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   Mesoscale Precipitation Discussion 1272   
   NWS Weather Prediction Center College Park MD   
   1015 AM EST Wed Dec 24 2025   
      
   Areas affected...Central Valley into the Sierra Nevada Foothills   
      
   Concerning...Heavy rainfall...Flash flooding likely   
      
   Valid 241515Z - 242215Z   
      
   SUMMARY...Significant atmospheric river activity continues to ride   
   up through the Central Valley and into the Sierra Nevada foothills   
   with heavy rainfall rates. Areas of flooding and some flash   
   flooding will continue to be likely.   
      
   DISCUSSION...The latest GOES-W IR satellite imagery shows an   
   impressive fanning out of cold cloud tops across the Central   
   Valley and into the Sierra Nevada as a full-latitude trough   
   offshore of the West Coast continues to channel a strong deep   
   layer atmospheric river inland across the region.   
      
   A surface cold front has been making steady progress eastward this   
   morning and has pushed east of the Bay Area and into the Central   
   Valley. Radar imagery shows a well-defined and dynamically forced   
   convective line just ahead of the cold front making its way closer   
   to the Sierra Nevada foothills with rainfall rates that are on the   
   order of 0.75" to 1"/hour. These rainfall rates are being   
   facilitated by MUCAPE values of as much as 100 to 250 J/kg ahead   
   of the cold front, but even more so by the very strong low-level   
   moisture convergence riding up through the Central Valley and with   
   impressively divergent flow aloft downstream of the offshore upper   
   trough.   
      
   Enhanced rainfall rates of up to 1"/hour will continue with these   
   convective elements ahead of the cold front as it advances   
   gradually eastward over the next several hours. The additional aid   
   of orographic ascent into the Sierra Nevada foothills will further   
   support the enhancement of rainfall rates. Given the strongly   
   anomalous degree of moisture through the column including some   
   tropical origins of the atmospheric river, the rainfall will be   
   quite efficient, with additional totals through mid-afternoon of   
   as much as 2 to 4 inches.   
      
   Additional areas of flooding and some flash flooding will be   
   likely, including portions of the Central Valley and into the   
   Sierra Nevada foothills. Localized urban flooding impacts will   
   continue to be a concern, and for areas in the terrain, localized   
   burn scar flash flooding will be a threat. Can't rule out some   
   landslide activity as well near areas of steep terrain.   
      
   Orrison   
      
   ...Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!=   
   DZ3fjg!53-BwiZrl9T80oxjrbDTjD5St9EgADRAizby-f106X4xaimzSLI6Rng1qTi5hYs__yXH=   
   NMi1oDBw2dNyDiht0mBs2c4$  for graphic product...   
      
   ATTN...WFO...HNX...LOX...MTR...REV...STO...   
      
   ATTN...RFC...RSA...NWC...   
      
   LAT...LON   40072068 39912039 39302013 38461984 37561932=20   
               36731859 36231827 35571843 35281882 35281945=20   
               35572006 35812034 36392080 37342095 39002111=20   
               39862104=20   
      
   =3D =3D =3D   
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