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   Mesoscale Precipitation Discussion 1276   
   NWS Weather Prediction Center College Park MD   
   405 PM EST Thu Dec 25 2025   
      
   Areas affected...Portions of Central and Northern CA   
      
   Concerning...Heavy rainfall...Flash flooding likely   
      
   Valid 252105Z - 260900Z   
      
   SUMMARY...New rounds of heavy showers and thunderstorms are   
   expected this afternoon through this evening for areas of central   
   and northern CA, including the Bay Area. Given the saturated soil   
   conditions and high streamflows, additional areas of flooding and   
   flash flooding are likely.   
      
   DISCUSSION...The latest GOES-W IR/WV suite shows a deep upper   
   trough and associated closed low offshore of the West Coast.   
   Shortwave impulses continue to round the base of the trough and   
   take aim on central and northern CA, with each impulse effectively   
   driving renewed atmospheric river activity into the state.   
      
   Solar insolation has facilitated sufficient levels of boundary   
   layer heating across the northern portions of the Central Valley   
   to result in SBCAPE values as high as 500 to 750 J/kg. This   
   heating coupled with relatively divergent flow aloft downstream of   
   the deep offshore upper trough/closed 500 mb low center will   
   facilitate an increase in convective showers in the near-term   
   across sizable areas of the Central Valley. Some locally organized   
   line-segments will be possible given the elevated bulk shear   
   parameters (30 to 50 kts), and this actually may become aligned   
   with the deep layer flow for some periodic instances of   
   cell-training.   
      
   However, of greater significance will be the approach of a   
   well-organized shortwave impulse rounding the base of the upper   
   trough heading into the early evening hours. This shortwave energy   
   which is well-depicted by the axis of cold convective cloud tops   
   in IR satellite imagery along 125W to 130W will be ejecting   
   northeast toward the northern CA coastal ranges in the 00Z to 03Z   
   time frame, including the Bay Area. Thereafter, this new surge of   
   energy and related Pacific-moisture transport should eject into   
   the Central Valley and Sierra Nevada foothills. The coastal ranges   
   and Sacramento Valley in particular will see the greatest IVT   
   magnitudes. The 12Z GFS/ECMWF solutions suggest IVT values   
   reaching into the 500 to 600 kg/m/s range with the aid of stronger   
   low to mid-level southwest flow.   
      
   The combination of this along with large-scale forcing/shear and   
   at least modest boundary layer instability should set the stage   
   for more organized bands of convection to approach the coastal   
   ranges and then advance inland going into the evening hours. Heavy   
   rainfall rates of 0.50" to 1"+/hour will be likely with these new   
   rounds of heavy shower and thunderstorm activity.   
      
   Additional rainfall totals by late this evening of 2 to 3 inches   
   will be possible locally for the coastal ranges and especially the   
   upslope areas of the northern Sierra Nevada foothills. Lesser   
   totals of as much as 1 to 2 inches cannot ruled out in at least   
   parts of the Sacramento Valley. By late this evening, this latest   
   surge of energy and moisture should settle farther south into   
   coastal ranges south of the Bay Area and adjacent areas of the   
   Central Valley.   
      
   Given the saturated soil conditions and high streamflows, these   
   additional rains are likely to result in more areas of areal   
   flooding and flash flooding, which will include urban flooding   
   impacts (San Francisco, Sacramento, Chico and Redding all at risk)   
   along with localized mud and landslide activity near areas of   
   higher terrain.   
      
   Orrison   
      
   ...Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!=   
   DZ3fjg!-ottk6EO9X3DhmXLkaq_LhrYqExDpVA8k2z7-Xyqo_Zc6oWbZCMKZU6FtSSO45tk8kVr=   
   vCEtkGhTo7CKh2iK1cj_1jE$  for graphic product...   
      
   ATTN...WFO...EKA...HNX...LOX...MFR...MTR...STO...   
      
   ATTN...RFC...RSA...NWC...   
      
   LAT...LON   41192196 40352163 39952136 39572073 39012031=20   
               38592000 37901945 37291920 36791979 35902040=20   
               35112065 35182118 35912190 37202260 37972308=20   
               39012405 39682433 40372428 40802375 40872295=20   
               41122238=20   
      
   =3D =3D =3D   
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