Mesoscale Precipitation Discussion 1240
NWS Weather Prediction Center College Park MD
831 AM EST Fri Nov 21 2025

Areas affected...Inland Deserts of Southern California and Lower
Colorado River Valley...

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

Valid 211330Z - 211930Z

SUMMARY...Prolonged moderate rain with embedded very weak showers
capable of .3-.5"/hr rates.  Scattered 1-1.5" totals pose longer
duration flash flooding concerns (3-6hr exceedance).

DISCUSSION...GOES-W WV suite shows core of deep upper-low has
wobbled over the LA Basin, but a strong upstream shortwave along
the western periphery of the upper-level closed low is digging
sharply resulting an combined effect of tightening the overall
flow across the deep cyclone while keeping the downstream older
center stationary over the next 6-12hr. As a result, the core of
the Atmospheric River will remain focused on NW Baja California
before cyclonically turning (fairly sharply) through the Imperial
Valley into the Mojave Deserts back toward the Tehachapi Range and
southern San Joaquin Valley. 

CIRA LPW prior to the local outage showed a fairly strong sfc to
850 and 850-700mb signal with 95th-99th percentile signals in each
layer, combine that with some increase in the surface to 850mb
layer from favorable southerly flow off the warm Sea of Cortez and
total PWats remain at or slightly above 1" into the Imperial
Valley and as high as .75" into the San Joaquin Valley. VWP and
RAP analysis suggest strong cyclonic curvature but also 20-30kts
of flux.  So while there is sub-100 J/kg of instability, the
vertical ascent through the strengthening western branch of the
TROWAL suggests highly anomalous flux to maintain moderate showery
activity through the deserts.  Rates of .33"/hr are more probable,
but an isolated rate of .5" is not impossible.  Still, persistent
weakly banded features will result in tracks of 1-1.5" totals in a
3-6hr time period, so any isolated, enhanced vertical cell within
the core may result in quicker stream/arroyo rises.   FFG values
being only .5"/hr and less than 1"/3hr and less than 1.5-2"/6hrs)
suggest a few scattered incidents of 'flash' flooding are
considered possible through early afternoon.

While less certainty, there are CAMs suggesting some filtered
insolation may be possible in the morning.  Near zero inhibition
through the profiles, suggest embedded widely scattered convective
elements may evolve in the 16-19z time period, particularly closer
to the warmer heat source of the Sea of Cortez across the Lower
Colorado and southern Imperial Valley.  Given the digging wave
upstream off the SW CA coast, upstream winds may be favorable for
some potential upstream redevelopment/back-building environment. 
Will continue to monitor those trends for any additional MPD later
toward the afternoon.  

Gallina

ATTN...WFO...HNX...PSR...SGX...VEF...

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

LAT...LON   36451689 36001550 35231468 34241409 33061394
            32331426 32491497 32691600 34241623 34841705
            34971779 35591825 36151792