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   FOUS30 KWBC 230828   
   QPFERD   
      
   Excessive Rainfall Discussion   
   NWS Weather Prediction Center College Park MD   
   328 AM EST Tue Dec 23 2025   
      
   Day 1   
   Valid 12Z Tue Dec 23 2025 - 12Z Wed Dec 24 2025   
      
   ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF=20   
   CALIFORNIA...   
      
   The S/SW to N/NE orientated atmospheric river is expected to   
   maintain scattered to widespread rainfall along the coastline and   
   inland to the Sierra Nevada range with rain for the lower   
   elevations and snow for the higher elevations. Cyclogenesis will   
   occur along the greatest gradient/boundary between the cold air=20   
   mass within the trough and the much warmer air mass in the=20   
   ridge/atmospheric river. As the low deepens the onshore flow will=20   
   align parallel to the coastline thus significantly increasing the=20   
   areal extent heavy rainfall and the higher terrain snow=20   
   accumulations.   
      
   The areas of greatest impact will be into the=20   
   northern Coastal Ranges and the Transverse Ranges. Both these=20   
   ranges are sufficiently aligned perpendicular to the south-   
   southwesterly flow to have a significant local upslope contribution   
   to rainfall rates. In the Transverse Ranges, local burn scars will   
   also worsen impacts from resultant flooding over and downstream of   
   any burn scars north and west of the Los Angeles Basin. The=20   
   renewed round of heavy rain will be in areas getting hard hit by=20   
   the current atmospheric river and will continue with the deepening=20   
   low offshore into the northern coastal ranges. A Slight Risk was=20   
   maintained for the Oregon border down through Los Angeles along   
   with a Marginal Risk encompassing much of the same area.   
      
   Campbell/Wegman   
      
      
   Day 2   
   Valid 12Z Wed Dec 24 2025 - 12Z Thu Dec 25 2025   
      
   ...THERE IS A HIGH RISK FOR EXCESSIVE RAINFALL ACROSS SOUTHERN   
   CALIFORNIA...   
      
   Highly anomalous 850-700 mb moisture flux on the order of 5+=20   
   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=20   
   south to north precip band in this anomalous moisture flux axis=20   
   with hourly rainfall rates of 0.50-1.5"+ likely. The potential for   
   high hourly rainfall totals will accentuate runoff over soil that=20   
   is relatively dry and or over recent burn scar areas. This will=20   
   pose a significant threat of flash flooding, landslides, rock falls   
   and mud slides. The greater than normal travel during the=20   
   Christmas Holiday will likely expose a potentially larger number of   
   people to these lift threatening hazards and was taken into=20   
   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=20   
   significant snow, some of the lower elevations could receive=20   
   additional rainfall of 1 to 4 inches, which is covered by a=20   
   sprawling Slight Risk area. Deep-layered moisture will also track=20   
   eastward across eastern California, southern Nevada and into=20   
   southern Utah and western Arizona where areal averages up to 1 to=20   
   2+ are forecast. A new Slight Risk area was raised for much of=20   
   southern Nevada, along the California/Nevada border and for western   
   Arizona. Rainfall rates will easily surpass 0.5-1.0 inch/hr rates,   
   particularly for California.   
      
   Campbell/Oravec   
      
      
   Day 3   
   Valid 12Z Thu Dec 25 2025 - 12Z Fri Dec 26 2025   
      
   ...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF=20   
   CENTRAL AND SOUTHERN CALIFORNIA...   
      
   The atmospheric river will continue to direct anomalous Pacific=20   
   moisture into California resulting in heavy rainfall across the=20   
   state. With the trough progressing inland, the Sierra Nevada Range   
   should see an significant/extended snow event in the higher=20   
   elevations with heavy rainfall expected in the lower elevations. A   
   Slight Risk area seems appropriate for this period given some of   
   the highest QPF forecast will be in the form of snow thus reducing   
   the threat for rapid runoff and flooding concerns. Southern   
   California can be considered on the higher end of Slight for this   
   period since it will be trying to recover from several inches of   
   rain expected for the Day 2 period.   
      
   Campbell   
      
      
   Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/94epoints.txt__;!!DZ3fjg!8OELiHv1PKn7Seic1J0n0T4CORqrntG6dEz20fFBeN8_=   
   m6DKUWwyMtJygSugQoRqHwMCmT_l54MuncOZMBFXo0bh_E8$=20   
   Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/98epoints.txt__;!!DZ3fjg!8OELiHv1PKn7Seic1J0n0T4CORqrntG6dEz20fFBeN8_=   
   m6DKUWwyMtJygSugQoRqHwMCmT_l54MuncOZMBFXXEZ1a-w$=20   
   Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/99epoints.txt__;!!DZ3fjg!8OELiHv1PKn7Seic1J0n0T4CORqrntG6dEz20fFBeN8_=   
   m6DKUWwyMtJygSugQoRqHwMCmT_l54MuncOZMBFXB7QBrEE$=20   
      
   =3D =3D =3D   
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