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   Mesoscale Precipitation Discussion 1269   
   NWS Weather Prediction Center College Park MD   
   229 AM EST Mon Dec 22 2025   
      
   Areas affected...portions of central and eastern California   
      
   Concerning...Heavy rainfall...Flash flooding possible   
      
   Valid 220729Z - 221529Z   
      
   Summary...Areas of moderate to heavy rainfall will continue for   
   several more hours across upslope/western sides of the Sierra,   
   prompting flood/flash flood concerns.   
      
   Discussion...Recent radar/observational data depict continued   
   areas of moderate to heavy rainfall along upslope sides of the   
   Sierra in central/northeastern California.  The rainfall continues   
   as part of a landfalling atmospheric river across the region, with   
   40 knots of 850mb flow oriented perpendicular to the Sierra   
   promoting abundant orographic ascent.  Additionally, 1-1.3 inch PW   
   values are continuing to enhance rainfall rates across the region.   
    Furthermore, the persistence of the atmospheric river has   
   resulted in a focused area of 3-6 inch rainfall totals over the   
   past 12 hours especially in the Tahoe and Lassen National Forests   
   (east of an axis from Chico to Sacramento).  Some instances of   
   flooding/flash flooding and landslides have occurred, which isn't   
   surprising given moderate MRMS Flash outputs and elevated   
   streamflows per the USGS Water Dashboard.   
      
   Ongoing trends should continue for several more hours.  850mb flow   
   should remain perpendicular to the Sierra through at least 15Z/7a   
   Pacific Time, prompting continued rainfall and runoff.  Models   
   suggest that flow should gradually slacken into the 20-25 knot   
   range during that time frame, which should decrease orographic   
   ascent and rain rates with time.  Nevertheless, ground conditions   
   continue to be sensitive and additional 0.25+ inch/hr rates are   
   likely to continue.  Flooding/flash flooding, debris flows, and   
   landslides continue to be possible.   
      
   Cook   
      
   ...Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!=   
   DZ3fjg!5iyvhhDbMLgpO0wdYJ0o3ApbhFqsnmdhB0ihXtI6v_mJfBQI739WZFXdnVdpfWrLxa66=   
   -c7qLMepHMB4TgFXu1YM5yk$  for graphic product...   
      
   ATTN...WFO...HNX...REV...STO...   
      
   ATTN...RFC...RSA...NWC...   
      
   LAT...LON   40692210 40622126 39852056 38821992 37971968=20   
               37641982 37902072 39222187 40052233=20   
      
   =3D =3D =3D   
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