Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
314 AM EST Wed Dec 31 2025
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 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt