scale
pattern is well agreed upon at this time by the various global=20
models.

This evolution will result in pronounced synoptic forcing across
the region from west to east, primarily due to height falls,
divergence, and periods of PVA. However, other forcing at play=20
will enhance deep layer ascent. This includes increasing LFQ
diffluence as a subtropical jet streak intensifies downstream of
the closed low, especially Wednesday into Thursday across NM/CO,
and periods of upslope flow embedded within isentropic ascent.
There has also been an increase this morning in post-system
deformation as a surge of wrap-around precipitation develops over
eastern CO Thursday morning and then pivots southward. These
parameters together should produce sufficient lift in an area of
increasing moisture to produce warning-level snow above generally
4000-5000 ft in the Sangre de Cristos, San Juans, and Jemez
Mountains. However, there remains some uncertainty as to the
movement of an accompanying dry slot, which could lower
accumulations in some areas.

WPC probabilities for 4+ inches on D1 are minimal, but by D2 expand considerably and amplify into the San Juans and Sangre de Cristos,
where they reach as high as 70-90% for 6+ inches, with locally over
1 foot possible in the highest terrain. Additionally WPC
probabilities are high (70-90%) for 4+ inches across the Raton Mesa
and higher elevations of I-25 near the NM/CO border. By D3 the=20
event begins to ramp down, but WPC probabilities for 4+ inches=20
above 30% continue in the Sangre de Cristos, and expand into parts=20
of the Palmer Divide as well.


...Pacific Northwest...
Day 3...

A trough moving eastward across the Northern Pacific will generate
an intensifying jet streak downstream, pivoting ascent and moisture
into the region after 00Z Friday. This will manifest as a corridor
of enhanced IVT as high as 500 kg/m/s (above the 90th CFSR
percentile according to NAEFS) reaching as far inland as Idaho
before the end of the period. This will drive an expanding area of precipitation spreading eastward, but with the accompanying WAA
driving snow levels to as high as 5500 ft, much of this will fall
as rain. However, in the Olympics and Cascades, a heavy wet snow is
likely, which could accumulate to more than 4 inches before 12Z
Friday as reflected by WPC probabilities reaching 30-50% in these
areas.


For the Days 1-3 period, the probability of significant icing
greater than 0.10" is less than 10 percent.


Weiss



...Winter Storm Key Messages are in effect. Please see current=20
Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_1.png__;!!DZ3fjg!89A0arGloreIKXQotadXhZgzOZWkTEDqObqSoaulhMJ-l= yNdVH-z-6fzkoR_y-dCV1OP1GMqdkfYjMeMmFHQVd3TBwA$=20

$$

=3D =3D =3D
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FOUS11 KWBC 282005
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
305 PM EST Tue Jan 28 2025

Valid 00Z Wed Jan 29 2025 - 00Z Sat Feb 01 2025

...Great Lakes, Northeast, Mid-Atlantic...=20
Days 1-3...

...Snow squalls racing across the Great Lakes tonight, then through
the Northeast on Wednesday may lead to hazardous travel...

A progressive area of low pressure will track through the Upper
Great Lakes this evening with both its warm front and cold front
featuring prominent roles in the development of heavy snow over the
next 24-36 hours. Regarding the warm front, modest 925-700mb WAA
aloft will overrun the sufficiently cold air-mass in place from
western NY on east through the rest of Upstate NY this evening.
This favorable WAA configuration aloft will support isentropic
ascent as far east as the New England coastline by early Wednesday
morning, while at the same time, the region resides beneath the
diffluent left-exit region of a 140kt 250mb jet streak. The areas
favored to see the heaviest snowfall through Wednesday are the Tug
Hill and Adirondacks which not only reside beneath the best
synoptic and mesoscale atmospheric drivers aloft, but WSWrly 40-50kt
850mb winds will also favorably upslope into these areas, thus
maximizing snowfall rates would support >1"/hr snowfall rates
tonight. Look for the White Mountains, Green Mountains, and
Berkshires to also witness periods of heavy snow tonight and
through Wednesday via the same processes as their NY neighboring
mountain ranges. WPC probabilities show moderate-to-high (50-70%)
chances for >12" of snow through Wednesday evening with similar
probabilistic chances exist over the Adirondacks for >8" of snow.
The taller peaks of the Berkshires, Green, and White Mountains
sport moderate chances (40-60%) for snowfall totals >6" through
Wednesday evening.

The improving FGEN along the New England coast Wednesday morning,
combined with the aforementioned left-exit region of the 250mb jet
streak overhead, will help spawn a new coastal low off the MA
Capes. Easterly flow north of the low will tap into Atlantic=20
moisture that in turn allows for periods of snow throughout much of
southern Maine and Downeast Maine. The progressive nature of the=20
storm system will keep periods of heavy snowfall short in duration,
but there are low chances (10-30%) for snowfall totals >4" along=20
the immediate Maine coastline.=20

Meanwhile, on the western flank of the low traversing the Great
Lakes tonight, an Arctic front will spark snow squalls over
northern MN that will race southeastward this evening through the
Great Lakes. By Wednesday morning, the front will head for the
eastern Great Lakes. Out ahead of the front, daytime heating will=20
help to steepen low-level lapse rates while 850mb winds, ranging=20
between 40-50kts, are above the 90th climatological percentile.=20
Areas most at risk for snow squalls stretch from northeast OH and=20
much of northern PA through Upstate NY and into central New=20
England. At the same time, periods of LES are forecast across=20
Michigan's U.P. and the northern tip of Michigan's Mitten. Even as=20
the snow squall threat winds down Wednesday evening, LES will=20
continue in northeast PA and west-central NY as brisk low-level=20
NWrly winds continue. LES bands should finally taper off by=20
Thursday morning as NWrly flow weakens. Day 1 WPC probabilities=20
show moderate chances (40-60%) for >4" of snowfall in the central=20
and eastern Michigan U.P., as well as localized northern areas of=20
Michigan's Mitten. For more on the potential snow squall impacts,=20
please visit our latest Key Messages below.

Following a brief reprieve Wednesday night and into Thursday
morning, the next round of unsettled weather arrives Thursday=20
night as a closed 500mb low over the Central Plains directs a=20
fetch of precipitation at the Mid-Atlantic. There remains modest=20
uncertainty in timing of the precipitation's arrival, how far north
QPF can reach into New England, and how much of a sub-freezing=20
air-mass remains as the storm's precipitation shield arrives on=20
Friday. That said, the mountainous terrain of the Northeast likely=20
sports the best chances for measurable snowfall late Friday, while=20
valleys and coastal areas will not be as favored to see heavy snow=20
given the lack of a sufficiently cold air-mass by the time the=20
storm arrives on Friday.=20


...Southwest through the Four Corners...=20
Days 1-2...

A large closed upper-low located over the Southwest is rather
anomalous for the time of year, sporting 500mb heights as low as
the 2.5 climatological percentile near the northern Gulf of
California coast. Temperatures are also rather cold by this time=20
of year's standards with 500-700mb temps generally below the 2.5=20 climatological percentile according to NAEFS over much of AZ
tonight and over western NM during the day on Wednesday. There is
just enough 700-300mb moisture aloft, combined with favorable
250-500mb diffluence aloft ahead of the closed low and modest
upslope ascent to support heavy snow in mountain ranges such as the
San Juans, the Mogollon Rim, the Gila Mountains, and as far east as
the Sangre De Cristo. The heaviest snowfall will reside in the San
Juans tonight through Wednesday, then late Wednesday into Thursday
in the Sangre De Cristo and as far north as the Palmer Divide as
easterly upslope flow helps with dynamic cooling of the atmospheric
column and supports heavier snow rates. Lower elevation snowfall=20
will be harder to come by given the lack of a cold continental
polar (cP) air-mass, but minor accumulations (<3") are possible in
the central High Plains. The tallest peaks of the San Juans and
Sangre De Cristos sport moderate-to-high chances (50-70%) for >8"
of snowfall with localized areas likely surpassing a foot by the
time the event concludes Thursday afternoon. The Palmer Divide does
sport low-to-moderate chances (30-50%) for snowfall totals >4", but
at elevations <6,000ft, probabilities are generally <20%. Still,
minor accumulations of 1-3" could result in slick travel conditions
for travelers along the I-25 corridor on Thursday.=20


...Pacific Northwest...
Day 3...

Multi-vort mid-level trough will approach the PacNW early Friday
with downstream moisture advection (IVT >90th percentile) and
rising snow levels (from ~2500-4000ft up to 3500-5500ft north to
south through the WA/OR Cascades). This will favor snow in the=20
higher elevations, mostly above pass level, especially as the WAA
drives in higher QPF. Guidance differs on how much snow levels will
rise, with a spread of 2500ft or so (near and higher than
Snoqualmie). For the higher elevations above 4000ft, WPC=20
probabilities for at least 8 inches of snow are high (>70%) through
00Z Sat (with additional precipitation thereafter). A heavy and wet
snow can be expected around the oscillating snow level, which is
shown in the WSSI-P snow load parameter.


For the Days 1-3 period, the probability of significant icing
greater than 0.10" is less than 10 percent.


Mullinax/Fracasso




...Winter Storm Key Messages are in effect. Please see current=20
Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_1.png__;!!DZ3fjg!4o-nP5-Iiez8E4mPjD6onPfrhtLiTl_5cXYvJBOrPdpp7= _5QIziYcdwW1HMtsNpSD-9Bx_zJtJRu6DRHQke0aQB9QdQ$=20

$$

=3D =3D =3D
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FOUS11 KWBC 290749
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
249 AM EST Wed Jan 29 2025

Valid 12Z Wed Jan 29 2025 - 12Z Sat Feb 01 2025

...Great Lakes, Northeast, Mid-Atlantic...=20
Days 1 & 3...

An Alberta Clipper type low will be racing east across New England
and into the Gulf of Maine to start D1, driving an arctic cold
front southward in its wake. This evolution will result in three=20
areas of moderate snowfall D1.

First, the guidance continues to indicate that the environment=20
will be favorable to support widespread and impressive snow squalls
today, especially across the interior Northeast and as far east as
I-95 from near Philadelphia, PA through Portland, ME. While=20
snowfall within any snow squalls will result in minimal=20
accumulations (less than 1" most areas), snowfall rates in excess
of 1"/hr as reflected by HREF probabilities in areas with
impressive overlap of 0-2km fgen and CAPE will create dangerous
driving due to highly variable visibilities and snow covered roads.
The strongest squalls are likely along and just behind the front,
and many areas will likely experience squalls through sunset on
Wednesday, and Key Messages (linked below) are in effect for this
event.

Behind the front, strong CAA across the Lakes will help support
another round of lake effect snow (LES) in the favored W/NW snow
belts, especially across the eastern U.P. of MI, and east of Lakes
Erie and Ontario. The heaviest accumulations are likely across the
Tug Hill and western Adirondacks (somewhat aided by upslope flow as
well), with lighter accumulations elsewhere. Overall, LES D1=20
should be modest, but WPC probabilities feature a high risk=20
(70-90%) for 4+ inches of snow in the Tug Hill, western=20
Adirondacks, portions of the Greens, and the higher elevations of=20
the Whites.

Finally, Downeast Maine will also likely experience a narrow
corridor of moderate to heavy snow as secondary low pressure
developing offshore beneath the LFQ of an upper jet streak angles
moist isentropic ascent onshore. The axis of moderate snow is
likely to be narrow, but a ribbon of snow accumulating to several
inches is likely, as reflected by WPC probabilities for 4+ inches
as high as 50% along the immediate coast.

Thursday appears quiet across the region, but this changes quickly
late Thursday night/Friday morning as a larger scale storm system
approaches from the SW. This system will emerge from the Central
Plains as a strong closed low, opening slowly as it approaches the
Mid-Atlantic Friday evening, and reaching the Atlantic coast by
Saturday morning. Impressive downstream divergence initially will
somewhat weak as the low fills, but will still overlap directed
moisture as a theta-e ridge surges northward from the Gulf of
Mexico leading to PWs that reach above the 90th climatological
percentile according to NAEFS. The antecedent airmass is modestly
cold for wintry precipitation, so much of the accompanying
precipitation will likely be rain across the Mid-Atlantic, but the
higher terrain of PA (around the Poconos) and into Upstate
NY/southern New England will likely receive some mixed
precipitation of sleet/freezing rain, with snow farther north.
There will also likely be some gradual cooling of the column from
north to south late in the forecast period in response to dynamic
effects and ageostrophic drainage as the low pulls away, resulting
in at least a brief period of heavy snow. WPC probabilities D3 for
heavy snow exceeding 4 inches are confined to the Adirondacks and
northern New England, where they are generally 10-50%.

There remains quite a lot of uncertainty into the placement of the
mixed precip zone, and how much moisture will lift northward, but=20
current WPC probabilities for freezing rain are 10-30% for more
than 0.1" of ice near the Poconos, with a broad swath of 30-70% for
0.01" of ice encompassing much of the interior northern Mid-
Atlantic and southern New England.


...Southwest through the Four Corners...=20
Days 1-2...

Anomalous closed low will continue to advect slowly northeast
across the Four Corners today before moving into the Central Plains
on Thursday. This feature will maintain amplitude, reflected by
500mb heights fall towards the 2.5 percentile according to NAEFS,
suggesting ascent will persist downstream as it moves. The most
impressive lift is likely immediately east of the upper low where=20
height falls and divergence combined with the diffluent LFQ of a=20 strengthening and poleward arcing jet streak, which is also where=20
the greatest moisture is expected as a PW plume surges and lifts=20 cyclonically into NM/CO. With a generally cold air mass in place,=20
and steep lapse rates beneath the upper low persisting, rounds of=20
heavy snow are expected to continue, especially in the higher=20
terrain above 5000 ft from the Mogollon Rim to the San Juans,=20
Sangre de Cristos, and even as far east as the Raton Mesa and=20
Palmer Divide. However, the moisture is expected to get cutoff and
pivot east into the Plains during D2, bringing an end to the=20
snowfall in the region.=20

Before that happens, heavy snow will accumulate in the higher=20
terrain of the Four Corners reflected by WPC probabilities D1 of=20
50-90% in the White Mountains of AZ, as well as the San Juans and=20
Sangre de Cristos. During D2, the focus shifts east, with heavy=20
snow likely confined to the Sangre de Cristos once again.

Farther east into the High Plains including the I-25 urban=20
corridor, wraparound snow aided by upslope could result in=20
impactful accumulations across these areas. Even just by D2 there=20
is uncertainty in the amount of available moisture due to extremely
different camps in the guidance, but WPC probabilities currently=20
suggest a 50-90% chance of at least 2 inches both D1 and D2,=20
greatest in the higher elevations in the vicinity of the Raton Mesa
and Palmer Divide.


