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   Mesoscale Precipitation Discussion 1274   
   NWS Weather Prediction Center College Park MD   
   302 AM EST Thu Dec 25 2025   
      
   Areas affected...portions of northern, central and southern CA   
      
   Concerning...Heavy rainfall...Flash flooding likely   
      
   Valid 250759Z - 251800Z   
      
   SUMMARY...Another round of heavy rain will approach central and   
   southern CA tonight into Christmas morning. Peak hourly rainfall   
   of 0.5 to 1.0 inches is expected, with isolated hourly totals over   
   1 inch. Due to recent heavy rainfall and areas of ongoing   
   flooding, renewed areas of flood/flash flooding are likely from an   
   additional 1 to 3 inches of rainfall (locally higher) through ~18Z.   
      
   DISCUSSION...GOES West water vapor/infrared satellite imagery from   
   07Z showed a mid-level vorticity max near 39N 127W with a   
   southward extending trough axis, advancing toward the northeast. A   
   surface low was associated with the vorticity max containing an   
   attached occluded/cold front extending to the south. Infrared   
   imagery and lightning data showed a line of showers/thunderstorms   
   just ahead of the front, advancing closer to the central CA coast.   
   This feature was located ahead of a broader closed low aloft, with   
   steepening lapse rates in the 850-500 mb layer edging eastward   
   central to northern CA.   
      
   While the primary moisture axis from Wednesday has moved into the   
   lower CO River Valley, layered PW imagery from OSPO showed a   
   secondary moisture axis just ahead of the offshore cold front with   
   blended TPW values of 0.8 to 0.9 inches. Instability along the   
   coast and within the Sacramento/San Joaquin Valley was less than   
   500 J/kg and should stay that way over the next 6-12 hours  with   
   some fluctuations up/down over time via recent RAP forecasts. RAP   
   guidance also showed 850-700 mb winds from the south to   
   south-southwest increasing into the 60-65 kt range from near San   
   Francisco Bay to the northern Sacramento Valley ahead of the front   
   which will likely be accompanied by a band of   
   showers/thunderstorms sweeping inland containing peak hourly   
   rainfall of 0.5 to 1.0 inches. The band will impact central to   
   northern coastal locations first, followed by inland locations   
   across the Sacramental Valley into the Sierra Nevada foothills.   
   The band of heavy rain will also begin to advance southward down   
   the Santa Lucia Range into the western Transverse Ranges between   
   12Z and 18Z. Post-frontal low level axes of convergence could   
   support additional narrow bands of showers/thunderstorms with   
   brief training across central to northern CA locations with peak   
   hourly rainfall near 0.5 inches.   
      
   Through ~18Z, peak additional rainfall of 1 to 3 inches is   
   expected (locally higher) for the Coastal Ranges into the Sierra   
   Nevada foothills (below snow levels of ~5000 ft). This additional   
   rainfall, coupled with brief high rates, should allow for areas of   
   renewed flooding/flash flooding across urban/terrain and low-lying   
   locations through 18Z. Due to saturated soils, the potential for   
   landslides/debris flows will exist, including any sensitive burn   
   scar locations.   
      
   Otto   
      
   ...Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!=   
   DZ3fjg!9Fy7N2vCbkgdsoukSqdAcxFlbzgguN4-zro9adZVuZDI_-2AgrwuRP_VHaaYvYkSzowB=   
   F8uDF9qksst-8nX1MlQ43cs$  for graphic product...   
      
   ATTN...WFO...EKA...HNX...LOX...MFR...MTR...STO...   
      
   ATTN...RFC...RSA...NWC...   
      
   LAT...LON   41222225 40912174 40102152 39872122 39182064=20   
               38632033 37621963 36411861 36311931 36131974=20   
               35721981 35191938 34581808 34011806 33771847=20   
               33841954 34132068 34802148 35772178 37082281=20   
               38992416 40242462 40682433 40842312=20   
      
   =3D =3D =3D   
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