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   FOUS30 KWBC 230056   
   QPFERD   
      
   Excessive Rainfall Discussion   
   NWS Weather Prediction Center College Park MD   
   756 PM EST Mon Dec 22 2025   
      
   Day 1   
   Valid 01Z Tue Dec 23 2025 - 12Z Tue Dec 23 2025   
      
   ...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL IN EFFECT FOR=20   
   PORTIONS OF NORTHERN CALIFORNIA...   
      
   ...01Z Update...   
      
   The atmospheric river impacting California continues to weaken this   
   evening as it drift northward through the northern Sacramento   
   Valley. Rainfall rates in excess of 0.25"/hr are rather sparse, but   
   earlier heavier rain has left much of the area saturated. HI-res   
   models show a lull in the QPF overnight through 12Z, with   
   additional QPF generally under 0.50" in most places and only near   
   1" in the northern Sierra. Have removed the Slight Risk area in   
   response to the diminished threat overnight. The next surge in   
   moisture/rainfall is expected after 12Z Tuesday.=20   
      
   Fracasso   
      
   Day 2   
   Valid 12Z Tue Dec 23 2025 - 12Z Wed Dec 24 2025   
      
   ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF   
   CALIFORNIA...   
      
   2100 UTC update: No changes made to the previous   
      
   Previous discussion...   
      
   The trajectory of the atmospheric river will become more S/SW to   
   N/NE as it shifts southward during this period as the trough   
   offshore in the Pacific also digs to the south. In contrast to   
   the air mass containing the atmospheric river, which will be warm   
   and moisture-laden, this contrast will support cyclogenesis along   
   the greatest gradient/boundary between the cold air mass within the   
   trough and the much warmer air mass in the ridge/atmospheric   
   river. As the low deepens the onshore flow will align parallel to   
   the coastline thus significantly increasing the areal extent heavy   
   rainfall and the higher terrain snow accumulations. The onshore   
   flow associated with the low itself will continue very heavy rain   
   and mountain snow across northern California, while the trailing   
   cold front moves into southern California starting Tuesday night.   
      
   The areas of greatest impact will be into the northern Coastal   
   Ranges and the Transverse Ranges. Both these 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 renewed round of heavy rain   
   will be in areas getting hard hit by the current atmospheric river   
   and will continue with the deepening low offshore into the   
   northern coastal ranges. A Slight Risk was maintained for the   
   Oregon border down through Los Angeles.   
      
   Campbell/Wegman   
      
      
   Day 3   
   Valid 12Z Wed Dec 24 2025 - 12Z Thu Dec 25 2025   
      
   ...THERE IS A HIGH RISK FOR EXCESSIVE RAINFALL ACROSS SOUTHERN   
   CALIFORNIA...   
      
   2100 UTC UPDATE:   
      
   The primary change to the previous day 3 Excessive Rainfall Outlook   
   was to add a high risk area along the west to east Transverse   
   Range region of Southern California after collaboration with WFO   
   LOX. Anomalous 850-700 mb moisture flux...5+ standard deviations   
   above the mean...will impact Southern CA late day 2 into day 3   
   ahead of the amplifying mid to upper level trof off the CA 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   
   .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.   
      
   Oravec   
      
      
   Previous discussion...Widespread heavy rainfall will ramp up even   
   further across the Transverse Range and surrounding locations of   
   southern California. Areal average of 3 to 5 inches are expected   
   but isolated local maximums may reach as much as 9 inches. A   
   Moderate Risk is in effect for a majority of Santa Barbara,   
   Ventura, Los Angeles, range, San Bernardino, Riverside and San   
   Diego Counties. 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, which is   
   covered by a sprawling Slight Risk area. Deep-layered moisture will   
   also track eastward across eastern California, southern Nevada and   
   into southern Utah and western Arizona where areal averages up to   
   1 to 2+ are forecast. A new Slight Risk area was raised for much of   
   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   
      
   Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/94epoints.txt__;!!DZ3fjg!7sTPOVrfXFOh3QFxcyY7QYAn-EtCWJHn2SmvnfcFs1kI=   
   Chga3QRQMyrg7b09TreZnSxd9Bi_lAhnYK9KXmiUX2HuwRI$=20   
   Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/98epoints.txt__;!!DZ3fjg!7sTPOVrfXFOh3QFxcyY7QYAn-EtCWJHn2SmvnfcFs1kI=   
   Chga3QRQMyrg7b09TreZnSxd9Bi_lAhnYK9KXmiUKuFkNQU$=20   
   Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/99epoints.txt__;!!DZ3fjg!7sTPOVrfXFOh3QFxcyY7QYAn-EtCWJHn2SmvnfcFs1kI=   
   Chga3QRQMyrg7b09TreZnSxd9Bi_lAhnYK9KXmiU3qam5hA$=20   
      
   =3D =3D =3D   
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