...Pacific and Interior Northwest...
Days 2-3...

A more active period begins in the West Thursday as an atmospheric
river (AR) begins to push onshore the coasts of WA/OR before
spilling inland on Friday. IVT within this AR has a high
probability (>60%) of exceeding 500 kg/m/s according to both ECENS
and GEFS probabilities, with this AR being surged onshore in=20
response to a deepening trough across the Pacific moving eastward.
The overlap of confluent mid-level flow with a surging Pacific jet
streak reaching the coast will drive the moisture onshore,=20
reflected by IVT eclipsing the 90th percentile Friday, with a core=20
above the 99th percentile in the northern Great Basin overnight=20
into Saturday morning. This increased moisture will manifest as an=20
expanding area of precipitation, but with WAA accompanying the AR,=20
snow levels will climb steadily to as high as 4000-5000 ft on=20
Friday before collapsing to as low as 1000 ft by the end of the=20
forecast period, lowest in WA state, behind a cold front.

The result of this will be increasing snowfall, primarily in the WA
Cascades and Olympics D2, expanding along the Cascades and into
northern CA, while concurrently reaching as far east as the
Northern Rockies and Salmon River/Sawtooth Ranges D3. SLRs will
likely be quite low ahead of the cold front, but should rise
steadily as snow levels crash late in the forecast period. This is
reflected by high WSSI-P probabilities for snow load late D2 into
D3, creating impacts at higher elevations. WPC probabilities are=20
moderate (30-50%) for 6+ inches D2 in the higher elevations of WA.=20
By D3 these probabilities increase and expand, with a high risk=20

90%) for more than 6 inches along the Cascades of WA and OR, the=20

Olympics, Northern Rockies, Salmon River, and Sawtooth Ranges.


Weiss



...Winter Storm Key Messages are in effect. Please see current=20
Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_1.png__;!!DZ3fjg!_6DWZg4T43ym23gfzWMYmZixow3aTP8tnQVTpa8YJ9ChZ= 19McCfg_OPK8WdfkQCQ05Q4G38sEhWXkW5W_vfADoHQCFg$=20

$$

=3D =3D =3D
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FOUS11 KWBC 292009
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
309 PM EST Wed Jan 29 2025

Valid 00Z Thu Jan 30 2025 - 00Z Sun Feb 02 2025

...Northeast & Northern Mid-Atlantic...
Days 2-3...

Any lingering snow squalls and LES bands ongoing this afternoon
will dissipate tonight as winds weaken gradually and high pressure
builds in from the southeast.=20

Thursday appears quiet across the region, but this changes Thursday night/Friday morning as a storm system approaches from the SW. By
12Z Friday, the positively tilted 500mb low located over the
Middle Mississippi Valley will have directed an exceptional IVT=20
over the eastern third of the U.S. with values topping 1,000 kg/m/s
over the TN Valley. However, the air-mass over the northern Mid-=20
Atlantic will have moderated to a point where most expected=20
snowfall will be confined to the mountain ranges of the Northeast=20
and throughout much of Maine. On Friday, boundary layer/surface=20
temperatures will initially start out <32F from central PA on=20
north through the Catskills, Southern Tier of NY, and the Lower=20
Hudson Valley, and into central New England while a burgeoning warm
nose aloft causes precipitation to fall as a sleet/freezing rain=20
mix. Farther north, boundary layer temperatures will remain below=20
freezing and allow for snow to be the primary precipitation type=20
from the Tug Hill and Adirondacks on east through the Green and=20
White Mountains.=20

As the primary low weakens Friday evening across NY, a new coastal
low will form east of the MA Capes Friday night. The low will be a
fast mover with periods of snow Friday night concluding by Saturday
morning. WPC probabilities show low-to-moderate chances (30-50%)
for snowfall >4" across the Adirondacks, Green and White Mountains,
and northern Maine with low chances (10-30%) for >6" in these same
areas. WPC probabilities depict low chances (10-30%) through=20
northern PA, the Poconos, and Catskills for ice accumulations=20

0.1". The Probabilistic WSSI (WSSI-P) shows low-to-moderate=20

chances (30-50%) for Minor Impacts in all of these aforementioned=20
areas.=20


...Southwest through the Four Corners...=20
Day 1...

An anomalous closed low will continue to advect slowly northeast=20
across the Four Corners this afternoon before moving into the=20
Central Plains on Thursday. This feature will maintain amplitude,=20
reflected by 500mb heights fall towards the 2.5 percentile=20
according to NAEFS, providing strong ascent downstream of the low as
it moves east. The most impressive lift is likely immediately east
of the upper low where height falls and divergence combined with=20
the diffluent LFQ of a strengthening and poleward arcing jet=20
streak, which is also where the greatest moisture is expected as a=20
PW plume surges and lifts cyclonically into NM/CO. Heavy snow will=20 accumulate in the higher terrain of the Southern Rockies,
highlighted by WPC probabilities that show moderate-to-high chances
(50-80%) for snowfall >8" of snowfall the San Juans and Sangre de=20
Cristos, as well as the Raton Mesa.

Farther east into the High Plains, including the I-25/I-70
corridors and Palmer Divide, wraparound snow aided by upslope may=20
result in impactful accumulations across these areas. Latest CAMs=20
guidance has keyed in on a narrow band of heavy snow from the=20
Palmer Divide and along I-25 on east towards the I-70 corridor
between 12-18Z Thursday. Latest HREF guidance shows moderate
chances (40-60%) for snowfall rates >1"/hr in that span, which
could lead to rapid accumulations on roadways and rapid reductions
in visibilities. WPC probabilities show moderate-to-high chanceS
(50-70%) for snowfall >6" along the I-25 corridor over the Palmer
Divide. There are low-to-moderate chances (30-50%) for snowfall
amounts >4" along I-25 in Elbert County on Thursday. It is
possible, should mesoscale banding manifest itself as CAMs suggest,
may result in localized amount of 8-10" from the Palmer Divide on
northeast to the I-70 corridor east of the Denver metro. Travelers
on these interstates and connecting thoroughfares should anticipate
the potential for hazardous travel Thursday morning in these=20
areas. Snow should conclude Thursday afternoon as the closed upper-
low tracks east.=20

...Pacific and Interior Northwest...
Days 2-3...

A more active period begins in the Northwest beginning on Thursday
as an atmospheric river (AR) arrives over the coasts of WA/OR=20
before spilling inland on Friday. IVT within this AR ranges between
300-500 kg/m/s Friday morning, which is as high as the 99th
climatological percentile per NAEFS. This increased moisture stream
will coincide with an expanding area of precipitation, but as WAA=20
increases as a byproduct of the AR's arrival, snow levels will=20
climb to as high as 4000-5000 ft on Friday before plummeting to as
low as 1000ft by the end of the forecast period, lowest in WA=20
state, behind a cold front. Upslope flow into the Cascades/Olympics
will continue through Saturday with heavy snow also expected=20
farther east into the northern Rockies. Farther south, the westerly
IVT will continue to pump copious amounts of Pacific moisture into
the West with the northern California ranges, most notably, seeing
the heaviest precipitation. While this is the case, snow levels=20
will be rather high with elevations >7,000ft most subjected to the
heavy snow in the Sierra Nevada, and >5,000ft in the=20
Trinity/Shasta.

WPC probabilities are highlighting the Cascades and Olympics above
3,500ft as having high chances (>70%) for snowfall totals >24"=20
through Saturday afternoon. A couple other regions likely to=20
measure snowfall in feet are the Sawtooth of central ID and the=20
Tetons where some of the peaks of these ranges are may surpass 30"=20
through Saturday night. Elsewhere, snowfall totals >12" are
moderate-to-high chance (50-70%) according to WPC probabilities in
the peaks of the Trinity/Shasta, the OR Cascades, and peaks of the
northern ID panhandle. There is concern for icy conditions at
lower elevations of northeast WA, northern ID, and western MT=20
where surface temps have rarely gotten above freezing in recent=20
days. Should snow transition over to a sleet/rain mix, ground=20
conditions are so cold that freezing rain could ensue on these=20
surfaces even with air temperatures above freezing. Expect wintry=20
conditions to linger through Sunday as the IVT to the south=20
persists and a more frigid air-mass inches its way south from=20
southwest Canada, resulting in more mountain snow and possibly icy=20 conditions in some valleys of the Pacific Northwest and=20
Intermountain West.

Mullinax



...Winter Storm Key Messages are in effect. Please see current=20
Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_1.png__;!!DZ3fjg!4T8K6qXV823lpoN5sq4BqzwEaU2fXh1CrA4XInAtgYuIQ= Ea4xsbpe0_y9gq5spQg9dVKVmmhql2RDEcVAj3udN2qbKw$=20

$$

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FOUS11 KWBC 300858
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
358 AM EST Thu Jan 30 2025

Valid 12Z Thu Jan 30 2025 - 12Z Sun Feb 02 2025

...Northeast & Northern Mid-Atlantic...
Days 1-3...


Deep low pressure over eastern CO this morning will eject east to
the Middle Mississippi Valley tonight with copious Gulf moisture
available will have directed an exceptional IVT over the eastern
third of the U.S. with values topping 1,000 kg/m/s over the TN
Valley. Pockets of freezing rain are likely late tonight into
Friday over northeast PA and southern NY with Day 1.5 ice probs for

0.1" in the 20-30% range. Farther north, boundary layer

temperatures will remain below freezing and allow for snow to be
the primary precipitation type Friday/Friday night from south of
Lake Ontario across the Adirondacks on east through the Green and
White Mountains and southern/eastern Maine.

As the primary low weakens Friday evening across NY, a new coastal
low will form east of the MA Capes Friday night. The low will be a
fast mover with periods of snow Friday night concluding by Saturday
morning. The EC remains more suppressed/quicker with the solution
resulting in less snow than the stronger/slower/snowier GFS. Day 2
WPC probabilities for >4" are 10-30% for lower elevations from
east of Buffalo and through eastern Maine and 30-60% in terrain of
the Adirondacks, Green and White Mountains. However, should banding
develop on the northern end of the precip shield, the motion would
be along the west-east orientation and lead to localized heavy
snow of several inches. Hopefully the variation in track and
magnitude improves in the next model suite.


...Southern Rockies and Central High Plains...
Day 1...

Mid-level is now over the CO High Plains where it will continue to
develop through this morning. Strong ascent around and under this
low will persist into the afternoon with moisture aiding snow,
heavy at times, for the southern Front Range and Sangre de Cristos
and Raton Mesa. Banding extends northeast from the Palmer Divide
with snow accum to the eastern CO border. Day 1 snow probs for >4"
after 12Z are 40-70% for these areas. Snow should conclude
this afternoon as the low tracks east.


...Pacific and Interior Northwest...
Days 1-3...

An atmospheric river (AR) crosses the WA/OR coasts tonight as an
approaching trough gets drawn into a deep low drifting south from
the southern AK coast. IVT within this AR ranges between 300-500
kg/m/s Friday morning, which is as high as the 99th climatological
percentile per NAEFS. WAA increases with the AR, causing snow
levels to rise tonight through Friday morning to 4000-5000ft for
the Cascades and west. However, height falls from both the trough
crossing and the approaching cold-core low causes snow levels to
plummet Friday night through Saturday. Snow levels reach sea level
in western WA on Saturday with a strong baroclinic zone over OR
and into the northern Rockies. Upslope flow into the
Cascades/Olympics will continue through Friday night before
diminishing to more moderate rates under the colder trough Saturday
night. Heavy snow persists through Saturday for the northern
Rockies. Farther south, the westerly IVT will continue to pump
copious Pacific moisture into the West with the northern California
ranges seeing the heaviest precipitation Saturday and continuing
into next week in a prolonged onshore flow. Snow levels will be
notably higher on that side of the baroclinic zone with snow levels

7,000ft in the Sierra Nevada, and >5,000ft in the Trinity/Shasta.

Day 1.5 WPC snow probabilities for >6" are high (60-90%) in the
WA Cascades and 50-80% in the OR/CA Cascades and northern
Bitterroots. These probabilities greatly expand Friday night with
Day 2.5 snow probs for >6" high (50-90% for the length of the
Cascades/High Sierra, Sawtooths, Bitterroots, and Absarokas/Wind
River around and south of Yellowstone.
There is concern for ice accretion at lower elevations of eastern
WA and northern ID, and western MT where surface temps have rarely
gotten above freezing in recent days. Should snow transition over
to a sleet/rain mix, ground conditions are so cold that freezing
rain would occur on these surfaces even with air temperatures above
freezing. Expect wintry conditions to linger through Monday as the
IVT to the south persists and a more frigid air- mass inches its
way south from southwest Canada, resulting in more mountain snow
and possibly icy conditions in some valleys of the Pacific
Northwest and Intermountain West.

Jackson



$$

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FOUS11 KWBC 301943
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
243 PM EST Thu Jan 30 2025

Valid 00Z Fri Jan 31 2025 - 00Z Mon Feb 03 2025

...Northeast & Northern Mid-Atlantic...
Days 1-3...


An anomalous closed upper-low over the Mississippi Valley Friday
morning will direct a broad and strong IVT at the Northeast U.S..
Rich moisture from the south and increasing low-mid level WAA aloft
will overrun boundary layer temperatures that start out below
freezing in the interior northern Mid-Atlantic and Northeast on
Friday. Pockets of freezing rain are most likely to occur from
northern PA on east through the Poconos, Upstate NY, and into
central New England beginning Friday morning and lasting for the
more northern locations through Friday evening. WPC probabilities
show low chances (10-30%, pockets of 40% in some areas) in north-
central PA and the Catskills for ice accumulations >0.1" through
Friday evening. Minor ice accumulations <0.1" are possible as far
south as the Lehigh Valley in PA, but with a quickly modifying air-
mass, a changeover to plain rain should limit the impact- potential
in these more southern areas. Meanwhile, the mountain ranges of
northern New York and northern New England will have better chances
of staying primarily snow for the duration of this event (Friday
afternoon through Saturday night). WPC probabilities show low-to-
moderate chances (30-50%) for snowfall totals >4" in these areas
with localized amounts within the peaks of the Adirondacks, Green,
and White Mountains topping 6" by early Saturday morning.


