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   CAZ000-211812-   
      
   Mesoscale Precipitation Discussion 1267   
   NWS Weather Prediction Center College Park MD   
   113 AM EST Sun Dec 21 2025   
      
   Areas affected...portions of northern and central California   
      
   Concerning...Heavy rainfall...Flash flooding likely   
      
   Valid 210612Z - 211812Z   
      
   Summary...Onset of a long-duration heavy rain hazard is underway   
   as an atmospheric river makes landfall across central and northern   
   California.  Upwind regions of high terrain could experience   
   periods of 0.5-1.0 inch/hr rain rates.  Flash flooding is likely   
   through the morning hours.   
      
   Discussion...Recent radar/MRMS data depicts an uptick in heavy   
   rainfall/rain rates across northern and central California   
   currently.  The greatest rates were occurring across coastal   
   ranges north/northwest of San Francisco, where strengthening,   
   west-southwesterly 850mb flow (around 40 knots) was oriented   
   perpendicular to ridgelines/terrain in that area.  The combination   
   of orographic ascent and 1.5 inch PW values located just upstream   
   of the area were supporting areas of 0.25-0.5 inch measured rain   
   rates over the past hour or so.   
      
   Models/observations suggest that the ongoing areas of heavy rain   
   will only expand and increase in intensity with time.  850mb   
   should increase into the 40-50 knot range over the next few hours   
   while impinging upon upwind slopes of the Sierra and northern   
   California coastal ranges.  This will allow for areas of 0.5 to 1   
   inch/hr rain rates to materialize in terrain-favored areas and   
   persist for 6+ hours.=20   
      
   USGS Streamflow and NASA SPoRT-LiS Soil Moistures suggest ground   
   conditions can handle initial rainfall with somewhat dry initial   
   soil conditions and modest flow in local streams.  Multiple hours   
   of heavier rainfall will likely make ground conditions more   
   favorable for excessive runoff and flood/flash flood impacts over   
   time.  The greatest risk for flood/flash flood instances will   
   occur from 09Z/1a Pacific Time, and multiple instances of   
   excessive runoff are expected through 18Z/10a.   
      
   Cook   
      
   ...Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!=   
   DZ3fjg!6UfaRemdCkGYpLIhlFPONIeYmwwruTOcraTfEqSSeTpLE3--jvLref83zgvivudYXlzW=   
   v-lQ7zarJ1Nt1ckhghWBPRM$  for graphic product...   
      
   ATTN...WFO...EKA...HNX...MFR...MTR...REV...STO...   
      
   ATTN...RFC...PTR...RSA...NWC...   
      
   LAT...LON   41962247 41542157 40852066 39602031 37971979=20   
               38042048 38322109 38922152 38772195 38162219=20   
               37902249 38402334 39852421 41912439 41932330=20   
   =20=20=20=20=20=20=20=20=20=20=20=20   
      
   =3D =3D =3D   
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