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   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.=20   
      
   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.=20=20   
   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.=20   
      
   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.=20=20   
      
   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.=20=20   
      
   Gallina   
      
   ...Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!=   
   DZ3fjg!6defLsDhQO-ON-LqODLd4v4g66xlCixSbO2_VOxQBeCuFq2HkSlxnEp1Y3e9UPBfwxAh=   
   InmHHKSqNelNU_uwQNCMueQ$  for graphic product...   
      
   ATTN...WFO...LOX...SGX...   
      
   ATTN...RFC...RSA...NWC...   
      
   LAT...LON   34551850 34481788 34071704 33471650 32801624=20   
               32581627 32531672 32561724 32981753 33301781=20   
               33581827 33971908 34331893=20   
      
   =3D =3D =3D   
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