As the primary low weakens Friday night across NY, a new coastal
low will form east of the MA Capes Friday night. Some guidance
shows an increasingly low-level easterly fetch to the north of the
low may deliver some Atlantic moisture over southern and central
Maine Friday night and into Saturday morning. Snow should taper off
by midday as the low races into the North Atlantic Saturday
afternoon. WPC probabilities show low chances (10-30%) for snowfall
totals >4" across southern and central Maine, which may result
minor impacts for travelers Saturday morning.


...Upper Great Lakes...
Days 2-3...

A fast moving upper level shortwave trough tracking across the
Northern Plains will coincide with the diffluent left-exit region
of a 250mb jet streak to provide modest ascent over the Upper
Midwest. In addition, as a warm front lifts north Saturday evening,
increasing 850-700mb WAA will provide additional low-level ascent
that results in periods of snow from northern MN on east across the
Upper Great Lakes. The combination of best lift and elevation is
currently being highlighted over northern WI and the southern MI
Upper Peninsula. By 06Z Sunday, a consolidating area of low
pressure will allow for southeast-to-easterly flow to allow for
added lake-enhancement along the MN Arrowhead. Light snow may ensue
over northern MI through the day on Sunday while the heavier
snowfall over northern MN/northern WI tapers off Sunday afternoon.
WPC probabilities show moderate chances (40-60%) for snowfall
amounts >6" along the MN Arrowhead and low-to-moderate chances for
snowfall totals >6" in parts of northeast Wisconsin through Sunday
morning.


...Pacific and Interior Northwest/Great Basin...
Days 1-3...

The start of an active and increasingly wintrier pattern for much
of the Pacific Northwest/Interior West/Great Basin is now in sight
as the lead atmospheric river event starts tonight. Initially mild
temperatures will limit heavier snow to the higher mountain
locations, but colder air will become entrenched as light to
modest precipitation continues to spread across the region. This
will support snow into the lowlands by the end of the period.

For D1, strong IVT (>90th percentile over much of the region
tonight into Fri) and WAA will drive snow levels up to 4000-5000ft
along the Cascades and northern Rockies/Great Basin (and over
6000-7000ft over NorCal into the Sierra). With over an inch QPF in
the terrain of the Cascades, well over a foot to two feet of snow
are likely well above pass level. Light to modest snow is likely
farther east into eastern WA/northern ID/western MT along with some
light icing due to very cold surface/ground temperatures even as
2m temps warm up to just over 32F.

Into D2-3, colder air will filter in (in typical fashion) then
rush in with a vengeance by Sunday as the upper low over coastal BC
sinks closer to the region. Though QPF will be lighter overall
over WA/OR as the moisture plume focuses into NorCal/northern
Sierra, SLRs will rise and snow levels will fall, bringing
accumulating and impactful snow to the passes D2 then into the
lowlands D3 and beyond. For areas north of CA/NV, continued onshore
flow ahead of the upper low will lead to less QPF each day but
more snow overall.

For the D2-3 period, WPC probabilities of at least 8 inches of
snow are high (>70%) above 1000-2000ft (north) to around 4000ft
(south) along the Cascades and across much of the Bitterroots,
Sawtooth, Tetons, Absarokas where 1-3ft of snow is likely. By D3
with colder air mostly in place (at least aloft and mostly to the
sfc), snow levels will drop to near sea level which will favor some accumulation depending on 2m temps and time of day. WPC
probabilities for at least an inch of snow D3 is non-zero through
the I-5 corridor from WA into OR though higher chances may be into
D4. To the south, with the strong AR settling into NorCal, snow
levels will remain on the higher side (6000-8000ft) into the
northern and central Sierra with WPC probabilities for at least 12
inches of snow >50% above these elevations. Low SLRs over much of
CA will promote a heavy and wet snow ("Sierra cement") esp Sat/Sat
night. Impacts per the WSSI may be at least moderate to major
(considerable disruptions).


Mullinax/Fracasso






$$

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FOUS11 KWBC 310915
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
415 AM EST Fri Jan 31 2025

Valid 12Z Fri Jan 31 2025 - 12Z Mon Feb 03 2025

...Northeast...
Days 1 & 3...

Mid-level low pressure over OK early this morning will shift ENE
over the Midwest today and continue to direct broad and strong
moisture advection to the east. A few pockets of freezing rain are
likely over northern PA and Upstate NY on the leading edge of the
warm air advection through this evening with a few hundredths of an
inch possible. Meanwhile, the northern edge of the precip shield
will feature snow bands east from Buffalo across the Adirondacks,
Green, and White Mountains, along with southern NH and far southern
Maine where Day 1 snow probabilities for >4" are around 20%,
perhaps 40% for the higher terrain. Uncertainty has reigned with
the banding potential, but the consensus has agreed on moderate
banding at best starting later this afternoon with max potential
around 6" despite the w-e orientation of the bands in the direction
of motion. Snow tapers off by sunrise Saturday.

The next shortwave trough is currently well off the PacNW Coast,
but rides a strong jet over the Great Lakes on Sunday and the
Northeast Sunday evening. This allows a warm front to lift over the
Northeast with a fairly quick hitting round of snow. Day 3 snow
probabilities for >4" are currently 20 to 40% for only the Tug
Hill, Presidential Range of the Whites in NH and southern Maine.


...Upper Great Lakes...
Day 2...

A fast moving upper level shortwave trough tracking across the
Northern Plains Saturday night will coincide with the diffluent
left- exit region of a 250mb jet streak to provide modest ascent
over the Upper Midwest with an eastward surface low track over
northern MN Saturday night. The warm front ahead of the low lifts
north Saturday evening, increasing 850-700mb WAA to provide
additional low- level ascent and periods of snow from northern MN
on east across the Upper Midwest Saturday afternoon/evening.
Northern WI and the southern MI Upper Peninsula see moderate snow
with locally enhanced lift on the North Shore of Lake Superior on
southeast-to- easterly flow. Light snow spreads over much of MI
early Sunday while the heavier snowfall over northern MN/northern
WI tapers off by Sunday afternoon. WPC snow probabilities for >4"
are 20-50% over northern WI/western U.P. and the MN Arrowhead with
60% probabilities for >6" along the North Shore escarpment.


...Pacific and Interior Northwest/Great Basin...
Days 1-3...

An active and prolonged wintry pattern for much of the Pacific Northwest/Interior West/Great Basin is expected through midweek. Mild temperatures in an atmospheric river (AR) will limit heavier snow
to the higher mountain locations with Cascades snow levels around
5000ft through this evening. Colder air under a trough axis and an
approaching cold-core upper low drifting down the BC coast will
become entrenched over the Northwest as precip rates decrease to
light to moderate Snow levels drop to sea level over western WA and
possibly northwest OR on Saturday where they remain through midweek.
The AR will remain focused into northern CA Saturday through
Tuesday with high snow levels (8000ft and up) on the south side of
a strong baroclinic zone.

For D1, strong IVT (>90th percentile over much of the region
today) and WAA will drive heavy precip and multiple feet of snow
above pass level in the Cascades. Day 1 Probabilities for >8" are
50-80% for the Bitterroots and Sawtooths, and 40-60% for the High
Sierra Nevada and western WY ranges.

Starting tonight colder air will filter in (in typical fashion)
then rush in with a vengeance by Sunday as the upper low over
coastal BC sinks closer to the region. Though QPF will be lighter
overall over WA/OR as the moisture plume focuses into
NorCal/northern Sierra, SLRs will rise and snow levels will fall,
bringing accumulating and impactful snow well below pass level by
Saturday afternoon then into the lowlands/Seattle and possibly
Portland metro by Saturday night. For areas north of CA/NV,
continued onshore flow ahead of the upper low will lead to less QPF
each day but more snow coverage from the cold.

Days 2 and 3 snow probabilities for >8" are focused over
OR/northern CA east across the north-central Rockies with the
heaviest snow continuing over the Sawtooths, Absarokas/Wind River
south through the Wasatch and higher ridges of northern Nevada.


Jackson


$$

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FOUS11 KWBC 311925
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
225 PM EST Fri Jan 31 2025

Valid 00Z Sat Feb 01 2025 - 00Z Tue Feb 04 2025

...Northeast...
Days 1 & 3...

Mid-level low pressure over the Mid-Mississippi Valley early this
afternoon will shift ENE over the Midwest today and continue to
direct broad and strong moisture advection to the east. This
feature will then become strung-out as an open, but sharp wave as
it exits the East Coast early Saturday. A few pockets of freezing
rain may linger over northern PA and Upstate NY through this
evening with a few hundredths of an inch possible. Meanwhile, the
northern edge of the precip shield will feature snow bands east
from Buffalo across the Adirondacks, Green, and White Mountains,
along with southern NH and far southern Maine. This expanding
northern snow shield is being driven by a ribbon of confluent flow
between the approaching shortwave and a deeper trough to the north
over southeast Canada, which promotes extended mid-level fgen and
dynamic cooling throughout the column at least initially tonight.
Cold air advection will shortly trail the system late tonight as a
surface low pressure system transfers to the coast. This may lead
to locally heavy east-west oriented bands of snow from Upstate NY
through southern VT/NH and northern MA, which is highlighted by 12z
HREF 1-hr snow rates up to 1". Day 1 snow probabilities (beginning
00z tonight) for >4" are around 10-30%, perhaps 40% for the higher
terrain. Snow tapers off by Saturday morning.

The next shortwave trough to impact the Northeast is currently
entering the PacNW, but rides a strong jet over the Great Lakes on
Sunday and reaches the Northeast Sunday evening. This allows a warm
front to lift over the Northeast with a fairly quick hitting round
of snow. Day 3 snow probabilities for >4" are currently 20 to 40%
for only the Tug Hill, Presidential Range of the Whites in NH and
southern Maine.


...Upper Great Lakes...
Day 2...

A fast moving upper level shortwave trough tracking across the
Northern Plains Saturday night will coincide with the diffluent
left- exit region of a 250mb jet streak to provide modest ascent
over the Upper Midwest with an eastward surface low track over
northern MN Saturday night. The warm front ahead of the low lifts
north Saturday evening, increasing 850-700mb WAA to provide
additional low- level ascent and periods of snow from northern MN
on east across the Upper Midwest Saturday afternoon/evening.
Northern WI and the southern MI Upper Peninsula see moderate snow
with locally enhanced lift on the North Shore of Lake Superior on
southeast-to- easterly flow. Light snow spreads over much of MI
early Sunday while the heavier snowfall over northern MN/northern
WI tapers off by Sunday afternoon. WPC snow probabilities for >4"
are 20-50% over northern WI/western U.P. and 40-60% in the MN
Arrowhead with 70% probabilities for >6" along the North Shore
escarpment.

Snell/Jackson

An unsettled and wintry pattern is underway for the Pacific
Northwest and will be quickly spreading into the interior/northern
Rockies this evening. Mild temperatures coincident with the
atmospheric river (AR) moisture plume axis will support snow levels
4000-5000ft across the Cascades this evening before dropping after
midnight as the cold air mass pushes in from the west. 160-170kt
jet will support broad lift on its poleward exit region across the
northern Great Basin into the Rockies tomorrow as QPF generally
decreases and focuses farther south into NorCal. There, snow levels
will be quite high (5000-8000ft from north to south) with plenty
of rain initially even up toward the Sierra passes before changing
over to snow. With a strong baroclinic zone settling into
NorCal/southeastern OR later this weekend, snow levels will remain
high over the Sierra.

For D1, the heaviest snow will fall over the Cascades while
increasing into lower elevations, eventually reaching below 500ft
Saturday. Ratios will be on the lower side given the strong Pacific
influence esp along/west of the Cascades. WPC probabilities for at
least 8 inches of snow are >50% above 2000-3000ft or so over the
WA-OR Cascades. Eastward, jet-enhanced snowfall will focus over the
Bitterroots and western MT into the Absarokas/Tetons/Wasatch with
WPC probabilities of at least 12 inches >50% above 6000-7000ft or
so from north to south.

For D2-3 on the colder side of the AR, QPF will decrease as the
jet extends into the western High Plains and the upper low over
coastal BC slowly migrates to near Vancouver Island by the end of
this forecast period. The moisture plume is forecast to dip
southward, stall, then lift back north a bit over the Sun-Mon
period, focusing the heaviest snowfall over the CA/OR border
northeastward across central ID and into western WY (lifting back
into southwestern MT Monday). To the north, snow levels will remain
low (near sea-level or valley floors for the interior) over WA/OR
though 2m temperatures may limit snowfall accumulation along the
I-5 corridor especially during the day. WPC probabilities for at
least 8 inches of snow D2-3 are >50% above 1500ft or so across
western WA/OR but above 5000ft or so into the northern Great Basin
and northern Rockies. Areas of the Tetons and Wind River Range may
see several feet of snow over the next few days.

To the south, higher snow levels will keep the highest snow totals
above 6000-8000ft from north to south across NorCal into the
Sierra as the warm side of the system remains more or less in place
there. Across northern NV and into UT, snow levels will remain
high as well with significant snow above 8000-9000ft.

For lowland areas, lighter QPF and marginal daytime temperatures
may preclude appreciable snow, but WPC probabilities for at least
two inches of snow are at least 20-30% over western WA into
northwestern OR.


Fracasso



$$

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FOUS11 KWBC 311954
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
254 PM EST Fri Jan 31 2025

Valid 00Z Sat Feb 01 2025 - 00Z Tue Feb 04 2025

...Northeast...
Days 1 & 3...

Mid-level low pressure over the Mid-Mississippi Valley early this
afternoon will shift ENE over the Midwest today and continue to
direct broad and strong moisture advection to the east. This
feature will then become strung-out as an open, but sharp wave as
it exits the East Coast early Saturday. A few pockets of freezing
rain may linger over northern PA and Upstate NY through this
evening with a few hundredths of an inch possible. Meanwhile, the
northern edge of the precip shield will feature snow bands east
from Buffalo across the Adirondacks, Green, and White Mountains,
along with southern NH and far southern Maine. This expanding
northern snow shield is being driven by a ribbon of confluent flow
between the approaching shortwave and a deeper trough to the north
over southeast Canada, which promotes extended mid-level fgen and
dynamic cooling throughout the column at least initially tonight.
Cold air advection will shortly trail the system late tonight as a
surface low pressure system transfers to the coast. This may lead
to locally heavy east-west oriented bands of snow from Upstate NY
through southern VT/NH and northern MA, which is highlighted by 12z
HREF 1-hr snow rates up to 1". Day 1 snow probabilities (beginning
00z tonight) for >4" are around 10-30%, perhaps 40% for the higher
terrain. Snow tapers off by Saturday morning.

