Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
314 AM EST Wed Dec 31 2025
Day 1
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 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 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt