FOUS30 KWBC 301701 CCA
QPFERD

Excessive Rainfall Discussion...CORRECTED
NWS Weather Prediction Center College Park MD
1201 PM EST Thu Jan 30 2025

Corrected for addition of Slight Risk for Iowa and vicinity.

Day 1 Valid 16Z Thu Jan 30 2025 - 12Z Fri Jan 31 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE ARKLATEX
INTO THE LOWER OHIO VALLEY AND FROM MISSOURI/IOWA INTO NORTHERN=20
ILLINOIS...

...1645Z update...

After coordination with WFO LOT, a targeted Slight Risk was added
from the Missouri/Iowa border into northern Illinois to account
for sensitive hydrologic conditions. The recent stretch of cold
temperatures during much of January has resulted in a deep frost
layer beneath the surface which will likely contribute to
additional runoff of rainfall than what would otherwise occur.
Warm advection rain followed by a developing deformation axis
later today into tonight should be accompanied by limited=20
rainfall rates (remaining mostly below 0.5 in/hr) given a lack of
instability but steady rain of near 1 inch is expected to fall in
a roughly 6 hour window which will likely result in some excess=20
runoff across portions of the Missouri/Iowa border into portions=20
of northern Illinois. While this outlook ends at 12Z Friday, the=20
event will be ongoing at that time with a continuation of runoff=20
concerns extending into the Day 2 ERO period beginning 12Z Friday
into northern Illinois.

Otto

...16Z update...

Adjustments for this update were minor and based on the latest
radar trends out of northeastern TX and the 12Z HREF suite of
guidance. The 12Z members of the HREF appear to be largely in line
with their 00Z counterparts with perhaps a subtle northward shift
with the highest QPF axis. In general, the HREF looks to be
handling the event well so far. Unidirectional southwesterly flow
will continue to allow for areas of training moderate to heavy
rainfall from the ArkLaTex into and across the lower/middle
Mississippi Valley through the day today. Instability is forecast=20
by a vast majority of the latest models to remain below ~500 J/kg=20
across much of the Slight Risk area which will limit hourly=20
rainfall rates to around an inch per hour, but repeating rounds of
0.25 to 0.75 in/hr will meet or exceed area flash flood guidance
values with areas of heavy rain translating eastward into the Ohio
Valley for early Friday morning.

Otto

...previous discussion follows...


Not much change to the inherited Slight risk stretching from
northeast TX, across central AR and into portions of TN and KY.
Both PWs and IVT are approaching climatological maximum values for
late January across this corridor, so a very dynamic system with
plenty of moisture to work with. Instability will be a limiting
factor, keeping rainfall rates and the overall flash flood
potential lower than it otherwise could be. The highest rates will
probably be early in the period over the southwestern portion of
the Slight risk (northeast TX into southwest AR), where hourly rain
could approach 1.5". Further downstream over AR into KY and TN
rainfall rates off the 06z HRRR and 00z HREF peak closer to
0.5-0.75"/hr.

Much of the Slight risk area is expected to see at least 2" of
areal averaged rainfall today into this evening, with the areas
from northeast AR into far western TN/KY having the highest
probabilities of 3"+ amounts. There is good overlap between the
higher 3" EAS probabilities from the HREF and the 06z HRRR max QPF axis...adding confidence in the potential for a 3-5" rainfall
swath from northeast AR into far western KY/TN. Rainfall rates here
will not be all that high to start out, with these higher totals
driven more by a persistent moderate rain. However by 00z guidance
indicates these areas could move into the warm sector and see more
robust convection with higher rate potential. If this occurs then a
greater coverage of flash flooding could evolve.

Stronger convection should develop over portions of southern AR
into MS by this evening. However this activity will be progressive
enough to limit the flash flood risk.

A Marginal risk was maintained in the developing comma head from
eastern KS into northern MO, southern IA and west central IL. Low
topped convection could locally increase rainfall rates, with a
broad 1-3" of rain forecast (highest probs of approaching 3" are in
far northeast KS into northwest MO).

Chenard


Day 2
Valid 12Z Fri Jan 31 2025 - 12Z Sat Feb 01 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
WEST VIRGINIA...

