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FOUS30 KWBC 291451
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
951 AM EST Mon Dec 29 2025

Day 1
Valid 16Z Mon Dec 29 2025 - 12Z Tue Dec 30 2025

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

Cook/Wegman

Day 2
Valid 12Z Tue Dec 30 2025 - 12Z Wed Dec 31 2025

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

Wegman

Day 3
Valid 12Z Wed Dec 31 2025 - 12Z Thu Jan 01 2026

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

A potent upper level shortwave trough will move east through the
day Wednesday and approach the southern California coast New Year's
night. As it approaches, the attendant surface low will tap into a
large plume of subtropical moisture that will advect north on the
southerly flow ahead of the trough. The trough itself will be
negatively tilted, supporting cyclogenesis. The surface low will
elongate thanks to the negatively tilted trough, allowing the
southerly flow to spread rainfall north along essentially the
entire California coast. However, with southerly flow, the east-
west oriented Transverse Ranges will be particularly well set up to
capture that moisture and wring it out in the form of upslope
rainfall, in much the same way as the Christmas rainfall event a
week ago.

The difference between the Christmas and New Year's rainfall events
in California will be the depth of moisture (less of it this time
around), and lack of instability this time. This combined with the
first part of the event having a southeasterly flow in the lower
levels should help to mitigate the total rainfall impacting the
L.A. Basin and the Transverse Ranges relative to the Christmas rainfall
event. Forecasted PWATs will be around 1.25 inches near the coast.
This is still 4 sigma above normal for this time of year, and
therefore still represents a highly unusual amount of available
moisture. The latest forecast has up to an inch of rain
overspreading much of the lower elevations of the L.A. Basin, while
1-3 inches of rain are forecast for the south- facing slopes of
the Transverse Ranges. Full reservoirs from the Christmas event,
some draining still ongoing due to wetter than normal soils for
this time of year, burn scars, and urban effects should all work to
worsen any flooding effects from the expected rainfall. Thus,
while forecasted rainfall amounts are lower, expect that there will
still be widely scattered instances of flash flooding, especially
in and around burn scars.

A Slight Risk upgrade was introduced with this update, in
coordination with LOX/Oxnard, CA forecast office. Fortunately,
unlike the Christmas rainfall event, for the aforementioned
reasons, do not expect to have additional upgrades in this area.
Short of some much larger forecast change that results in much
higher forecasted rainfall amounts into SoCal, a possibility with a
low probability of occurring, this should be the final risk
category upgrade. Nonetheless the forecast should continue to be
monitored for the latest updates.

Wegman


Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/94epoints.txt__;!!DZ3fjg!4Gr9D2QxpOCj1EAf-2uL1DtATfbyP3llOY6wCkG4G2Pk=
ugudNBfXZrq7ulUj5z5LBHCTC3TaiEwbq0YGujfUNijz3Ws$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/98epoints.txt__;!!DZ3fjg!4Gr9D2QxpOCj1EAf-2uL1DtATfbyP3llOY6wCkG4G2Pk=
ugudNBfXZrq7ulUj5z5LBHCTC3TaiEwbq0YGujfUFZgVCQ8$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/99epoints.txt__;!!DZ3fjg!4Gr9D2QxpOCj1EAf-2uL1DtATfbyP3llOY6wCkG4G2Pk=
ugudNBfXZrq7ulUj5z5LBHCTC3TaiEwbq0YGujfUTZJ6_xE$=20

=3D =3D =3D
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