The next shortwave trough to impact the Northeast is currently
entering the PacNW, but rides a strong jet over the Great Lakes on
Sunday and reaches the Northeast Sunday evening. This allows a warm
front to lift over the Northeast with a fairly quick hitting round
of snow. Day 3 snow probabilities for >4" are currently 20 to 40%
for only the Tug Hill, Presidential Range of the Whites in NH and
southern Maine.


...Upper Great Lakes...
Day 2...

A fast moving upper level shortwave trough tracking across the
Northern Plains Saturday night will coincide with the diffluent
left- exit region of a 250mb jet streak to provide modest ascent
over the Upper Midwest with an eastward surface low track over
northern MN Saturday night. The warm front ahead of the low lifts
north Saturday evening, increasing 850-700mb WAA to provide
additional low- level ascent and periods of snow from northern MN
on east across the Upper Midwest Saturday afternoon/evening.
Northern WI and the southern MI Upper Peninsula see moderate snow
with locally enhanced lift on the North Shore of Lake Superior on
southeast-to- easterly flow. Light snow spreads over much of MI
early Sunday while the heavier snowfall over northern MN/northern
WI tapers off by Sunday afternoon. WPC snow probabilities for >4"
are 20-50% over northern WI/western U.P. and 40-60% in the MN
Arrowhead with 70% probabilities for >6" along the North Shore
escarpment.

Snell/Jackson

...Western U.S. into the Northern Plains...
Days 1-3...

An unsettled and wintry pattern is underway for the Pacific
Northwest and will be quickly spreading into the interior/northern
Rockies this evening. Mild temperatures coincident with the
atmospheric river (AR) moisture plume axis will support snow levels
4000-5000ft across the Cascades this evening before dropping after
midnight as the cold air mass pushes in from the west. 160-170kt
jet will support broad lift on its poleward exit region across the
northern Great Basin into the Rockies tomorrow as QPF generally
decreases and focuses farther south into NorCal. There, snow levels
will be quite high (5000-8000ft from north to south) with plenty
of rain initially even up toward the Sierra passes before changing
over to snow. With a strong baroclinic zone settling into
NorCal/southeastern OR later this weekend, snow levels will remain
high over the Sierra.

For D1, the heaviest snow will fall over the Cascades while
increasing into lower elevations, eventually reaching below 500ft
Saturday. Ratios will be on the lower side given the strong Pacific
influence esp along/west of the Cascades. WPC probabilities for at
least 8 inches of snow are >50% above 2000-3000ft or so over the
WA-OR Cascades. Eastward, jet-enhanced snowfall will focus over the
Bitterroots and western MT into the Absarokas/Tetons/Wasatch with
WPC probabilities of at least 12 inches >50% above 6000-7000ft or
so from north to south.

For D2-3 on the colder side of the AR, QPF will decrease as the
jet extends into the western High Plains and the upper low over
coastal BC slowly migrates to near Vancouver Island by the end of
this forecast period. The moisture plume is forecast to dip
southward, stall, then lift back north a bit over the Sun-Mon
period, focusing the heaviest snowfall over the CA/OR border
northeastward across central ID and into western WY (lifting back
into southwestern MT Monday). To the north, snow levels will remain
low (near sea-level or valley floors for the interior) over WA/OR
though 2m temperatures may limit snowfall accumulation along the
I-5 corridor especially during the day. WPC probabilities for at
least 8 inches of snow D2-3 are >50% above 1500ft or so across
western WA/OR but above 5000ft or so into the northern Great Basin
and northern Rockies. Areas of the Tetons and Wind River Range may
see several feet of snow over the next few days. Farther east over
the western High Plains into the northern Plains, combination of
the jet extension and increasing N-S baroclinicity will support frontogenetically-forced snowfall in a west-to-east band from near
the Black Hills across SD late Sun into early Mon (north of a
forming area of low pressure). WPC probabilities for at least 4
inches of snow there D3 are 25-55% with potential for some higher
accumulations in any bands that may set up parallel to the frontal
zone.

To the south, higher snow levels will keep the highest snow totals
above 6000-8000ft from north to south across NorCal into the
Sierra as the warm side of the system remains more or less in place
there. Across northern NV and into UT, snow levels will remain
high as well with significant snow above 8000-9000ft.

For lowland areas in WA/OR, lighter QPF and marginal daytime
temperatures may preclude appreciable snow, but WPC probabilities
for at least two inches of snow are at least 20-30% over western WA
into northwestern OR.


Fracasso






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FOUS11 KWBC 010925
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
425 AM EST Sat Feb 1 2025

Valid 12Z Sat Feb 01 2025 - 12Z Tue Feb 04 2025


...Western U.S. across the Northern Plains...
Days 1-3...

...Prolonged heavy snow Oregon, Idaho, western Wyoming tonight
through Tuesday night with locally extreme impacts...

...This heavy snow spreads south down the Sierra Nevada Monday
night into Wednesday...

A prolonged Atmospheric River (AR) and cold-core low maintain an
extreme wintry pattern over much of the western U.S. through the
middle of next week. The cold core low over the Queen Charlotte
Sound will drift south over Vancouver Island through Sunday and
linger around/west of Washington through the middle of the week.
Height falls from troughs rounding the low are bringing a sharply
colder profile western WA/northwestern OR this morning with snow
levels quickly descending to sea level over the PacNW Lowlands/Seattle/Portland. As seen in recent years, these cold core
lows often have little QPF in global guidance with much of the low
land snowfall coming in narrow/transient bands that are just now
starting to be realized in CAMs. As of now general snow totals from
Seattle through Portland are generally around 2", but given the
three-day cold spell, localized higher totals can be expected.

A strong baroclinic zone in the AR has been shunted south to far
northern CA. Mild temperatures in the AR moisture plume axis with
snow levels over the Sierra Nevada over 8000ft with heavy precip
that lingers into Sunday night. A narrow stripe of heavier/
prolonged snow occurs on the northern side of the baroclinic zone
where sufficient moisture meets colder air. Southern OR/ID through
western WY sees pivoting bands/swaths of moderate to heavy snow in
this stalled pattern tonight through Tuesday night. Day 1 snow
probs for >12" are fairly spread out from the WA Cascades, highest
OR Cascades and High Sierra (above pass level) along the Wasatch
and western WY ranges.

The northern side of the baroclinic zone becomes the focus for
western heavy snow late tonight. Day 2 snow probs for >12" is
30-50% over much of southern OR into far northern CA, the
Sawtooths, and all western WY ranges. Similar probs merely drift
north for Day 3 making for the risk for several feet of snow in
48hrs in these areas.
As the trough over the NW becomes more positively tilted, height
falls are directed south over CA Monday night with Day 3 snow probs
for >12" 40-80% over the Shasta/Siskiyou and northern/central
Sierra Nevada.

Banding under the zonal jet Sunday night spreads east over the
Dakotas with intensity forecast to weaken as they cross MI/the
Great Lakes Monday, but that should continue to be monitored. Day
2.5 snow probs for >4" are 30-70% from southeast MT east over
northern SD.


...Upper Great Lakes and Northeast...
Days 1-3...

A progressive shortwave trough shifts east over the Northern
Plains today ahead of a diffluent left-exit region of a 130kt jet
streak to provide ascent and allow snow to develop later this
morning over MN on increasing 850-700mb WAA. Northern WI and the
MI Upper Peninsula see moderate snow this afternoon/evening with
locally enhanced lift on the North Shore of Lake Superior on
southeasterly flow. Light snow spreads over much of MI early Sunday
while the heavier snowfall over northern MN/northern WI tapers off
by Sunday afternoon. Day 1 WPC snow probabilities for >6" are
40-80% from greater Duluth through the North Shore with 30-60% for

4" for the rest of the Arrowhead of MN and over most of the U.P.

The WAA continues to shift east over the Northeast Sunday afternoon
into Monday. Day 2 snow probs for >4" are 20-30% for the Tug Hill
and higher Greens/Whites.

The next wave strengthens a bit over the Northeast Monday night
with Day 3 snow probs for >4" 20-50% over the Adirondacks, and
northern VT/NH into northern Maine.


Jackson

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FOUS11 KWBC 011902
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
202 PM EST Sat Feb 1 2025

Valid 00Z Sun Feb 02 2025 - 00Z Wed Feb 05 2025


...Western U.S. across the Northern Plains...
Days 1-3...

...Atmospheric River (AR) event to bring prolonged heavy mountain
snow to parts of the West and Rockies...

Combination of a strong upper low over coastal British Columbia
and another upper low north of Hawaii moving eastward will maintain
onshore flow into the West over the next several days, focusing a
plume of moisture into NorCal and the northern Great Basin to the
northern Rockies. The moisture axis will ebb and flow north then
south as the two players jockey for prominence but the net result
will be lighter snow in the lower elevations north of NorCal,
moderate to heavy snow in the mountains (especially from NorCal
across southeastern Oregon into central Idaho and western WY) with
very heavy snow at the highest peaks. Much of the Sierra will
remain on the warmer side of the AR event which will retain higher
snow levels.

For Day 1, the focus will be over central Idaho into western WY
with moderate to heavy snow in the mountains, generally above
5000-7000ft depending on location. Snow will gradually lessen
across the WA and northern OR Cascades but not shut off
completely. Into Day 2, the approach of the upper lows will help
raise heights a bit over the interior, helping to nudge the
moisture axis northward and increase QPF over NorCal into southern
OR. Downstream, extended jet and an approaching arctic front from
the north will help spread snow into southern/southeastern MT and
into the High Plains/northern Plains in the fast flow. Guidance
continues to waver on the north-south QPF axis across ND/SD, but
potential remains for several inches of snow where modest FGEN sets
up (albeit briefly). WPC probabilities for at least 4 inches of
snow D2 are moderate (40-70%) along the ND/SD border.

Into D3, the northern upper low will meander just west of
Vancouver Island as the southern ex-upper low moves into NorCal
with another surge of moisture that will favor the northern to
central Sierra. The moisture plume will move inland and sink a bit
southward, allowing snow levels to fall a bit but still promoting
heavy snow above 7000ft where WPC probabilities for at least a foot
of snow just D3 are >50%. Over the northern Great Basin to the
northern Rockies, the moisture plume will continue to lift
northward as well in response to the upstream digging heights,
favoring central Idaho into western Montana atop the cold air mass
in place from D2.

Three-day snowfall totals will exceed 1-3 feet in the higher
mountain elevations with lighter snow of an inch or two (or a few)
on the valley floors in most locations across OR/ID. Much more snow
is likely over most of Montana due to the increased low-level
convergence via the arctic front beneath the moisture plumes,
supporting light and fluffy snow. Over western WA/OR, precipitation
will be light and mostly snow or a mix of some rain/snow during
the day with marginal temperatures. This should lead to light
accumulations through the I-5 corridor where WPC probabilities of
at least two inches of snow during the next three days are >20-50%
(especially away from the water and with at least some elevation).


...Upper Great Lakes and Northeast...
Days 1-3...

A progressive shortwave trough slides east over the Northern
Plains through tonight ahead of a diffluent left-exit region of a
130kt jet streak. This will continue to provide ascent along with
modest 850-700mb WAA and allow for an area of light to moderate
snow initially across MN, northwest WI, and the MI Upper Peninsula
to eventually shift east across the Upper Great Lakes into Sunday.
The heaviest snow on D1 is forecast on the North Shore of Lake
Superior across far northeast MN on southeasterly flow, where WPC
probabilities for at least 6 inches of snow are greater than 70%.
Otherwise, light snowfall spreading over northern WI and MI is
expected to remain under 4 inches, with scattered 20-40% WPC
probabilities for more than 4 inches in the MI Upper Peninsula.

The WAA continues to shift east over the Northeast Sunday afternoon
into Monday, with largely light snowfall and the potential for
light icing (less than 0.1") in the Allegheny Mts of PA and
locations east of the Continental Divide where low-level cold air
damming is more favorable. Day 2 snow probs for >4" are 20-30% for
the Tug Hill and higher Greens/Whites.

The next wave enters the Upper Great Lakes on D2 (Monday) as a
subtle shortwave riding along mostly zonal, but fast flow across
the north- central United States creates the potential for a stripe
of moderate to locally heavy snowfall stretching eastward out of
the Dakotas. The first system on Sunday is expected to push a cold
front well south across the central Plains and Midwest on Monday,
with briefly strong 850-700mb fgen and a strengthening upper jet
streak over the Great Lakes. This will prompt an east-west oriented
band of snowfall across central MN, northern WI, and northern MI
on Monday. WPC probabilities on D2 of at least 4 inches of snow are
between 20-40% across this region. By D3 (Tuesday) this 850-700mb
fgen axis slides into the Interior Northeast and along with
trailing CAA and upslope flow into northern NY, VT, NH, and ME,
locally heavy snow is possible. WPC probabilities for at least 4
inches of snow are high (70-80%) from the Adirondacks to the
northern White Mts.


Fracasso/Snell


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FOUS11 KWBC 020914
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
414 AM EST Sun Feb 2 2025

Valid 12Z Sun Feb 02 2025 - 12Z Wed Feb 05 2025


...Western U.S. across the Northern Plains...
Days 1-3...

...Atmospheric River (AR) and upper low maintain prolonged heavy
snow from Oregon/California border to the Northern Rockies through
Monday...

Deep upper low persists over Vancouver Island through Monday before
drifting southwest off the PacNW coast with troughing lingering
there through Thursday. Another upper low moving east to the north
of Hawaii moving eastward will maintain onshore flow into the West
over the next several days, focusing the moisture axis from CA/OR
border through southern Idaho through western WY terrain through
Monday before shifting south over CA Monday night through Tuesday
night. Tight baroclinic zone north of this moisture axis maintains
lower elevation snow with enhanced totals in terrain through this
time.

48hr probs for >2' ending 12Z Tuesday are 50-90% over terrain from
the Shasta/Siskiyou. Fremont Mtns in southern OR, the Sawtooths,
and Yellowstone through Wind Rivers. Beneficial precip for this
area, but major impacts to any passes in this area with snow levels
around 4000ft in the heavier snow. Snow levels are 5000-8000ft in
the core of the AR moisture axis over northern CA/NV/UT. However,
on Monday afternoon height falls as troughing expands over the NW
low brings snow farther south into CA. Heavy snow reaches the
northern Sierra Nevada by Monday evening with a slow progression of
heavy snow down the length of the Sierra with hourly rates
exceeding 2"/hr into Wednesday. Day 3 probs for >1' are 60-95% for
the length of the Sierra Nevada with a few feet in the High Sierra
(snow levels generally 5000-6000ft in the heaviest snow.

