AWUS01 KWNH 211245
FFGMPD
CAZ000-211830-

Mesoscale Precipitation Discussion 1239
NWS Weather Prediction Center College Park MD
744 AM EST Fri Nov 21 2025

Areas affected...Southern California

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

Valid 211245Z - 211830Z

SUMMARY...Risk of slow moving, narrow updrafts capable of
.75"-1"/hr rates likely to continue to a few more hours. Isolated
1-2" totals in/near urban locations in SoCal suggest localized
flash flooding remains possible.

DISCUSSION...A well-defined closed low is parked directly over
southern California with an upstream highly anomalous
shortwave/vorticity center descending southward along the western
periphery of the low. This and favorable cyclonically curved
geography of the coast and mountain ranges will help to keep
surface to boundary layer cyclone relatively stationary for the
next few hours just along/south of Long Beach, CA. The cold front
and associated warm conveyor have pressed eastward into far NW
Baja California (state) of Mexico before angling through Imperial
Valley and lower Colorado River Valley. However, the western
branch of the TROWAL and occluded front remain banked up against
the Orange county and San Diego county beaches.

GOES-W WV suite shows the core of the upper-level jet remains
south near the San Diego country border with Mexico, allowing for
solid cyclonically curved left exit ascent across much of SoCal
providing solid ascent for updrafts that do develop with the solid
surface/low level moisture convergence. The limiting factor will
likely be unstable air/vertical ascent through convective
processes. MUCAPEs have dropped to around 500 J/kg but are now
mostly driven through cold advection aloft. The upstream
shortwave will dull this advection for a few hours but may allow
the lapse rates to maintain this weak 250-500 J/kg instability.
As such, updrafts are likely to remain isolated and generally
narrow, though with isallobaric response to the updraft, moisture
convergence should allow for the .75-.9" total PWats (loaded
mainly below 700mb) to support rates of .75-1"/hr.

As such, a localized 1-2" total still remains possible for the
next few hours. Naturally low FFG and/or urban hydrophobic
conditions will result in solid run-off and maintain a risk of
widely scattered incident or two of possible flash flooding
through the morning.

It is also possible (more likely toward 16-18z) that a few
showers/cells may try to develop with some weak clearing through
the Imperial Valley along the western branch of the TROWAL. These
cells would be also similarly slow moving but likely translating
NW rather than NE as with the coastal cells; but there is less
certainty with that evolution and a secondary MPD may be required
if trends continue to improve.

Gallina

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

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

LAT...LON 34551850 34481788 34071704 33471650 32801624
32581627 32531672 32561724 32981753 33301781
33581827 33971908 34331893

$$
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