Mesoscale Precipitation Discussion 1238
NWS Weather Prediction Center College Park MD
112 AM EST Fri Nov 21 2025

Areas affected...coastal southern CA

Concerning...Heavy rainfall...Flash flooding possible

Valid 210611Z - 211210Z

Summary...Highly localized flash flooding will be possible from
slow moving showers/thunderstorms capable of producing hourly
rainfall up to ~1 inch. This threat will exist along portions of
southern CA near the coast through at least 12Z.

Discussion...GOES West water vapor imagery at 06Z showed
a mid-level closed low centered over the central CA coast, slowly
moving south. Several smaller scale vorticity maxima were observed
within the cyclonic flow, with one of interest located over/near
Santa Cruz Island. Cooling cloud tops were noted east of the
vorticity center, northwest of a surface low within a mesoscale
deformation zone. The 1009 mb low was located about 30 miles west
of Santa Catalina at 06Z with an occluded/cold front extending
southward. A weak plume of low level moisture transport was
located ahead of the front, parallel to the southern CA coastline
where a few showers have recently picked up in intensity with
localized training along the Orange County coast.

Weak instability up to a couple hundred J/kg was located along the
southern CA coast from Santa Barbara to San Diego counties as
noted in recent ACARS/model soundings. As the mid-level low
continues to translate down the CA coast, the plume of low level
moisture ahead of the front will push inland along with the
initial round of stronger showers. Later in the night, some
steepening of mid-level lapse rates should increase instability up
to about 500 J/kg along the immediate coast and closer to 1000
J/kg offshore. The surface low is forecast to edge closer to the
Los Angeles/Orange County coasts through 12Z. With this movement,
another round of slow moving showers and thunderstorms is expected
to track toward the coast from the offshore waters, with potential
for highly localized hourly (or sub-hourly) rainfall between 0.5
and about 1 inch, due to slow movement.

These spotty higher rainfall rates could result in flash flooding
of urban areas or other sensitive terrain.

Otto

ATTN...WFO...LOX...SGX...

ATTN...RFC...RSA...NWC...

LAT...LON   34581936 34531776 33821649 32841623 32541631
            32531634 32251753 32241756 32731790 33131881
            33501962 34001978