Cold continues over western WA/OR with occasional snow in the
Seattle/Portland metro areas through Tuesday night. A few to
several inches at a time are possible given the banding potential under/adjacent to the upper low just to the north.

The tilting axis that brings snow to CA Monday night also shifts
snow across the northern Rockies through MT through Tuesday night.
Day 3 snow probs for >4" are high over the Sawtooths and Absarokas
(adding to the extreme totals of the previous two days) with 40-60%
probs over the Bitterroots and all western MT ranges into the
north-central MT Plains.


...Northern Plains, Great Lakes, Northeast...
Days 1-3...

Zonal flow sends two shortwave troughs east over the northern tier
through Monday night.

Shortwave trough axis over MN shifts east across the Great Lakes
and New England through tonight. Final snow band works over the
North Shore in MN through mid-morning with locally heavy rates from southeasterly flow lifting over the Arrowhead. Expanding precip
shield over MI rest of the morning where a couple inches are
possible with more moderate snow across the Northeast late
afternoon through the evening. Local terrain enhancements over the
Tug Hill/southern Adirondacks/Berkshires/Greens/Whites and coastal
Maine where Day 1 snow probs for >4" are 20-40%.

The next shortwave trough enters the northern Plains this afternoon
with east-west oriented snow bands forming in the left exit region
of a 130kt+ jet streak producing a narrow axis of moderate snow
from southeast MT and along the SD/ND border where Day 1 snow probs
for >4" are 30 to 60%. These bands maintain their strength as they
shift east across central MN/northern WI and the northern L.P. late
tonight through Monday morning where Day 1.5 snow probs for >4" are
30-60%.

This jet induced swath of snow shifts across Ontario Monday, then
grazes northern NY and northern New England Monday night. Day 2
snow probs for >4" are 40-80% for the northern Adirondacks,
northern VT/NH, and northern Maine.


Jackson


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FOUS11 KWBC 021932
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
232 PM EST Sun Feb 2 2025

Valid 00Z Mon Feb 03 2025 - 00Z Thu Feb 06 2025


...Western U.S. across the Northern Rockies...
Days 1-3...

...Atmospheric River (AR) and upper low maintain prolonged heavy
snow from Oregon/California border and Sierra Nevada to the
Northern Rockies through Wednesday...

Deep upper low (under the 1st climatological percentile tonight per
the GEFS) persists over Vancouver Island through Monday before
drifting southwest off the PacNW coast with troughing lingering
there through Thursday. Another upper low moving east to the north
of Hawaii will maintain onshore flow into the West over the next
several days, focusing the primary moisture axis from CA/OR border
through southern Idaho and western WY terrain before a brief break
later on Wednesday. Along the West Coast, this moisture axis will
shift south over CA beginning on Tuesday as the next upper low
approaching interacts with the larger upper system churning off the
PacNW. The prolonged nature of anomalous IVT (+4 to 8 standardized
anomaly per the 00z NAEFS) will likely lead to a corridor of very
high snowfall totals in excess of 2 feet, with the most anomalous
snowfall totals expected across southern OR east of the Cascades as
highlighted by ECMWF EFI values of 0.95-1.0. A tight baroclinic
zone north of this moisture axis will also maintain lower elevation
snow with enhanced totals in terrain through this time.

72hr probs for >2' ending 00Z Thursday are 50-90% over terrain
from the Shasta/Siskiyou, Fremont Mtns in southern OR, the
Sawtooths, and Yellowstone through Wind Rivers. Beneficial precip
for this area, but major impacts to any passes in this area with
snow levels around 4000ft in the heavier snow. Snow levels are
5000-8000ft in the core of the AR moisture axis over northern
CA/NV/UT. However, on Monday afternoon height falls as troughing
expands over the NW low brings snow farther south into CA. Heavy
snow reaches the northern Sierra Nevada by Monday evening with a
slow progression of heavy snow down the length of the Sierra with
hourly rates exceeding 2"/hr into Wednesday. Day 2 and Day 3 probs
for >1' are 60-95% for the length of the Sierra Nevada with totals
over 3' likely in the High Sierra (5000-6000ft).

Cold continues over western WA/OR with occasional snow in the
Seattle/Portland metro areas through Tuesday night. A few to
several inches at a time are possible given the banding potential under/adjacent to the upper low just to the north. Total 72hr
probabilities for at least 2 inches are 20-40% for Seattle and
40-60% in Portland.

The moisture plume extending across the northern Great Basin and
into the Northern Rockies on Day 1 will gradually lift north on Day
2 and Day 3 as the tilting axis that brings snow to CA Monday
night also shifts snow across the northern Rockies through MT
into Wednesday. Day 1 snow probs for >8" are high (70-90%) over
the Sawtooths, Absarokas, and much of the greater Yellowstone
region of western WY, with 40-60% probs over all the southern and
southwestern MT ranges. Low probs (10-30%) for at least 8" extend
east into the adjacent northern High Plains of southeastern MT. By
Day 2 these heavy snow probabilities shift north into the
Bitterroot Range and all of the northern ID and western MT ranges.
For Day 2 and Day 3 the highest WPC probabilities for at least 8"
remains in the Sawtooths, but with 40-60% probabilities into
northern ID and western MT.


...Northern Plains, Great Lakes, Northeast...
Days 1-2...

Zonal flow sends two shortwave troughs east over the northern tier
through Monday night followed by a third over the Northern Plains
on Wednesday.

Shortwave trough axis over the Great Lakes speeds across New
England tonight with light to locally moderate snow spreading
across the Interior Northeast and New England before sunrise on
Monday. Local terrain enhancements over the Tug Hill/southern Adirondacks/Berkshires/Greens/Whites and coastal Maine where Day 1
snow probs for >4" are 20-40%. Elsewhere, amounts are generally
expected to remain in the 1-3" range throughout New England.
Additionally, some light icing is possible across central/southern
PA, but with amounts expected to remain below 0.1".

The next shortwave trough entering the northern Plains this
afternoon will trigger east-west oriented snow bands forming in
the left exit region of a 130kt+ jet streak stretching across the
northern Plains, Upper Midwest and Great Lakes through Monday
afternoon. This will lead to a narrow axis of moderate snow along
the SD/ND border through central MN, northern WI, and the northern
MI Lower Peninsula, where Day 1 snow probs for >4" are 30-60%.
These bands slide mainly north of and weaken near northern New
England as precipitation crosses the region Monday night along a
cold front. However, upslope and some lake enhancement off Lake
Ontario lead to Day 1-2 probabilities for at least 4" of 30-60% for
the northern Adirondacks, northern VT/NH, and northern ME.

By Day 3 another short wave trough exits east of the Northern
Rockies and spreads another round of snow into the northern
Plains. For now probabilities of at least 4" are low (10-30%)
across northern MT and northern ND with the expectation that this
snowfall would expand eastward across the Upper Midwest Wednesday
night.


...Midwest to Mid-Atlantic...
Day 3...

By the end of Day 3 (Wednesday) a couple of subtle short waves
within a weak trough traversing the Midwest will help blossom a
precipitation shield to the north of a stalled stationary front
expected to drape from the southern Mid-Atlantic to the middle
Mississippi Valley. With surface high pressure reaching across the
Great Lakes and more prominent over the Northeast, while above
average 500mb height anomalies span the region, icing is
anticipated to the main winter weather hazard between the Midwest
and Mid-Atlantic through Wednesday night. WPC probabilities for at
least 0.1" are currently low (10-30%) from northern MO and
southeast IA across northern IL and IN. Additionally, low
probabilities show up across the Allegheny Mts of western MD and
WV, with these probabilities expected to increase and expand
throughout the Mid-Atlantic once the forecast period includes
Thursday.


Snell/Jackson





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FOUS11 KWBC 030807
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
307 AM EST Mon Feb 3 2025

Valid 12Z Mon Feb 03 2025 - 12Z Thu Feb 06 2025


...Western U.S. across the Northern Rockies...
Days 1-3...

...Atmospheric River (AR) and upper low maintain prolonged heavy
snow from Oregon/California border and Sierra Nevada to the
Northern Rockies through Wednesday...

Current WV satellite imagery pinpoints our two distinct features
that have been controlling the ambient weather pattern across the
Western half of the CONUS. The first is an analyzed ULL centered
over Vancouver island that has provided significantly lower heights
across the PAC Northwest. The steady onshore component of the mean
flow has provided waves of small mid-level perturbations to meander
onshore with light precip in-of the Cascades of WA/OR. The main
player in the ULL positioning is the aid it has provided for our
second "player" within atmospheric schema; a progression of
shortwave troughs moving ashore within a west to east aligned
stationary boundary bisecting NorCal through the Northern Sierra,
extending inland to as far east as the Absaroka and Wind River
Ranges in western WY. Each shortwave pulse has aided in increased
IVT advection with a highly anomalous (+5 to +8 deviations via
NAEFS) transport of PAC moisture into the confines of the above
locations and everywhere in-between (Northern NV thru ID). This
direct moisture source has provided a steady ground for moderate to
heavy snowfall within the higher terrain confined over the above
locations with the upslope component within the Shasta/Siskiyou
areas aiding in heavy snow totals >12" above 6500ft.

This pattern will maintain general continuity given the prolonged
structure of the AR wave train with some adjustment in the aligned
flow likely to occur later this afternoon. A more amplified
shortwave trough coupled with a southwestern retrograde of the ULL
off the BC coast will provide a slightly more meridional component
to the flow with a greater southwest-northeast alignment shifting heavier precip further south compared to the past 24-36 hrs. This will
generate a better signal for heavy snow within the Northern Sierra
with areas along I-80 getting into the greater potential for
significant snowfall Monday night and beyond. The pattern evolution
will manifest itself for a period of 24-36 hrs before the primary
shortwave trough currently centered over the Pacific finally shunts
eastward with a landfall timing centered around Tuesday night,
kicking out of the area later Wednesday morning. This will lead to
significant snowfall across the Northern and Central Sierra on
Tuesday through that Wednesday morning time frame totals >12"
highly likely within the latest snowfall probability fields for D1
and D2 (50-80%).

Further inland, the pattern progression will provide some changes
in the overall time frames of impact as the realignment of the mean
flow will allow for the heavier snowfall encompassing portions of
ID through western WY to scale back in intensity with more
lingering light to moderate snowfall anticipated late Monday night
through much of Tuesday. Current snowfall totals across the
Absaroka and Wind River Ranges have been pretty significant since
the beginning of the pattern evolution with the inland extension
of the AR so any additional accumulation will still provide
impacts, but will at least curb the heaviest precip threat to
points further northwest. Conditions will deteriorate over Central
ID into the Northwest Rockies in MT thanks to the flow alignment
and shortwave perturbations advecting overhead. Probs for >8" over
those areas will rise to 70-90% during the D2 time frame, a
considerable adjustment compared to the previous periods.

The aforementioned shortwave over the Pacific will move inland of
CA with sights downstream over the Northern Rockies during the D3
time frame leading to a secondary pulse of heavier snowfall back
across the interior with an emphasis on the Absaroka and Wind River
Ranges once again. Associated upper-level speed max with the
disturbance will nose inland with the Northern Rockies situated
within the LER of a 125kt jet streak providing ample support for a
better defined QPF maxima that will situate itself over the above
areas. High probs are forecast for >8" (40-70%) on D3 likely
bringing totals over the course of 5-days to 3 to 6' thanks to the
onslaught of heavier precip and broad upslope flow within the
interior ranges of the west.


...Great Lakes, Upper Midwest, & Northeast...

Day 1...

Persistent zonal flow regime will continue through D1 with an
analyzed speed max over the Northern Plains this morning pushing
eastward within the west to east aligned flow. Nose of the 25H jet
will push through the Upper Midwest, eventually nosing into the
northern Lower Peninsula of MI with a swath of snowfall breaking
out just before the beginning of the period. As the jet core moves
overhead, increasing ascent under the influence of the jet will
generate a period of moderate to heavy snowfall as regional omega
improves significantly within the 850-600mb layer as progged by
recent bufr soundings in the zone of interest. A solid 3500' of
favorable DGZ depth provides a sufficient layer for better crystal
growth that will promote a better defined dendrite scheme during
peak snowfall later this morning and afternoon before the setup
wanes. The progressive nature of the event will limit the max
potential below double-digits, however west-east aligned banding
signatures within recent CAMs signal areas that could benefit from
prolonged longitudinal banding with the storm motion also focused
on a similar west to east alignment. Areas that can situate within
persistent banding will likely experience rates ~1"/hr for several
hours before the event fades as primary forcing shifts eastward
into Ontario and eventually the Northeast U.S. Snowfall probs for

4" are forecast between 40-70% across the Door Peninsula of WI,

extending east through the northern half of the Lower MI peninsula
with a maxima focused downwind of Grand Traverse Bay, including the
town of Gaylord.

Further downstream over Upstate New York and New England, the same
jet presence upstream over the Great Lakes will shift focus into
the interior Northeast with increasing ascent generally aligned
over areas downwind of Lake Ontario within the Tug Hill and
Adirondacks, eventually encompassing Northern New England with the
greatest impacts in-of the Green and White Mountains of VT/NH. The
strongest signal for heavy snow is within the Adirondacks and
adjacent North Country with probs for >4" upwards of 70-90% within
those zones with 40-70% extending back towards the Tug Hill and
points west near Pulaski and north along I-81. Snow levels across
New England will be sufficient for snowfall above 1000', however
greater impacts will be mainly relegated to zones above 2000' with
a strong gradient in the snow probability fields for >4" with <20%
in those lower elevations with a solid 50-80% within the highest
elevations of the Green and White Mountains.

Light ice accumulations will be found across areas of Northern PA
and Western NY within a shallow moist layer present south of the
main axis of lift, especially in the post-frontal regime that will
transpire as the disturbance exits through Northern New England.
Totals will be solidly <0.1" with a majority of the ice accretion
likely relegated to a glaze to perhaps 0.01-0.05" at max.

Day 3...

Secondary disturbance(s) originating upstream across the Northern
and Central Rockies will eject eastward within a very progressive
upper flow evolution. Northern wave over WY/MT will progress east-
northeast across the Northern Plains with snowfall breaking out
downstream within a increasingly diffluent mid-level pattern.
Light to moderate snow will transpire over Dakotas with the
greatest impactful snowfall likely to occur over the Upper Midwest
in MN, especially as the disturbance moves into the Arrowhead by
the end of D3. Modest probabilities of >4" exist within northern MN
and the Arrowhead with a swath of 40-60% located over Duluth and
points northeast, as well as the northern most reaches of MN with
the highest probs long the Canadian border.


