TZUTC: 0000
MSGID: 168131.weather@1:2320/105 2dbd0957
PID: Synchronet 3.21a-Linux master/123f2d28a Jul 12 2025 GCC 12.2.0
TID: SBBSecho 3.28-Linux master/123f2d28a Jul 12 2025 GCC 12.2.0
BBSID: CAPCITY2
CHRS: ASCII 1
FORMAT: flowed
FOUS30 KWBC 311534
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1034 AM EST Wed Dec 31 2025

Day 1
Valid 16Z Wed Dec 31 2025 - 12Z Thu Jan 01 2026

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

...16Z Outlook Update...
The ongoing forecast is largely on track, with areas of moderate to
heavy rainfall expected across the Transverse Ranges especially
after 06Z tonight.=20=20

The only minor change to the ongoing forecast for this update was
to broaden Marginal across portions of San Luis Obispo and Santa
Barbara counties, where greater rainfall totals (around/over 1
inch) could occur atop sensitive areas/burn scars during the
forecast period). Isolated areas of excessive runoff cannot be
ruled out in these areas today and tonight.

See the previous discussion below for more forecast information.

Cook

...Previous Discussion...

A slow-moving upper level low centered well west of the California
coast will move towards California through the Day 1/New Year's
period. The upper low will remain negatively tilted as it
approaches the coast. This will allow a surface low, already formed
at the start of the period, to move north along the coast.
Southerly winds ahead of the low will drive plentiful Pacific
moisture into southern California. Upslope into the Transverse
Ranges will wring out much of that moisture, resulting in multiple
inches of rainfall, especially New Year's night, into the L.A.
Basin and adjacent Transverse Ranges. Once the center of the
surface low moves north of the Bay Area, its trailing cold front
will move into southern California. This will turn the flow to out
of the southwest, which will further enhance the upslope flow into
the Transverse Ranges, causing heavier rainfall Wednesday night.

The inherited Slight Risk for much of the L.A. Basin and higher-end
Slight for the Transverse Ranges themselves was left largely
unchanged with this update. The forecast weather pattern remains
largely unchanged, with more parallel southeasterly flow into the
Ranges supporting mostly light rain through much of New Year's
Eve and into the evening, then the rain turning heavier around the
stroke of midnight for the L.A. Basin. While soils across Southern
California have dried out a fair bit since the Christmas rainfall
event, both soils and rivers are still running above normal even
several days later. Thus, the renewal of heavy rainfall Wednesday
night should quickly refill any rivers, with much of the lighter
rain during the day Wednesday saturating the soils in time for the
heavier rains Wednesday night to largely convert to runoff. Thus,
the Slight Risk looks to be in good shape, with no major changes to
rainfall amounts expected requiring any further major changes to
the excessive rainfall forecast.

Wegman

Day 2
Valid 12Z Thu Jan 01 2026 - 12Z Fri Jan 02 2026

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

Heavy rain will be ongoing at the start of the period Thursday
morning across much of southern California. As the upper level
shortwave moves into the coast, the winds will turn out of the
west-southwest, or exactly orthogonal to the Transverse Ranges.
This will maximize the upslope enhancement of the rainfall just as
the maximum forcing is moving ashore. Through 18Z, this period New
Year's morning will be when the heaviest rain of the event is
expected into southern California. PWATs between 1 and 1.25 inches
will be between 3 and 4 sigma above normal for this time of year, a
sign of how much moisture will be available to convert into
rainfall and runoff from the Transverse and to a lesser extent,
Peninsular Ranges. Once the shortwave moves ashore and inland by
Thursday evening, the combination of weakening of both upper level
and surface lows, as well as rapid weakening of the onshore west-
southwesterly flow behind the low, will lead to rapid diminishing
of the rainfall across Southern California, such that by Thursday
evening, little if any rainfall will be left.

