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   Excessive Rainfall Discussion   
   NWS Weather Prediction Center College Park MD   
   156 PM EST Fri Dec 26 2025   
      
   Day 1   
   Valid 16Z Fri Dec 26 2025 - 12Z Sat Dec 27 2025   
      
   ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF   
   SOUTHERN CALIFORNIA...   
      
   1600Z Update...   
      
   The Marginal Risk area has been expanded north to include much of   
   northern CA, including the coastal ranges, Bay Area, and northern   
   portions of the Central Valley. The arrival of a fairly strong   
   500/700 mb low center just offshore will be driving areas of heavy   
   showers going through the afternoon hours. While this energy will   
   be generally weakening with time, the addition of at least modest   
   boundary layer heating along with some steepening of the mid-level   
   lapse rates should favor some convective elements capable of   
   producing localized rainfall rates of 0.50" to 0.75"/hour. Some   
   localized additional rainfall totals of 1 to 2 inches will be   
   possible, and especially across the northwest CA coastal range.   
   These additional rains may foster some additional localized runoff   
   concerns given the wet/saturated soil conditions and elevated   
   streamflows. No changes have been made to the Slight Risk area   
   farther south across southern CA as the threat of heavy rain will   
   continue here into at least the early evening hours.   
      
   Orrison   
      
      
   Previous discussion...   
      
   Few changes were needed for this morning's update. A positively-   
   tilted upper level trough will push ashore and inland across the   
   Pacific Northwest today. This will keep the flow at all levels   
   unidirectional out of the southwest across Southern California, on   
   the leading edge of a 140 kt jet. As the trough pushes east, less   
   and less of it will be over the Pacific, resulting in a diminishing   
   of the onshore flow with time. Waves of rainfall will push ashore   
   into southern California through the day. Lingering instability   
   will support rates that may be heavy at times. This is most likely   
   as the storms interact with the terrain of the eastern Transverse   
   Ranges, from L.A. north and east. It is in this area that the   
   rainfall forecast peaks at around 3 inches at the highest   
   elevations. Meanwhile most of the L.A. Basin picks up another 1-2   
   inches. 1-2 inches of rain would be problematic on a normal day,   
   but following multiple days of multiple inches of rain per day,   
   making the 1-2 inches expected today lighter by comparison...expect   
   scattered instances of flash flooding to redevelop again today.   
   While all areas of coastal Southern California and the Los Angeles   
   basin will pick up some rain, those highest totals will be north   
   and east of the downtown area. Thus, from downtown L.A. north and   
   east, a higher end Slight is in effect. The flooding impacts will   
   be the result of continued drainage of prior rainfall over   
   Christmas Eve and Day, as well as urban effects. A Moderate Risk   
   may need to be considered from Pasadena east through San Bernardino   
   if rainfall amounts exceed expected totals.   
      
   Elsewhere, the Slight remains in place down the coast to about San   
   Diego, as the eastward moving bands of rain impacting the L.A.   
   Basin should struggle to continue further south. Meanwhile the   
   Marginal for limited shower activity and/or lowering snow levels in   
   the foothills of the Sierra Nevada remains in place unchanged with   
   this update. For Central and Northern California, it's probable   
   that there will be more total precipitation in these parts of the   
   state compared to further south, but because of lowering snow   
   levels due to cold air influx with the aforementioned trough   
   moving in, much of it should fall as snow at the higher elevations,   
   reducing the flooding threat downstream.   
      
   By this evening, the flow will flip around to the northwest,   
   ushering in drier air and finally ending the rainfall threat across   
   Southern California likely for the rest of 2025.   
      
   Wegman   
      
   Day 2   
   Valid 12Z Sat Dec 27 2025 - 12Z Sun Dec 28 2025   
      
   The probability of rainfall exceeding flash flood guidance is less   
   than 5 percent.   
      
   Santorelli   
      
   Day 3   
   Valid 12Z Sun Dec 28 2025 - 12Z Mon Dec 29 2025   
      
   The probability of rainfall exceeding flash flood guidance is less   
   than 5 percent.   
      
   An area of rainfall across much of the Midwest is expected with a   
   developing low moving into the Great Lakes. The combination of   
   prior dry soil conditions, rapid movement, poor instability, and   
   cold air following the rain should all work to keep this event   
   largely beneficial. Flooding concerns in this area at this time=20   
   are too low for even a marginal risk.   
      
   Santorelli/Wegman   
      
      
   Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/94epoints.txt__;!!DZ3fjg!94U5mbYYygS2sn9KgtYxd_HQW6l7TMw8XWfcKYq7rLYP=   
   2u3M53GPsbmuwydNOdPBk1CKOJHI19aRLBB1c6EQoa8dbgQ$=20   
   Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/98epoints.txt__;!!DZ3fjg!94U5mbYYygS2sn9KgtYxd_HQW6l7TMw8XWfcKYq7rLYP=   
   2u3M53GPsbmuwydNOdPBk1CKOJHI19aRLBB1c6EQ6CfUsS8$=20   
   Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/99epoints.txt__;!!DZ3fjg!94U5mbYYygS2sn9KgtYxd_HQW6l7TMw8XWfcKYq7rLYP=   
   2u3M53GPsbmuwydNOdPBk1CKOJHI19aRLBB1c6EQC8MLRmo$=20   
      
   =3D =3D =3D   
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