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   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.=20   
      
   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.=20   
   Will continue to monitor those trends for any additional MPD later   
   toward the afternoon.=20=20   
      
   Gallina   
      
   ...Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!=   
   DZ3fjg!_-WjtQ6vmZP70V-mcQSPBPEHSb5UPtn6MaaXF4h2x6fY00XJ7ZrT2AvFASaI-gjDsv3r=   
   8Ao-f3IcG6w7HoujVkdzYxE$  for graphic product...   
      
   ATTN...WFO...HNX...PSR...SGX...VEF...   
      
   ATTN...RFC...RSA...STR...NWC...   
      
   LAT...LON   36451689 36001550 35231468 34241409 33061394=20   
               32331426 32491497 32691600 34241623 34841705=20   
               34971779 35591825 36151792=20   
      
   =3D =3D =3D   
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