...West Virginia...
A Slight risk was issued for portions of central and eastern
WV...primarily for areas where significant snowpack still exists.
The latest NOHRSC analysis indicates upwards of 2-4" of snow water
equivalent over the terrain of eastern WV and the latest model
consensus is for 1-3" of rainfall over this area. With
temperatures and dewpoints forecast to at least get into the 40s
(maybe 50s in spots) the rain on snow should trigger a fairly
efficient melting of the snowpack. This is likely to result in
numerous instances of areal flooding and at least some river
flooding.

Rainfall rates are generally forecast to stay in the 0.25" to 0.5"
an hour range, which is probably not enough to generate flooding
of a more flash variety. However there are indications in both the
06z HRRR and 00z HREF that embedded convective elements Friday
could locally push hourly rainfall a bit over 0.5". Given the
amounts of runoff that will be occurring from the combined snow
melt and rain, any of these higher rates could locally result in a
flash flood threat as well.


...Northern Illinois and Indiana...
A Marginal risk was maintained across this area as comma head
rainfall drops up to 1.5" of rain in a 6hr period Friday. Typically
this degree of rain would not result in impacts, however frozen
ground will likely enhance runoff potential. Given the extra
runoff this will generate, amounts around 1.5" in 6 hrs could be
enough to trigger at least some flood concerns across this
corridor.


...West Coast...
An atmospheric river will bring heavy rainfall from western WA
southward into northern CA Friday into Friday night. The IVT plume
is generally progressive and weakening through the period, so not
expecting major rainfall totals or impacts. Generally forecasting a
1-3" rainfall (locally higher in the favored terrain), which should
mainly drive an isolated minor flood threat.

The IVT axis does start to stall as it shifts into northern CA, and
so could be looking at totals more in the 2-4" range across the
terrain here. Will need to monitor for a potential Slight risk
upgrade across portions of the northern CA coast, but for now will
stick with the Marginal. Leaning towards this day 2 rain mainly
helping saturate conditions leading towards a greater flood risk
on later days. However should things trend quicker and/or wetter
an upgrade may need to be considered.

Chenard


Day 3
Valid 12Z Sat Feb 01 2025 - 12Z Sun Feb 02 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
CALIFORNIA...

A pretty significant model change has been noted over the past
couple cycles across the eastern Pacific into the west coast. The
earlier model consensus was for a stalling of a moderate to strong
AR over northern CA and southwest OR resulting in significant
rainfall amounts. However recent model runs have unanimously
shifted further south with this AR, now stalling it more over
central to north central CA. There has also been a weakening trend
in the AR, so QPF values are not as high as earlier forecast.
However, with the southward shift the max rainfall axis is now over
areas that tend to be more sensitive to heavy rainfall and related
flood impacts. Thus even with the drop in QPF, think a southward
shift of the Slight risk is warranted. The Slight risk now
stretches from Mendocino county south into the San Francisco area
and as far south as Santa Cruz county. It also extends inland
across the Sacramento Valley and into the Sierra Nevada.

Rainfall along the coast will start on Friday and continue into
the day Saturday. A gradual saturating of soils are expected to
lead to potential greater impacts on Saturday, warranting the
Slight risk. Two day rainfall totals are currently forecast in the
2-4" range. Two day rainfall amounts of 1-3" are expected within
Sacramento valley locations, with upwards of 4-6" within the
Sierra Nevada (most of which will be rain given high snow levels).

Chenard


Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7OaPx0J68M44d-ArU2mleNiDjmo5d_i-AE5HwruQYIt2= YxX2nXYtwoKdhHYHnsJ9UEdgVY2x6Z5N_kEQ0bAtU7x0EcM$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7OaPx0J68M44d-ArU2mleNiDjmo5d_i-AE5HwruQYIt2= YxX2nXYtwoKdhHYHnsJ9UEdgVY2x6Z5N_kEQ0bAtTBM5nzo$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7OaPx0J68M44d-ArU2mleNiDjmo5d_i-AE5HwruQYIt2= YxX2nXYtwoKdhHYHnsJ9UEdgVY2x6Z5N_kEQ0bAtB6QMQfs$=20

=3D =3D =3D
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