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   NVZ000-CAZ000-ORZ000-220620-   
      
   Mesoscale Precipitation Discussion 1268   
   NWS Weather Prediction Center College Park MD   
   123 PM EST Sun Dec 21 2025   
      
   Areas affected...central to northern CA   
      
   Concerning...Heavy rainfall...Flash flooding likely   
      
   Valid 211820Z - 220620Z   
      
   Summary...A long-duration heavy rain event will continue through   
   tonight. 12 hour rainfall totals of 2 to 4 inches for the Coastal   
   Ranges and 3 to 7 inches for the Sierra Nevada are expected   
   through 06Z (locally higher possible). Peak hourly rainfall of   
   0.50 to 0.75 inches is likely with isolated values up to 1 inch,   
   likely translating into flood/flash flood impacts across the   
   region.   
      
   Discussion...Moderate to heavy rain was occurring as of 18Z from   
   roughly Point Arena, CA to southern OR with peak hourly rainfall   
   between 0.5 and 0.75 inches, but with values locally up to 1 inch.   
   An atmospheric river containing PW values between 1.0 and 1.4   
   inches along the northern CA coast and southwesterly 850-700 mb   
   winds of 40-50 kt were present. 24 hour gauge reports of 3-6   
   inches (King Range) and 4-7 inches (north-central Sierra Nevada)   
   were observed through 18Z and the northern CA region has   
   experienced 200 to 400 percent of normal over the past week. There   
   have been several reports of flooding and landslides over the past   
   few hours within the northern Coastal Ranges and a couple of   
   landslides reported within the Sierra Nevada.   
      
   The approach of a positively tilted shortwave trough axis off of   
   the Pacific Northwest coast will continue to advance ESE over the   
   next 12 hours, allowing for a slow southward movement to the axis   
   of highest moisture transport down the CA coastline. As this   
   occurs, PWs are forecast to lower slightly and 850-700 mb winds   
   should weaken about 10 kt through 06Z with IVT values of 800   
   kg/m/s lowering into the 600-700 kg/m/s range. Steady peak hourly   
   rainfall values of 0.50 to 0.75 inches are likely to continue over   
   the next 6-12 hours. 12Z HREF probabilities showed 40 to 80   
   percent probabilities of exceeding 0.5 in/hr through 02Z for the   
   central to northern CA Coastal Ranges north of San Francisco and   
   40 to 90+ percent for the north-central to northern Sierra Nevada   
   through at least 06Z. Meanwhile, hourly probabilities for 1+   
   inches in an hour are near or less than 10 percent through the   
   overnight.   
      
   Given wet antecedent conditions over the past week, including 24   
   hour rainfall, the addition of another 2 to 6+ inches is likely to   
   result in additional flooding, landslides/mudslides and debris   
   flows. Where overlap of high rain rates occurs with sensitive burn   
   scars or urban areas, more rapid flooding/flash flooding can be   
   expected.   
      
   Otto   
      
   ...Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!=   
   DZ3fjg!-N3fonV1wnIiFTFf7U_Ok7Dt2UfKXjgzF4Wstna-jHDUM2PoHkO-k6Iapux8XqvGKO4P=   
   j7M3a0cP32dAi1_XlY33J7s$  for graphic product...   
      
   ATTN...WFO...EKA...MFR...MTR...REV...STO...   
      
   ATTN...RFC...PTR...RSA...NWC...   
      
   LAT...LON   42022284 41832164 41492099 40542042 39901995=20   
               38651967 38301964 38142001 38202058 38022121=20   
               37712152 37302170 36992188 36762237 37242297=20   
               37532336 38612413 40252494 41812489=20   
      
   =3D =3D =3D   
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