...Midwest to Mid-Atlantic...
Day 3...

Second disturbance referenced in the section above will motion more
west to east across the Plains into the Ohio Valley and Mid
Atlantic by the second half of D3. High pressure over the Great
Lakes will shift eastward into the Northeast U.S by the beginning
of D3 with cold air funneling south into the Mid Atlantic with
guidance signaling a classic CAD wedge pattern initiating east of
the Appalachian front. Limited blocking downstream over the North
Atlantic will preclude any chance for the high pressure to hang on
across the region with the continued eastward progression of the
surface ridge eventually shifting off the Atlantic seaboard.
Despite its positioning, time frame of interest for precipitation
across the Ohio Valley and Northeast will be occurring after peak
diurnal heating with wet bulb temperatures into the upper 20s and
low 30s by the time precipitation breaks out across areas of IN/OH
over into the Central Mid-Atlantic. Primary concern with the
pattern is the low to mid-level WAA regime as the surface
reflection encroaches from the west leading to a mixed ptype likely
correlating to more icy (IP/ZR) hydrometeors compared to plain
snow that we will see for points well to the north. Recent trends
within deterministic have been for colder wedge to hang on more
thoroughly for places east of the Apps with the highest ice probs
located in- of the Laurel Highlands due to elevation factors
leaning colder at precip onset, as well as a secondary ice maxima
over the northern Ohio Valley where cold air will be able to hang
on through much of the event given stronger WAA pattern displaced
further downstream into the Mid-Atlantic comparatively.

Ice probs of >0.1" are very high (80+%) across west-central PA
with the focus in-of the Laurel Highlands where even some modest
20-40% probs for >0.25" of ice accretion are forecast as of the
latest WWD progs. Northern Ohio Valley doesn't carry the
probabilistic maxima for >0.25" like the mountains to the east,
however >0.1" ice accretion is sufficiently within the 40-70%
range, very much a non-trivial depiction that will have to be
monitored closely. Lower probs exist within the Central Mid
Atlantic east of the Blue Ridge, however a similar prob of 40-70%
for >0.1" of ice exists for locations northwest of the fall line
across MD extending into southern PA with lower, non-zero probs
further east into the I-95 corridor and adjacent lowlands. Setup is
delicate with multiple variables involved including depth of cold
air, strength of LLJ providing WAA pattern, as well as timing of
precip onset. These factors will determine the magnitude of ice
potential in-of the Mid-Atlantic and Ohio Valley with guidance
starting to hone in on specifics as we move closer to the event.

Kleebauer


$$

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FOUS11 KWBC 031954
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
254 PM EST Mon Feb 3 2025

Valid 00Z Tue Feb 04 2025 - 00Z Fri Feb 07 2025

...Western U.S. across the Northern Rockies...=20
Days 1-3...

...Atmospheric River (AR) and upper low to sustain prolonged heavy
snow from Oregon/California border and Sierra Nevada to the=20
Northern Plains through Wednesday...

A positively-tilted upper-low off the Pacific NW coast trapped=20
beneath a strong upper ridge over Alaska is responsible for an=20
active jet stream pattern that is resulting in a highly active
storm track to open the month of February. The 200mb jet stream
pattern today sets the diffluent right-entrance region over
interior Northwest with a strong 1040mb+ high located over
southwest Canada (NAEFS shows the dome of high pressure tops the
97.5 climatological percentile over North Dakota by Tuesday AM).
This expansive area of high pressure will work to lock a cold
Canadian air-mass in place along and east of the Continental
Divide, while also causing easterly upslope flow to enhance
precipitation rates along the leeward-facing slopes of the Northern
Rockies. In tandem with the busy Pacific jet stream pattern, which
will deliver wave after wave of IVT surges into the western U.S.,
periods of heavy mountain snow will common from the West Coast=20
ranges (Sierra Nevada, OR/CA coastal ranges) on east to the=20
Sawtooth, Bear River, Bitterroot, Teton, Absaroka, Wind River, and=20
Lewis Ranges.

In terms of the time line, a stream of precipitation associated=20
with low pressure in the Great Basin is tied to period of mountain
snow this afternoon from the northern Sierra Nevada on north and=20
east to the Northern Plains. By Tuesday morning, the next wave of=20
low pressure approaches the CA coast with IVT values topping the=20
99.5 climatological percentile over the Sierra Nevada by Tuesday=20
afternoon. Strong WAA will keep the heaviest snowfall in the=20
Trinity/Shasta and northern Sierra NEvada to elevations >6,000ft,=20
while in the central and southern Sierra Nevada, snowfall will be=20
heaviest at elevations >7,000ft Tuesday afternoon and through=20
Tuesday evening. This same stream of moisture will keep heavy snow=20
in the forecast in parts of southeast OR, the Blue Mountains, and=20
both the Sawtooth and Bitterroots through Tuesday night. By=20
Wednesday morning, heavy snow will be common in the Teton and Wind
River Ranges. A brief reprieve in the heavy snowfall (sans some=20
ongoing upslope flow into the Cascades, Olympics, and coastal=20
ranges along the Pacific NW coast) will arrive Wednesday afternoon=20
and linger into Wednesday night, but the next Pacific storm system=20
arrives on Thursday with colder 700mb temps over northern CA this=20
time around per NAEFS. This should allow for heavier snowfall to=20
occur at lower elevations throughout the northern California=20
(excluding elevations <3,000ft) with some light snow accumulations=20
as low as 1,000ft western OR.

Through Thursday afternoon, WPC probabilities paint a very wintry
picture for much of the northwestern U.S.. Snow will be measured in
feet from the Sierra Nevada >7,000ft and the Trinity/Shasta/Salmon
mountains of northern CA >5,000ft through the Sawtooth, Bitterroots,
Absaroka, Tetons, and Wind River Ranges. The Sierra Nevada,=20
Sawtooth, and Tetons in particular could are forecast to see as=20
much as 3-5 feet of snowfall through Thursday. For western OR/WA,=20
parts of the Cascades, Olympics, and coastal ranges could see up to
a foot of snow with even light accumulations possible along the=20
I-5 corridor from the Willamette Valley on north through the=20
Seattle metro area. In fact, WPC probabilities through Tuesday=20
afternoon show low chances (10-30%) for snowfall >2" from the=20
Olympia on south to just north of the Columbia River. In the=20
Northern Plains, WPC probabilities show high chances (70-90%) for=20

4" of snowfall for areas east of the Lewis Range and surrounding=20

the Big Snowy/Little Belt mountains on east through northwest=20
North Dakota. In fact, these area event show low-to-moderate=20
chances (30-50%) for snowfall amounts >8". The WSSI shows the worst
impacts (Extreme Impacts mostly due to Snow Amount and Snow Load))
for the Sierra Nevada above 7,000ft. These areas are likely to=20
contend with dangerous to even impossible travel conditions, as=20
well as extensive closures and disruptions. Major Impacts=20
(considerable disruptions and dangerous travel conditions) are=20
highlighted in the Sawtooth, the Trinity/Shasta/Salmon of northern=20
CA and southern OR, and even some isolated areas of the OR coastal=20
ranges. Lastly, Moderate Impacts (hazardous driving conditions) are
most common in these areas lower elevated areas (still generally=20
above 2,000ft) from northern CA and the WA/OR coastal ranges=20
through southeast OR and into the northern Rockies.=20

Mullinax


...Northeast and Upper Great Lakes...=20

Day 1...

Persistent zonal flow regime will continue through D1 with an
analyzed speed max over the Great Lakes this afternoon pushing=20
eastward toward New England within the west to east aligned flow.

Over Upstate New York and New England, this jet presence upstream=20
over the Great Lakes will shift focus into the interior Northeast=20
with increasing ascent generally aligned over areas downwind of=20
Lake Ontario within the Tug Hill and Adirondacks, eventually=20
encompassing Northern New England with the greatest impacts in-of=20
the Green and White Mountains of VT/NH. The strongest signal for=20
heavy snow is within the Adirondacks and adjacent North Country=20
with probs for >4" upwards of 70-90% within those zones. Snow=20
levels across New England will be sufficient for snowfall above=20
1000', however greater impacts will be mainly relegated to zones=20
above 2000' with a strong gradient in the snow probability fields=20
for >4" with <20% in those lower elevations with a solid 50-80%=20
within the highest elevations of the Green and White Mountains.=20


...Upper Midwest...
Days 2-3...

Disturbance originating upstream across the Northern and Central=20
Rockies will eject eastward by midweek within a very progressive=20
upper flow evolution. Northern wave over WY/MT will progress east-=20
northeast across the Northern Plains and Upper Midwest with=20
snowfall breaking out downstream within a increasingly diffluent=20
mid-level pattern. Light to moderate snow will transpire over=20
north-central MT, far northern ND, and into northern MN with the=20
greatest impactful snowfall likely to occur over the Arrowhead of=20
MN. Modest probabilities (30-50%) of >4" exist from north- central=20
MT on D2 and passes just north of the ND-Canada border before=20
expanding into the Arrowhead of MN.


...Midwest to Mid-Atlantic...
Days 2-3...

System referenced in the section above will motion more west to=20
east across the Plains into the Ohio Valley and Mid Atlantic by the
late-D2 into much of D3. High pressure over the Great Lakes will=20
shift eastward into the Northeast U.S by Wednesday night with cold
air funneling south into the Mid Atlantic and guidance signaling a
classic CAD wedge pattern initiating east of the Appalachian=20
front. Limited blocking downstream over the North Atlantic will=20
preclude any chance for the high pressure to hang on across the=20
region with the continued eastward progression of the surface ridge
eventually shifting off the Atlantic seaboard. Despite its=20
positioning, time frame of interest for precipitation across the=20
Ohio Valley and Northeast will be occurring after peak diurnal=20
heating with wet bulb temperatures into the upper 20s and low 30s=20
by the time precipitation breaks out across areas of IN/OH over=20
into the Central Mid-Atlantic. Primary concern with the pattern is=20
the low to mid-level WAA regime as the surface reflection=20
encroaches from the west leading to a mixed ptype likely=20
correlating to more icy (IP/ZR) hydrometeors compared to plain snow
that we will see for points well to the north. Recent trends=20
within deterministic have been for colder wedge to hang on more=20
thoroughly for places east of the Apps. This colder trend is=20
partly in response to greater confluence downstream across the Gulf
of Maine. The highest ice probs located in- of the Laurel=20
Highlands of PA, western MD, and the WV Panhandle due to elevation
factors leaning colder at precip onset, as well as a secondary but
weaker ice maxima over the northern Ohio Valley where cold air=20
will be able to hang on through much of the event given stronger=20
WAA pattern displaced further downstream into the Mid- Atlantic=20 comparatively.=20

Ice probs of >0.1" are very high (80+%) across west-central PA=20
with the focus in-of the Laurel Highlands where even some modest
30-60% probs for >0.25" of ice accretion are forecast as of the=20
latest WWD progs. Northern Ohio Valley doesn't carry the=20
probabilistic maxima for >0.25" like the mountains to the east,=20
however >0.1" ice accretion is sufficiently within the 50-80%=20
range, very much a non-trivial depiction that will have to be=20
monitored closely. Ice probabilities also exist within the Central
Mid Atlantic east of the Blue Ridge, however a similar prob of=20
50-70% for >0.1" of ice exists for locations northwest of the fall
line across MD extending into southern PA with lower, non-zero=20
probs further east into the I-95 corridor and adjacent lowlands.=20
Setup is delicate with multiple variables involved including depth=20
of cold air, strength of LLJ providing WAA pattern, as well as=20
timing of precip onset. These factors will determine the magnitude=20
of ice potential in-of the Mid-Atlantic and Ohio Valley with=20
guidance starting to hone in on specifics as we move closer to the=20
event. While there may be some snow at onset across the Mid-
Atlantic on Wednesday and throughout the Interior Northeast and New
England as WAA increases on D3, probabilities for >4" remains low
and confined to parts of VT and ME.

Given the relatively large geographic coverage of at least 0.10"=20
ice potential and the increasing chances for over 0.25" of ice=20
accretion in parts of WV, MD, and PA... Key Messages have been=20
initiated for this event.

Kleebauer/Snell


...Winter Storm Key Messages are in effect. Please see current=20
Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_1.png__;!!DZ3fjg!76BKJeK4WDZz8dhcSrSdtbYfnXFwGLJx2JenAkneZ5sUX= Rl-uSZVW7Vs20Evp249u0dna6xfOcz8r1HLrQDUphRKLNo$=20

$$

=3D =3D =3D
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FOUS11 KWBC 040859
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
359 AM EST Tue Feb 4 2025

Valid 12Z Tue Feb 04 2025 - 12Z Fri Feb 07 2025

...Western U.S. across the Northern Rockies...=20
Days 1-3...

...Atmospheric River (AR) and upper low maintain heavy snow from=20 Oregon/California border through the Northern Rockies and down the
Sierra Nevada through tonight. A separate low tracks through
California Thursday night bringing further heavy snow for the
Sierra Nevada...

A positively-tilted trough around an upper low off Vancouver Island
will continue to drift southwest off Pacific NW beneath a strong=20
upper ridge over Alaska through Wednesday before weakening in place
through Thursday. A zonal jet streak exceeding 130kt punches
through north-central CA today with the increasingly diffluent=20
left exit region over northern CA/NV today and across the=20
northern Great Basin/Rockies tonight. This will maintain the swath
of heavy snow over far northern CA into southern OR and northeast
through the Bitterroots. Day 1 snow probs for additional >18" are
40-70% for the Klamath/Siskiyou, Sawtooth/Salmon River and
Bitterroots with 70-90% probs for the Sierra Nevada above 6500ft.=20=20
A strong 1042mb high centered over the Canadian Prairies will=20
maintain the cold air on easterly upslope flow to enhance snow=20
rates along the leeward slopes of the Montana Rockies where Day 1
probs for >4" are 30-60%.

Low pressure ahead of the jet streak reaches the CA coast this
afternoon and northern NV tonight, bringing heavy precip to the
Sierra Nevada. Strong WAA with the low will keep snow levels=20
6000-7000ft. Heavy snow crosses the Teton and Wind River Ranges=20
late tonight into Wednesday.=20
Onshore flow will cause moderate snow over the Cascades and=20
coastal ranges Wednesday (where Day 2 snow probs for >8" are=20
50-80%) while the rest the Northwest enjoys a reprieve in snow.=20
However, the next Pacific storm system arrives Thursday with lower
heights and snow levels 3000-5000ft over northern CA Heavy snow=20
with prolonged 1-2"/hr rates is expected along the length of the=20
Sierra Nevada Thursday afternoon through the night. Day 3 snow
probs for >18" are 50-80%.


