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   CAZ000-260600-   
      
   Mesoscale Precipitation Discussion 1275   
   NWS Weather Prediction Center College Park MD   
   110 PM EST Thu Dec 25 2025   
      
   Areas affected...Portions of Central and Southern CA   
      
   Concerning...Heavy rainfall...Flash flooding likely   
      
   Valid 251810Z - 260600Z   
      
   SUMMARY...Additional atmospheric river surge to impact central and   
   southern CA going into the evening hours. Locally heavy rainfall   
   rates coupled with extremely sensitive conditions on the ground   
   from previous rainfall will promote additional areas of flash   
   flooding with locally dangerous and life-threatening impacts   
   possible.   
      
   DISCUSSION...GOES-W visible satellite imagery shows a secondary   
   cold front and associated atmospheric river surge advancing down   
   across areas of central and southern CA with a particular focus on   
   the Transverse Range and nearby areas of the Los Angeles Basin.   
   While this surge of moisture is certainly more modest compared to   
   yesterday, the additional rainfall associated with this will be   
   falling on areas that have seen as much as 6 to 12+ inches of rain   
   over the last 36 hours, and especially in the San Gabriel and San   
   Bernadino Mountains.   
      
   The latest CW3E IVT analyses suggest as much as 500 kg/m/s of IVT   
   will be moving through the Transverse Range and Los Angeles Basin   
   region going through this afternoon and into at least the early   
   part of this evening. Lesser magnitudes will continue to impact   
   areas farther north extending into the southern Sierra Nevada.   
   There is some meaningful instability with MUCAPE values near 500   
   J/kg situated along the offshore portion of the front, and the   
   latest GOES-W and CMORPH2 microwave-driven data does show some   
   convective showers with heavier rainfall rates offshore. These   
   showers are likely to move inland over the next several hours, and   
   the orthogonal orientation of the low to mid-level flow in general   
   relative to the terrain should further support locally enhanced   
   rainfall rates.   
      
   A look at the 12Z HREF guidance and experimental guidance   
   including the latest NSSL/MPAS and WoFS solutions suggests a slow   
   southeast advance of shower activity down into the Los Angeles   
   Basin along with the adjacent terrain. Some rainfall rates going   
   through early this evening are likely to exceed a 0.50"/hour with   
   some spotty, but stronger convective showers near the terrain   
   potentially fostering rates as high as 1"/hour. Additional   
   rainfall totals going through early this evening may reach as high   
   as 2 to 4 inches over the higher terrain, with 1 to 2 inches in   
   the lower elevations.   
      
   Given the new rounds of heavy rainfall, and extremely sensitive   
   antecedent conditions, additional areas of flash flooding are   
   likely with potential for dangerous and life-threatening impacts.   
   This will include localized urban flash flooding concerns, a   
   threat for new mud and rockslide activity, and also debris flows   
   near and adjacent to any burn scar areas.   
      
   Orrison   
      
   ...Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!=   
   DZ3fjg!9tCd1fNUm3drPEqqNgsb-dyEzZuFTsPMoi3SvR4sV8id1rDF_qoI-bDx33w-rZvHuMCg=   
   pTnQ6c56bFOr857iq0_-lF0$  for graphic product...   
      
   ATTN...WFO...HNX...LOX...MTR...SGX...   
      
   ATTN...RFC...RSA...NWC...   
      
   LAT...LON   37191917 36801870 36261839 35631845 35031837=20   
               34701782 34331738 33891750 33751824 33851892=20   
               34211976 34382045 34732060 35202058 35902039=20   
               36871969=20   
      
   =3D =3D =3D   
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