The Slight Risk inherited is largely unchanged both from the
previous forecast and from the Day 1/Wednesday Slight Risk area.
Both days' Slight Risks will be splitting the two halves of the
same event, so little should be different. There is a higher-end
Slight in effect for most of the Transverse Ranges from about
Malibu east through San Bernardino. This will largely be for the
morning hours, as once the rain weakens, the flooding threat
associated with additional rainfall should also rapidly diminish,
likely resulting in an early downgrade of the Slight Risk Thursday
afternoon.

Further north, along the track of the aforementioned upper level
shortwave, rainfall will persist for much longer into the northern
Coastal Ranges and much of the Sierra Nevada foothills, especially
those along the Sacramento Valley. However, this area has more
thoroughly recovered from the Christmas rainfall event and also is
better equipped naturally to handle long duration rainfall. As
such, only an isolated flash flooding threat, consistent with the
unchanged Marginal, is expected.

Wegman

Day 3
Valid 12Z Fri Jan 02 2026 - 12Z Sat Jan 03 2026

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

Light rain over most of northern California from about 06Z Friday
through 18Z Friday will end as another large upper level low begins
to make its way towards northern California and Oregon. This upper
low, while bigger, will be less amplified/elongated as compared
with the last upper low. That should focus most of the rainfall
into northern California. Further, the surface low will be a
vertically stacked one, whose occluded front has wrapped clear
around the low center. It is this occluded front making its way
ashore that will restart any heavier rain into the southward facing
slopes of the coastal ranges, Klamath, and the northern Sierra
Nevada foothills. Rainfall totals will generally fall in the 2-3
inch range in most of the mountains, with around an inch into the
Sacramento Valley. This amount of rain should be enough to raise
river levels with perhaps an isolated instance or two of flash
flooding in some of the flashier creeks and streams, but overall
impacts should still be minimal, despite some precipitation
occurring prior to this on Day 2. Much of northern California is
well-suited to handle rainfall amounts such as these, so widespread
impacts are not expected. The area's forecast will be monitored
for any potential Slight Risk upgrades needed.

Wegman


Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/94epoints.txt__;!!DZ3fjg!7o8Eg9AOCNMgaJC05jFAIC1fkLbfj77oMUDCFRAo6PPM=
j1sM1Yy5BqelH3I-e4JsONR7Lq7-msUc_9z02JkDybKSlnk$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/98epoints.txt__;!!DZ3fjg!7o8Eg9AOCNMgaJC05jFAIC1fkLbfj77oMUDCFRAo6PPM=
j1sM1Yy5BqelH3I-e4JsONR7Lq7-msUc_9z02JkDvBeINxA$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/99epoints.txt__;!!DZ3fjg!7o8Eg9AOCNMgaJC05jFAIC1fkLbfj77oMUDCFRAo6PPM=
j1sM1Yy5BqelH3I-e4JsONR7Lq7-msUc_9z02JkDwSaCW4g$=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)
SEEN-BY: 1/19 100 120 16/0 18/0 19/10 37 105/81 106/201 116/116 123/0
SEEN-BY: 123/25 126 130 180 755 3001 3002 4040 128/187 129/14 134/100
SEEN-BY: 135/115 142/104 153/143 148 149 151 757 7715 154/10 110 201/0
SEEN-BY: 203/0 218/700 840 220/6 221/1 6 360 222/2 226/30 227/114
SEEN-BY: 229/110 134 206 300 307 317 400 426 428 470 664 700 705 240/5832
SEEN-BY: 250/1 266/512 275/1000 280/5003 291/111 301/1 320/119 219
SEEN-BY: 320/319 2119 322/757 762 326/101 335/364 341/66 342/200 396/45
SEEN-BY: 423/81 460/58 633/280 712/848 1321 902/26 2320/0 105 304
SEEN-BY: 3634/0 12 27 57 58 60 61 119 5020/400 5075/35
PATH: 2320/105 3634/12 153/757 221/6 1 320/219 229/426