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FOUS30 KWBC 301854
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
154 PM EST Tue Dec 30 2025

Day 1
Valid 16Z Tue Dec 30 2025 - 12Z Wed Dec 31 2025

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

Cook/Wegman

Day 2
Valid 12Z Wed Dec 31 2025 - 12Z Thu Jan 01 2026

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

...21Z Outlook Update...
The ongoing forecast is on track and no changes are needed. Areas
of 1-3 inches of rainfall are expected in the Transverse Ranges=20
during the 06-12Z Thursday period that should result in instances=20
of excessive runoff, mud/rock slides, and debris flows. USGS=20
analyses depict enhanced streamflows suggesting that the region=20
still hasn't recovered from prior rainfall and further enhancing=20
the flood/flash flood risk.

Additional details are in the previous discussion below.

Cook

...Previous Discussion...

A large upper level Pacific low will track east and approach the
coast of Southern California on New Year's Eve. The low will
gradually weaken with time, and remain under a ridge in the
polar jet, thus keeping the low cut off from the main jet stream.
The low will draw an unusually moist, tropical air mass northward
up the Mexican coast and into Southern California. The predominant
850 flow will be out of the southeast for much of the day
Wednesday, but will turn more out of the south-southwest by New
Year's Night. This pattern will keep rainfall into the Transverse
Ranges, and much of southern California for that matter, rather
light through the day. However, once the flow turns to become more
orthogonal to the coast Wednesday night, rainfall rates will
increase into the Transverse Ranges. The orthogonal turn of the
flow will correlate with the closest approach of both the upper
level trough and corresponding surface low. These will also enhance
rainfall rates into the mountains.

Hydrologically, while Southern California has had some time to
recover from the Christmas rainfall event, it's still too soon
after the event for full drainage to have occurred. Forecast
rainfall has increased in the past 24 hours, due in part to a
closer approach of the surface low to the coast relative to the
forecast last night. As such, a higher end Slight was issued for
much of the Transverse Ranges, where 2-4 inches of rain are
forecast through the period. Much of that rain will fall between
06Z and 12Z Thursday morning. Meanwhile, in the adjacent L.A.
Basin, 1-2 inches of rain are expected. Between burn scars and
urban concerns, this amount of rain should lead to scattered
instances of flash flooding. Thus, the Slight Risk inherited
remains in place. Further increases to the rainfall forecast may
require an additional upgrade to a Moderate Risk with future
updates.

Wegman

Day 3
Valid 12Z Thu Jan 01 2026 - 12Z Fri Jan 02 2026

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

...21Z Outlook Update...
The ongoing forecast philosophy is on track. Areas of 2-4 inch
rainfall amounts are expected during the first half of the forecast
period (12Z Thu through 00Z Fri) across the eastern Transverse
Ranges with slightly lesser amounts across the Los Angeles Basin
and Peninsular Ranges. Several instances of excessive runoff,
rock/mud slides, and flash flooding are expected.

See the prior discussion below for additional information.

Cook

...Previous Discussion...

A Slight Risk upgrade was introduced with this update, for many of
the same areas as the Day 2 Slight Risk. At the start of the period
New Year's morning, the brunt of the event will be well underway
across Southern California. Heavy rain will continue into the
morning hours as the surface low and upper level trough finally
move ashore during the day Thursday. Thus, the heaviest rains are
expected Thursday morning. Behind the low Thursday afternoon and
into the overnight, the rain should largely end for most, tapering
to a few showers here and there. Thus, this Slight Risk is really
for the first half of the day, and should be able to be dropped
after that. Nonetheless, an additional 1-2 inches of rain on top of
the rains from the overnight will continue to cause flooding
concerns across Southern California. So while amounts in total will
be less Thursday than on Wednesday, the continuation of Wednesday's
rain on into Thursday will keep the commensurate impacts ongoing as
well.

Further north, a Marginal Risk remains in place for portions of the
northern California coastal ranges and extending south along the
Sierra Nevada foothills. The rain in these regions Thursday and
Thursday night will be much more evenly spread in time. However,
these areas are much more equipped to handle the expected rain,
especially since full recovery from the Christmas rainfall event
has already occurred. An isolated instance or two of flash flooding
can't be ruled out, hence the Marginal, but overall impacts into
central and northern California should be minimal.

Wegman


Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/94epoints.txt__;!!DZ3fjg!_KD6BVs7araMuBjXlUNcwSI6-UhFhETmLSSp9HEnttKQ=
g4IPk73AfnYpTFqwOIirtzUEaAShaXTaTW-su9s7zY5BnU0$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/98epoints.txt__;!!DZ3fjg!_KD6BVs7araMuBjXlUNcwSI6-UhFhETmLSSp9HEnttKQ=
g4IPk73AfnYpTFqwOIirtzUEaAShaXTaTW-su9s78Q3NIHM$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/99epoints.txt__;!!DZ3fjg!_KD6BVs7araMuBjXlUNcwSI6-UhFhETmLSSp9HEnttKQ=
g4IPk73AfnYpTFqwOIirtzUEaAShaXTaTW-su9s7Vcg5rgk$=20

=3D =3D =3D
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