FOUS30 KWBC 270206 CCA
QPFERD

Excessive Rainfall Discussion...CORRECTED
NWS Weather Prediction Center College Park MD
906 PM EST Fri Dec 26 2025

Day 1
Valid 01Z Sat Dec 27 2025 - 12Z Sat Dec 27 2025

Corrected for geographical typo in Day 1

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CALIFORNIA MAINLY FOR THIS EVENING...

0100Z Update...
With radar continuing to show decreasing coverage and intensity of
rainfall across California and decreasing signals from the HREF=20 probabilities for rainfall exceeding 1- and 3-inch amounts=20
exceeding flash flood guidance...have opted to downgrade from a=20
Slight Risk area to a Marginal Risk area. Reported rainfall rates=20
and those derived by radar have been decreasing during the late=20
afternoon with only isolated spots realizing 0.25 inches per 3=20
hours. Given how water-logged some places have become over the=20
past couple of days...any additional rainfall has the potential to
result in new flooding/run off concerns or at least prolong the=20
amount of time needed to drain off existing floodwater. On the=20
other hand...given the recent AEP values have been less than=20
impressive during the afternoon and the diminishing coverage of=20
rainfall rates and coverage of rainfall...removed the Slight risk=20
but kept the Marginal risk area where a moisture plume remained=20
with the expectation that loss of daytime heating will further aid
the reduction of rainfall rates and areal coverage.

Bann

1600Z Update...

The Marginal Risk area has been expanded north to include much of
northern CA, including the coastal ranges, Bay Area, and northern
portions of the Central Valley. The arrival of a fairly strong
500/700 mb low center just offshore will be driving areas of heavy
showers going through the afternoon hours. While this energy will
be generally weakening with time, the addition of at least modest
boundary layer heating along with some steepening of the mid-level
lapse rates should favor some convective elements capable of
producing localized rainfall rates of 0.50" to 0.75"/hour. Some
localized additional rainfall totals of 1 to 2 inches will be
possible, and especially across the northwest CA coastal range.
These additional rains may foster some additional localized
runoff concerns given the wet/saturated soil conditions and=20
elevated streamflows. No changes have been made to the Slight Risk=20
area farther south across southern CA as the threat of heavy rain=20
will continue here into at least the early evening hours.

Orrison


Previous discussion...

Few changes were needed for this morning's update. A positively-
tilted upper level trough will push ashore and inland across the
Pacific Northwest today. This will keep the flow at all levels
unidirectional out of the southwest across Southern California, on
the leading edge of a 140 kt jet. As the trough pushes east, less
and less of it will be over the Pacific, resulting in a diminishing
of the onshore flow with time. Waves of rainfall will push ashore
into southern California through the day. Lingering instability
will support rates that may be heavy at times. This is most likely
as the storms interact with the terrain of the eastern Transverse
Ranges, from L.A. north and east. It is in this area that the
rainfall forecast peaks at around 3 inches at the highest
elevations. Meanwhile most of the L.A. Basin picks up another 1-2
inches. 1-2 inches of rain would be problematic on a normal day,
but following multiple days of multiple inches of rain per day,
making the 1-2 inches expected today lighter by comparison...expect
scattered instances of flash flooding to redevelop again today.
While all areas of coastal Southern California and the Los Angeles
basin will pick up some rain, those highest totals will be north
and east of the downtown area. Thus, from downtown L.A. north and
east, a higher end Slight is in effect. The flooding impacts will
be the result of continued drainage of prior rainfall over
Christmas Eve and Day, as well as urban effects. A Moderate Risk
may need to be considered from Pasadena east through San Bernardino
if rainfall amounts exceed expected totals.

Elsewhere, the Slight remains in place down the coast to about San
Diego, as the eastward moving bands of rain impacting the L.A.
Basin should struggle to continue further south. Meanwhile the
Marginal for limited shower activity and/or lowering snow levels in
the foothills of the Sierra Nevada remains in place unchanged with
this update. For Central and Northern California, it's probable
that there will be more total precipitation in these parts of the
state compared to further south, but because of lowering snow
levels due to cold air influx with the aforementioned trough
moving in, much of it should fall as snow at the higher elevations,
reducing the flooding threat downstream.

By this evening, the flow will flip around to the northwest,
ushering in drier air and finally ending the rainfall threat across
Southern California likely for the rest of 2025.

Wegman

Day 2
Valid 12Z Sat Dec 27 2025 - 12Z Sun Dec 28 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Santorelli

Day 3
Valid 12Z Sun Dec 28 2025 - 12Z Mon Dec 29 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

An area of rainfall across much of the Midwest is expected with a
developing low moving into the Great Lakes. The combination of
prior dry soil conditions, rapid movement, poor instability, and
cold air following the rain should all work to keep this event
largely beneficial. Flooding concerns in this area at this time
are too low for even a marginal risk.

Santorelli/Wegman


Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8Xs7kvA94gMOKx_Qtm2QhI4P-4myh6917pS8E_A0k0QX= F3kCp1QvESYEnMuQr5YwVmAUgCWwNX5eYg-1GogyIFGPpZM$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8Xs7kvA94gMOKx_Qtm2QhI4P-4myh6917pS8E_A0k0QX= F3kCp1QvESYEnMuQr5YwVmAUgCWwNX5eYg-1GogyM5v2eCI$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8Xs7kvA94gMOKx_Qtm2QhI4P-4myh6917pS8E_A0k0QX= F3kCp1QvESYEnMuQr5YwVmAUgCWwNX5eYg-1Gogy9BPPAgY$=20

=3D =3D =3D
To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM.

--- SBBSecho 3.28-Linux
* Origin: Capitol City Online (1:2320/105)