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   Excessive Rainfall Discussion   
   NWS Weather Prediction Center College Park MD   
   228 PM EST Wed Dec 24 2025   
      
   Day 1   
   Valid 16Z Wed Dec 24 2025 - 12Z Thu Dec 25 2025   
      
   ...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL ACROSS SOUTHERN   
   CALIFORNIA...   
      
   1600Z Update...   
      
   A strong atmospheric river event continues to impact California as   
   a full-latitude trough offshore of the West Coast advances   
   gradually east and sends a cold front inland across the coastal   
   ranges and the Central Valley. Changes to the previous outlook   
   include trimming especially the Slight Risk area across portions of   
   central and northern CA where the heaviest rains have now shifted   
   off to the south and east. Some minor and generally cosmetic   
   adjustments were made to the Marginal Risk area. Additional shower   
   activity will impact much of northern California going into the   
   overnight hours as additional shortwave energy embedded within the   
   larger scale trough impacts the region.   
      
   Farther south for southern California, no changes have been made to   
   the High Risk. Heavy and persistent rainfall rates into the 0.50"   
   to 1"/hour range, and some spotty peak rates perhaps to near   
   1.5"/hour will continue to be a threat which is advertised by the   
   12Z HREF guidance along with the recent HRRR and WoFS guidance.   
   Please consult WPC's MPDs for additional short-term information on   
   the evolution of this strong atmospheric river which will continue   
   to bring dangerous and locally life-threatening flooding and flash   
   flooding impacts.   
      
   Orrison   
      
      
   Previous discussion...   
      
   Highly anomalous 850-700 mb moisture flux on the order of 5+   
   standard deviations above the mean will yield extremely heavy   
   rainfall ahead of the amplifying mid/upper trough located offshore   
   of the California coast. The heavy rainfall totals for Southern CA   
   will likely occur over an 18 hour period, with widespread 4-7 inch   
   totals, with isolated max amounts of 9"+. Expect a well defined   
   south to north precip band in this anomalous moisture flux axis   
   with hourly rainfall rates of 0.50-1.5"+ likely. The potential for   
   high hourly rainfall totals will accentuate runoff over soil that   
   is relatively dry and or over recent burn scar areas. This will   
   pose a significant threat of flash flooding, landslides, rock falls   
   and mud slides. The greater than normal travel during the   
   Christmas Holiday will likely expose a potentially larger number of   
   people to these lift threatening hazards and was taken into   
   consideration with the upgrade to high risk.   
      
   Although snow levels will be lowering across the Sierra Nevada and   
   adjacent locations and much of the highest elevations will have   
   significant snow, some of the lower elevations could receive   
   additional rainfall of 1 to 4 inches. The Slight Risk areas were   
   combined to cover heavy rain tracking eastward across eastern   
   California, southern Nevada and into southern Utah and western   
   Arizona where areal averages up to 1 to 2+ are forecast.   
      
   Campbell   
      
      
   Day 2   
   Valid 12Z Thu Dec 25 2025 - 12Z Fri Dec 26 2025   
      
   ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF   
   SOUTHERN CALIFORNIA...   
      
   20Z Update: No major changes to the previous forecast as the heavy   
   rainfall anticipated across portions of Southern California, mainly   
   the San Gabriel and San Bernardino mountains remains the outlook   
   through Christmas Day. Considering ongoing flash flooding and   
   mud/landslide activity in the region from 6+ inches of rainfall,   
   any additional rains will warrant attention for considerable   
   impacts as more heavy rain bands move into the region. A higher end   
   MDT is forecast for those mountain areas as they will be the most   
   susceptible to flooding considering the antecedent environment.   
   Rainfall rates of 0.5-1"/hr expected in heaviest cells, plenty   
   enough to cause scattered flash flooding prospects.=20   
      
   Kleebauer   
      
   ..Previous Discussion..=20   
      
   A Moderate Risk remains in effect for a large portion of the coast   
   from San Luis Obispo through much of the Transverse Ranges north   
   of L.A. Expect much of the heaviest rain to occur on D2/Wednesday,   
   but by Christmas Day, there will still be one more front that will   
   cross the area from west to east, which will result in another   
   round of heavy rainfall in southern California. Since this next   
   front will arrive soon after the heaviest rains from Wednesday,   
   expect there will be outsized impacts from the heavy rainfall   
   associated therewith. Showers and maybe even a few thunderstorms   
   will impact much of coastal southern California and the adjacent   
   mountains. While each one will be fast moving, the rivers and   
   streams all across southern California will likely already be   
   flooding, so any additional rainfall, especially where heavy, will   
   make ongoing flooding worse.   
      
   Campbell/Wegman   
      
      
   Day 3   
   Valid 12Z Fri Dec 26 2025 - 12Z Sat Dec 27 2025   
      
   ...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS SOUTHERN   
   CALIFORNIA...   
      
   20Z Update: No changes to the previous forecast SLGT risk as the   
   waning atmospheric river will linger through Friday afternoon   
   before finally ending. Any additional rainfall could initiate flash   
   flooding due to widespread saturated grounds with some ongoing=20   
   flooding still plausible from the previous periods of heavy   
   rainfall. Final totals of 8-12+ inches are likely over parts of the   
   San Bernardino mountains for the entire event, so any rainfall   
   would pose problems, even of the moderate variety.   
      
   Kleebauer   
      
   ..Previous Discussion..   
      
   The atmospheric river will be waning however given the significant   
   accumulations leading up to this period soils remain hyper   
   sensitive to any additional precipitation, particularly across the   
   Transverse Range and surrounding locations. As such, a Slight Risk   
   area was maintained for this part of the state along with a   
   Marginal Risk that extends up the coast toward San Francisco and   
   along the foothills of the Sierra Nevada Range.   
      
   Campbell   
      
      
   Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/94epoints.txt__;!!DZ3fjg!_gT2YbZIttiUbRl3xdyLhfA9TfhOV4_bwGWRnrAR_bEg=   
   yNOh-sdIG7Fc5poEUKs1bV_8D67jVwk9SXx3MWIDbg-fzeg$=20   
   Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/98epoints.txt__;!!DZ3fjg!_gT2YbZIttiUbRl3xdyLhfA9TfhOV4_bwGWRnrAR_bEg=   
   yNOh-sdIG7Fc5poEUKs1bV_8D67jVwk9SXx3MWIDbnBI2Lc$=20   
   Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/99epoints.txt__;!!DZ3fjg!_gT2YbZIttiUbRl3xdyLhfA9TfhOV4_bwGWRnrAR_bEg=   
   yNOh-sdIG7Fc5poEUKs1bV_8D67jVwk9SXx3MWID07nq2yg$=20   
      
   =3D =3D =3D   
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