...Upper Midwest/Great Lakes...=20
Days 2-3...

Reinforcing shortwave trough that crosses OR tonight on the nose=20
of a powerful zonal jet will track over the northern Great Plains
Wednesday night. Warm air advection ahead of the wave allows=20
snowfall to breakout Wednesday afternoon over central MN, expanding
over northern MN/WI through the evening before tracking over the
U.P. into Thursday. Day 2.5 snow probs for >4" are 20-50%, highest
over the MN Arrowhead.

Jackson


...Midwest to Mid-Atlantic...
Days 2-3...

Very progressive shortwave ejecting eastward off the Pacific will=20
make quick headway to the east crossing the Continental Divide by=20
Wednesday morning with sights downstream on the Ohio Valley and=20
Mid-Atlantic by the end of D2. Further downstream, high pressure=20
will usher eastward out of the Great Lakes with a cold front=20
progression through the Mid- Atlantic and Northeast, solidifying a=20 relatively formidable polar airmass with a classic CAD wedge=20
situated east of the Appalachian front by Wednesday morning.=20
Consensus of forecast soundings across the region signal a shallow,
yet firmly entrenched airmass by Wednesday afternoon with a modest
CAA regime ongoing until later that evening. Limited blocking=20
across the North Atlantic will allow the surface ridge to propagate
eastward fairly quickly leading to a shift to modest return flow=20
by the very end of the D2 period allowing for warmer air to=20
protrude the boundary layer from south to north. This is important=20
as the approach of the aforementioned trough will couple with the=20
surface high off the Atlantic seaboard creating a modifying low to=20
mid- level airmass as the two work in tandem.

There's a growing consensus within the 00z deterministic suite for
a bout of mixed precipitation developing downstream across the=20
Ohio Valley and Mid-Atlantic within the increasing mid-level=20
difluence pattern ahead of the mean trough. Smaller shortwave=20
perturbations will also eject out ahead of the trough axis which=20
could spawn some scattered light precip ahead of the main QPF axis=20
that will approach more during the overnight time frame on=20
Wednesday night into early Thursday morning. The initial ptype may=20
start as a very brief period of snow for places further northeast=20
away from the nose of the budding 85H LLJ (50-55kts) initiating=20
over the Shenandoah. Any snowfall will quickly transition over to=20
sleet and/or freezing rain/drizzle before the main swath of precip=20
makes its way through the Appalachian and Allegheny front into the=20
rest of the Mid- Atlantic.

=46rom here, the approach of the shortwave trough will create a=20 strengthening v-vector component of the wind field allowing for=20
enhanced meridional flow capable of advecting much warmer air=20
poleward within the 925-650mb layer as indicated via bufr soundings
from locations across the Mid- Atlantic. Further west over the=20
Appalachian front extending from northern WV up through the Laurel=20 Highlands, a very shallow yet stern surface cold air pattern will=20
lock in and become very difficult to erode despite the increasing=20
warm air depth being advected overhead. The main thermodynamic=20
process for warming will likely have to come from latent heat=20
release of ice accretion processes to slowly maneuver the=20
temperature to near freezing by the end of the D2 window when much=20
of the precipitation will be shutting off. Wet bulb temps in the=20
higher elevations and elevated valleys west of the Blue Ridge will=20
likely be within 24-27F creating a large gap for any latent heat=20
processes to effectively kick the shallow cold layer out of the=20
lowest portions of the PBL. This is one of the main reasons this=20
area is the focus for the most significant ice accumulation=20
forecasts thus far with agreement among much of the deterministic=20
suite and national blend. Areas east of the Blue Ridge will likely=20
see the surface pattern erode a bit faster, especially those east=20
of the fall line within the Piedmont. Places between the Blue Ridge
and the fall line will be slower to erode as is customary within=20
these synoptic scale evolutions, especially when you factor in the=20
fresh CAA pattern that transpired less than 24 hrs before.=20

Ice probs of >0.1" remain very high (80+%) across west-central PA=20
with the focus in-of the Laurel Highlands where now 50-70% probs=20
for >0.25" of ice accretion are now forecast as of the latest WWD=20
progs. Northern Ohio Valley doesn't carry the probabilistic maxima=20
for >0.25" like the mountains to the east, however >0.1" ice=20
accretion is sufficiently within the 50-80% range, very much a non-
trivial depiction that has been consistent within NWP outputs. Ice probabilities also exist within the Central Mid Atlantic east of=20
the Blue Ridge with a similar prob of 50-70% for >0.1" of ice=20
existing for locations northwest of the fall line across MD=20
extending into southern PA. Newest WWD progs have introduced small=20
areas in-of the higher terrain of north-central MD within the Parrs
Ridge and Catoctin Mtn domains. These areas also have=20
probabilities for >0.25" now upwards of 30-40% for the event,=20
something we'll have to monitor closely as we move closer to the=20
event. Lower non-zero probs (20- 50%) exist for totals >0.1"=20
further east into the I-95 corridor and adjacent lowlands. Setup is
delicate with multiple variables involved including depth of cold=20
air, strength of LLJ providing WAA pattern, as well as timing of=20
precip onset. These factors will determine the magnitude of ice=20
potential in-of the Mid-Atlantic and Ohio Valley with guidance=20
starting to hone in on specifics as we move closer to the event.=20
While there may be some snow at onset across the Mid-Atlantic on=20
Wednesday and throughout the Interior Northeast and New England as=20
WAA increases on D3, probabilities for >4" remains low (10-40%) and
confined to mainly higher elevations of NY, VT, NH and ME.

Given the relatively large geographic coverage of at least 0.10"=20
ice potential and the increasing chances for over 0.25" of ice=20
accretion in parts of WV, MD, and PA... Key Messages continue for=20
this event.

Kleebauer



...Winter Storm Key Messages are in effect. Please see current=20
Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_1.png__;!!DZ3fjg!55ZGO0kmXlJMSOLOdySuP5k5U9km6UPbTmZ4wOEja5ADf= _PR_mVVma02h8M9SV2dBtxRuD6lr4WJvAgzyiBLP_f_wnk$=20

$$

=3D =3D =3D
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FOUS11 KWBC 041927
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
227 PM EST Tue Feb 4 2025

Valid 00Z Wed Feb 05 2025 - 00Z Sat Feb 08 2025

...Pacific Coast to the Northern Rockies...=20
Days 1-3...

An active period of weather will bring repeating rounds of active
wintry weather to much of the west, as a series of Atmospheric
Rivers (ARs) and varying snow levels occur through the week.

The forecast period begins with an impressive closed low
positioned just west of the WA/B.C. coast. 500mb height anomalies
within this low are progged by NAEFS to fall below the lowest ever
recorded in the CFSR climatology offshore these states by Wednesday
morning, indicating the intensity of this feature. In response,
downstream ascent will be impressive through mid-level divergence
and an intensifying jet streak exceeding 140kts Wednesday pivoting
into central CA. The overlap of this jet streak with confluent
mid-level flow downstream of the closed low will surge moisture
into the West as an AR, with both GEFS and ECENS probabilities
indicating a high risk (>90% chance) for 500 kg/m/s IVT lifting
into northern CA. With impressive mid and upper level winds
driving this AR, spillover moisture could also be significant,
aided by a wave of low pressure shifting east towards the Northern
Rockies beneath a spoke of vorticity shedding from the main low.
This will help spread an axis of moderate to heavy snowfall from
central CA through the northern Great Basin and into the Northern
Rockies D1. WPC probabilities are high (>70%) for more than 6
inches along the Sierra, across much of the northern CA ranges
including the coastal range, across the Blue Mountains, much of the
Northern Rockies, and into the Wind Rivers/Tetons. As much as 4-6
feet of snow is possible in the Sierra, with 1-2 feet possible in
the higher terrain of the other ranges. Additionally, with snow
levels crashing to the surface, lowland snow and coastal range snow
will also be impactful in Oregon.

The wave of low pressure D1 will push a cold front eastward behind
it, which when combined with the approach of the upper low will
lower heights and steepen lapse rates considerably late D1 and into=20
D2. This will result in snow levels falling to less than 500 ft as=20
far south as central Oregon, which when combined with the steep=20
lapse rates (dry adiabatic through the mid-levels as reflected by=20
regional forecast soundings) and increasing synoptic ascent, could=20
result in heavy snow into the coastal ranges and even the lowlands=20
of WA/OR and northern CA. Available moisture with this second wave=20
is somewhat suppressed from the prior day, but should still support
rounds of moderate to heavy precipitation falling as snow. There=20
is a lot of uncertainty involving the snowfall amounts due to the=20
likelihood for more periodic rounds of snow rather than a=20
continuous plume of precipitation, but theta-e lapse rates AOB=20
0C/Km will support convective rates which could quickly and=20
efficiently cause snowfall accumulations. Despite the transient=20
nature of heavy snow showers, WPC probabilities across the lowlands
of OR and southern WA, as well as some of the lower elevations of=20
northern CA reach 50-70% for 2 inches, and around 30% for the=20
Portland, OR metro area. The coastal ranges will likely receive=20
significant snowfall D2 reflected by a 50-70% chance for 4+ inches. Additionally, the continued moisture fetch onshore will bring=20
heavy snow again into the OR Cascades and northern CA ranges where=20
WPC probabilities for 6+ inches are at least 70%.

Then, during D3 /00Z Fri - 00Z Sat/ the closed low offshore WA/OR
opens, but remains amplified in response to a maxima and=20
accompanying shortwave trough rotating around it, through the base=20
of the trough, and onshore northern CA once again. This will be=20
accompanied by a renewed surge of deep layer ascent through=20
secondary jet intensification and downstream height=20
falls/divergence, to push another swath of heavy precipitation=20
onshore. Much of this will be in the vicinity of a surface low=20
which will develop just downstream of this vorticity lobe, and this
low is progged to move progressively northeast through the Great=20
Basin, reaching the Central Rockies by the end of the forecast=20
period. Snow levels will rise south and east of this low, but as it
tracks along a stationary front, there will likely be continued=20
low snow levels to the north, with an axis of frontogenesis and=20
accompanying enhanced ascent into a deepening DGZ driving a swath=20
of heavy snowfall across the area. A weaker AR will accompany this=20
low, reflected by GEFS and ECENS probabilities for 250 kg/m/s IVT=20
peaking around 70%, but this will still be sufficient to produce=20
heavy snow, especially in regions of upslope or the strongest fgen.
The global models still feature quite a bit of variability in=20
placement and timing for this second AR and low, so confidence in=20
is somewhat modest this far out. However, WPC probabilities are
already high (above 70%) for 12+ inches in the Sierra D3,
especially above 4000 ft. Heavy snow exceeding 6 inches is also
likely (>70%) for the ranges of NW WY.



...Upper Midwest/Great Lakes...=20
Day 2...

The lead shortwave ejecting downstream of the closed low west of
WA/OR will spin east out of Montana, reaching the Northern Plains
by Thursday morning and then racing east within pinched/progressive
flow to the Great Lakes Thursday night. This feature will help
spawn a weak wave of low pressure moving from the Northern Plains,
with accompanying downstream WAA and isentropic ascent resulting in
an expanding area of precipitation falling as snow on Thursday. The
overlap of synoptic lift with this isentropic ascent could result
in axis of heavy snow, especially along the international border
with Canada in MN and into WI and the U.P. of MI where the DGZ
depth is greatest as reflected by moderate probabilities (50-70%)
for at least 50mb of DGZ depth. The system is progressive which
will limit overall accumulations, but heavy rates of 1"/hr at times
will still produce heavy snow exceeding 4" as reflected by WPC
probabilities of 30-70% in the Arrowhead of MN, as well as the
Western U.P. of MI and Bayfield Peninsula of WI.


...Midwest to Northeast...
Days 2-3...

A fast moving upper-level disturbance over the Central Plains
Wednesday AM will be the primary feature in providing upper-level=20
ascent from the Midwest Wednesday evening to the Northeast on
Thursday. The progressive 500mb shortwave trough will be located
beneath the divergent left-exit quadrant of a roaring 150kt 250mb
jet streak located over the Rockies. At lower-levels, an 850mb
ridge over the subtropical Atlantic will work in tandem with
longwave 850mb troughing over the Northern Plains to create a 50kt
850mb jet over the Mississippi/Ohio River Valleys Wednesday night
that will act to deliver modest low-mid level moisture. NAEFS shows
IVT values by 06Z Thursday are topping 750 kg/m/s over the OH
Valley, which is above the 97.5 climatological percentile in the
CFSR climatology. In addition, 850-700mb WAA and 290K isentropic=20
glide amidst SWrly flow will lead to an gradual incline of the FGEN
axis over the OH Valley on east to the Mid- Atlantic Wednesday=20
evening. Last but not least in the overview of the key contributors
in this icy setup is the presence of a colder/dry air-mass that
will be anchored initially by a 1028+mb high that is above the 90th
percentile according to NAEFS. The high will be moving well
offshore by Thursday morning, but the linger cold-air damming
signature (CAD) will stick around for much of the event.

In these classic "overrunning a CAD wedge" setups, most guidance
tends to erode the <32F near-surface wet-bulb temperatures too
quick. Timing-wise, an icy wintry mix looks to get started in=20
northern IL around 00Z Thurs with some freezing drizzle, sleet, and
even light snow breaking out across the Ohio Valley and Mid-
Atlantic between 00-06Z Thurs. The heaviest FZRA occurs in the=20
Lower Great Lakes states between 03-09Z Thursday, then from the=20
central Appalachians to southeast PA and northern MD between=20
06-15Z. Sub- freezing surface temperatures will stick around=20
longest in the Laurel and Potomac Highlands, the Blue Ridge=20
Mountains, and into parts of the Shenandoah Valley where sub-=20
freezing air is harder to erode. By midday Thursday, a coastal low=20
begins to take shape off the Mid-Atlantic coast that would allow=20
for some Atlantic moisture to be advected into the Northeast=20
Thursday afternoon. The air-mass is more supportive of snow in=20
southern New England and as far north as the Adirondacks. While=20
most totals should generally be in the 2-4" range in these areas,=20
should the coastal low strengthen sooner, some mesoscale banding to
the north of the coastal low could occur over southern New=20
England.=20

Most guidance has come into better agreement on the timing
and amounts with a swath of >0.1" of ice accumulations from central
IL and across the Lower Great Lakes states to southeast PA. WPC
probabilities have focused on the Laurel and Potomac Highlands, as
well as parts of the Alleghenies and Blue Ridge as having the
better odds of seeing >0.25" ice accumulation. The Laurel Highland,
most notably, sport moderate chances (40-60%) for ice accumulations

0.25". Most ice accumulations that are observed will generally

fall below 0.25" but even amounts around 0.1" can result in
hazardous travel conditions. WPC probabilities show at least
moderate chance probabilities (>50%) for >0.1" of ice for northern
IN, northern OH, much of western PA (excluding areas south of
Pittsburgh), western VA, and along the Mason-Dixon line on east ot
I-83. There could be some portions of the western DC/Baltimore
suburbs that receive as much as 0.1" of ice as well for the
Thursday morning rush hour. The WSSI does depict Moderate Impact
potential (hazardous driving conditions and disruptions to daily
life such as closures/delays) in the DC/Baltimore metro areas,=20
particularly west of I-95 and north of I-66. Moderate Impacts are
also shown in the Laurel and Potomac Highlands, the Blue Ridge
Mountains, and the northern extent of the Shenandoah Valley. All
other areas previously mentioned from northern IL on east through
the Lower Great Lakes, central PA, and into New England are
forecast to see Minor Impacts from this event, highlighting the
potential for some slick roads and troublesome travel conditions.

Given the relatively large geographic coverage of at least 0.10"=20
ice potential and the increasing chances for over 0.25" of ice=20
accretion in parts of WV, MD, VA, and PA... Key Messages continue=20
for this event.

Weiss/Mullinax




...Winter Storm Key Messages are in effect. Please see current=20
Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_1.png__;!!DZ3fjg!_gWTm10JCuUOHHJgr_CLDVIFJZ6S5pfdlnOoqN6hW_00q= Vq5hpB-ASusc25nG7Hn1QGVuPX4vPtu85GAtg2w1tYI1zo$=20

$$

=3D =3D =3D
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FOUS11 KWBC 052136
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
436 PM EST Wed Feb 5 2025

Valid 00Z Thu Feb 06 2025 - 00Z Sun Feb 09 2025

...Pacific Coast through the Northern Rockies and Northern Plains...
Days 1-3...

...Active winter period across the west ends with a potent low=20
that tracks across California Thursday night, the northern Rockies=20
Friday, and the northern Plains Friday night...

The stormy pattern that has been ongoing across the West for
several days now will come to an end by this weekend. But until
then, periods of mountains and lowland snow will stick around the
Pacific Northwest this afternoon and evening thanks to an elongated
and positively tilted upper level trough over the northwest
Pacific. Farther south, a potent 500mb shortwave trough caught up
in the SWrly flow aloft will race towards California on Thursday.
NAEFS shows this system will have a healthy AR associated with it
and direct a >500 kg/m/s IVT (climatological percentiles topping
the 99th percentile) into the western U.S. Mandatory heights
associated with this trough are all below the 10th climatological
percentile as well, which should lead to a lowering of the snow
levels as it moves ashore on Friday. The heaviest snowfall in the
central and southern Sierra Nevada will generally reside above
6,000ft, while the northern Sierra Nevada and the
Shasta/Salmon/Siskiyou see heavy snowfall as low as 4,000ft in=20
some cases.=20

As the storm moves inland Friday morning, a cold Canadian air-mass
anchored by a 1030+mb high over the northern High Plains will be in
place, allowing for snow to the remain the primary precip type from
eastern Oregon and the Blue Mountains through the Sawtooth,
Bitterroots, Absaroka, Tetons, and Wind River Ranges. Snowfall=20
will be heavy in these ranges, including ranges farther south such=20
as the Bear River and Uinta ranges thanks to a highly anomalous IVT

300 kg/m/s (>99.5 climatological percentile at 18Z Fri) and these

ranges placement beneath the left-exit region of a 90kt 500mb jet=20
streak. Snow will push east through central and eastern Montana=20
Friday afternoon, then into the Dakotas by Friday night. Snowfall=20
rates will increase along the ND/SD border Friday night and into=20
Saturday morning as 850-700mb frontogenesis (FGEN) ensues and WAA=20
aloft supports heavy snow.

WPC probabilities through Saturday morning show high chance
probabilities (>70%) for snowfall totals >24" above 7,000ft in the
central and southern Sierra Nevada, and above 6,000ft in the
northern Sierra Nevada and Shasta ranges. Similar high chance
probabilities for >12" of snowfall are present in the Siskiyou and
Salmon mountains, as well as the Tetons, Bear River, and Wind=20
River Ranges. The Blue, Sawtooth, and Absaroka above 6,000ft are
also forecast to receive as much as a foot of snow, while the High
Plains of central and eastern Montana have moderate-to-high chances
(50-70%) for snowfall totals >4". The WSSI shows Major Impacts
(dangerous to impossible travel, widespread closures and
disruptions) along the spine of the Sierra Nevada, in the Siskiyou,
Salmon, Shasta, and southwest OR coastal ranges. There are
expansive areas of Moderate Impacts (hazardous driving conditions)
from the northern OR coastal ranges and along the OR Cascades on
east through most of the aforementioned Northern Rockies Ranges.
Minor Impacts are also being depicted at the moment in central and
eastern Montana for late Friday into Saturday.


...Northern Plains through the Great Lakes and Ohio Valley...=20
Days 1-3...

Potent shortwave trough (700mb heights falling below the 1st
percentile of the CFSR climatology according to NAEFS) will advect=20
across the Dakotas and into Minnesota Thursday and Thursday night=20
before racing east and out of the area by Friday morning. Rapid=20
height falls and impressive PVA downstream of a strung out lobe of=20
vorticity rounding the base of this trough will combine with the=20
LFQ of a modestly poleward arcing jet streak to produce large scale
ascent into the region. At the same time, moisture will increase=20
as modestly veered flow leads to robust moist isentropic ascent at=20
285-290K which will result in an expanding area of precipitation.=20
This precip will occur entirely as snow from ND through MI as the=20
column is quite cold, and the WAA accompanying this isentropic=20
ascent will help deepen the DGZ impressively (SREF probabilities=20
for 100mb of DGZ depth 30-70%, highest along the Canadian border).=20
This will result in high SLR snow accumulating rapidly as snowfall=20
rates may exceed 1"/hr at times (30-50% chance according to the=20
HREF). The system is progressive, limiting total accumulations, but
WPC probabilities indicate a 50-70% chance of at least 4 inches in
NE MN, including the Arrowhead, and into the western U.P. of MI.

Farther downstream and to the east, the accompanying WAA will
overrun a retreating high pressure to the east, leading to an axis
of moderate mixed precipitation from the Corn Belt into the Ohio
Valley. The WAA will lift rapidly northward, but will drive a brief
period of robust fgen, in the 925-700mb region, leading to a period
of moderate to heavy sleet/freezing rain before precipitation
changes over to rain. WPC probabilities D1 are as high as 50-70%
for 0.1" of ice, highest across north/central Ohio.

After a brief respite on D2, the progressive flow across much of
the CONUS will surge another winter storm system eastward into the
Northern Plains to begin D3. This system is progged to be much more
impressive as the potent driving trough ejects from the Pacific
Northwest and into the Northern Plains by Saturday morning. The
overall mid-level pattern across the Northern Plains is progged to
remain generally flat, but rapid height falls and a potent
subtropical jet streak reaching 150kts will help push IVT exceeding the
90th percentile from NAEFS into the region. At the same time,
strengthening WAA downstream of a low pressure moving across the
Central Plains will draw higher moisture northward, and the overlap
of this WAA with the diffluent portion of the upper jet streak will
produce a stripe of impressive 850-600mb fgen, efficiently lifting
ascent into a deepening DGZ (SREF probabilities for 100mb of depth
exceeding 30%). This will result in a swath of snow from the
Dakotas through Wisconsin, with an embedded band of heavier snow
likely. There is still quite a bit of longitudinal (timing) and
latitudinal (placement) uncertainty amongst the global members, but
ensembles suggest high confidence for this event and impactful snow
is likely across the Dakotas and into Minnesota/Wisconsin. WPC
probabilities have increased dramatically today, and now support a
high risk (>70%) for more than 6 inches D3 near Aberdeen, SD,
surrounded by a larger expanse of >50% from far western SD through
western WI.

Then farther to the southeast, the WAA will lift a warm nose more
robustly immediately downstream of the surface low into the Upper
Midwest and Ohio Valley. This will cause a stripe of mixed
precipitation to develop before the end of the period from Illinois
through eastern Ohio, and WPC probabilities increase once again,
now featuring a 30-50% chance for at least 0.01" from near
Davenport, IA through Cleveland, OH.


...Mid-Atlantic and Northeast...=20
Days 1-3...

Generally flat and fast flow within the mid-levels will dominate=20
the eastern half of the CONUS, keeping systems progressive into the
weekend. This will allow for a series of winter storms to impact
the region.

The first of these will develop tonight as a modest shortwave
trough ejects from the Central Plains and lifts E/NE across the
Ohio Valley, reaching New England Thursday evening. A weak jet
streak will accompany this feature as it moves east, producing some
subtly enhanced deep layer lift, but most of the ascent will be
driven instead by impressive 850mb warm advection, which will
intensify especially over New England Thursday afternoon. This WAA
overrunning a retreating high pressure will drive intensifying
isentropic ascent, especially within the 290-300K layer, and
moisture will be copious as reflected by mixing ratios exceeding
6g/kg. This will allow PWs surging northward from the Gulf of
Mexico to exceed the 90th, and in some places the 97th percentile
of the CFSR climatology according to NAEFS ensemble tables. At the
same time, a wave of low pressure will develop offshore and move up
the coast from NC to New England on Thursday, additional enhancing
ascent across the region.

As WAA/isentropic ascent increases, precipitation will overspread
the area from SW to NE, with a mixture of rain, snow, freezing
rain, and sleet expected. The p-type challenges are significant
with this event.=20

Across the Mid-Atlantic states, into Upstate NY, and as far north
as southern New England, precipitation will feature a mix of all
p-types, and sorting out the predominant is challenging. The column
initially starts cold enough that everywhere north of the Mason-
Dixon line could start as a burst of snow within the WAA, with
sleet the initial p-type farther south. What happens thereafter
makes the difference with respect to impacts. Although the high
retreats quickly, a wedge of high pressure will remain embedded
south into the Mid-Atlantic, and initially surface wet-bulb
temperatures are well below 0C due to large T-Td depressions. This
suggests that as precip begins, the wedge will be reinforced and
hold firmer than guidance indicates, despite the overwhelming=20
influence of the WAA. The high-res models have warmed a little bit=20
today compared to previous runs, but the initial cold depth is=20
still well above the 75th% percentile for freezing rain for parts=20
of VA/MD/PA, suggesting a long period of sleet before WAA wins out=20
turning precip over to ZR. Of course, farther north into PA and=20
Upstate NY may experience more IP than ZR, while the higher terrain
of WV/MD/PA (Appalachians, Alleghenies, and Laurel Highlands) have
the best chance for damaging freezing rain accretions due to being
above the cold layer. Overall, the system is progressive, but=20
significant QPF falling into the cold layer will produce=20
significant icing for the Mid-Atlantic with hazardous commutes=20
likely Thursday morning. WPC probabilities are moderate (30-50%)
for 0.25" in the central Appalachians, surrounded by a much larger
area of at least 50% for 0.01" or more across much of the Mid-
Atlantic and into southern New England. This event continues to be=20
the source of key messages linked below (Keymessage_1).

Farther north into Upstate NY and New England, much of the
precipitation is expected to fall as snow. The intensifying WAA=20
will result in a band of heavy snow as the resultant 850-700mb fgen
drives intense ascent into the DGZ just above. This will cause a
burst of snow lifting SW to NE Thursday, first in the Poconos, then
all points northeast, including New York City and Boston. Snowfall
rates within this burst could reach 1-2"/hr as reflected by the WPC
prototype snowband tool and HREF probabilities, but rapid=20
translation of this band northeast will somewhat limit total=20
accumulations. Still, substantial impacts are likely, and WPC
probabilities are high (>70%) for more than 4" of snow in the
higher terrain from the Adirondacks into the Greens and Whites,
with lesser accumulations likely in the lower elevations.

As this first low pulls away into Canada, a second wave of low
pressure moving across Ontario will drive a potent cold front
eastward from the Great Lakes and through New England Friday
morning/aftn. This cold front will have a two-pronged effect on
winter weather. First, the passage of the cold front may incite
some convective snow showers or snow squalls from Upstate NY
through Maine as low-level fgen and some modest 0-2km instability
combine in an area of elevated RH. This is reflected by the SnSq
parameter exceeding 1 Friday morning in this area. While
accumulations within any squalls will be limited, brief intense
snowfall rates could cause hazardous travel.=20

Then behind this front, increasing CAA on W/NW flow will cause
modest lake effect snow (LES), especially downstream of Lake
Ontario where water temps are still marginally warm (+5) while Lake
Erie is completely ice covered at this point. With 850mb temps
falling to as low as -15C, this will steepen lapse rates
sufficiently to produce heavy snow into the Tug Hill and western
Adirondacks, although total accumulations are expected to be
somewhat modest as reflected by WPC probabilities that are 50-70%
for 4 inches D1 and D2.

Finally, as the active pattern continues, another shortwave trough
will dig out of the Central Plains Saturday, reaching the eastern
Great Lakes/Ohio Valley by the end of the forecast period. In a
pattern similar to that on Thursday, an accompanying surface wave
will develop across the Ohio Valley and then track progressively
eastward. The downstream WAA will again be impressive, and surge
another round of mixed precipitation into the Mid-Atlantic states.
There is still quite a bit of uncertainty as to both timing and
placement of the heaviest snow and icing amounts Saturday, but
current WPC probabilities indicate a moderate risk (30-50%) for at
least 0.01 inches from central VA through southern PA.


Weiss/Mullinax


...Winter Storm Key Messages are in effect. Please see current=20
Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_1.png__;!!DZ3fjg!5tPhRn2Zs7FAIu5VEjJyezLhh0L0FF0Y7B7p7Cjz_8zGx= rF-KHEmuJ7rmyzHAssvmGm33xyd99gfS9LjWbDw3jIVJcY$=20

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_2.png__;!!DZ3fjg!5tPhRn2Zs7FAIu5VEjJyezLhh0L0FF0Y7B7p7Cjz_8zGx= rF-KHEmuJ7rmyzHAssvmGm33xyd99gfS9LjWbDw9DeXYzM$=20

$$

=3D =3D =3D
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