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   FOUS30 KWBC 221550   
   QPFERD   
      
   Excessive Rainfall Discussion   
   NWS Weather Prediction Center College Park MD   
   1050 AM EST Mon Dec 22 2025   
      
   Day 1   
   Valid 16Z Mon Dec 22 2025 - 12Z Tue Dec 23 2025   
      
   ...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL IN EFFECT FOR=20   
   PORTIONS OF CENTRAL AND NORTHERN CALIFORNIA...   
      
   ...16Z Update...   
      
   The atmospheric river impacting California is in a bit of a lull   
   today. IVT values, a measure of the moisture advection from off the   
   Pacific, will be dropping rapidly through the day. This means that   
   the wind will be less and less cooperative with advecting moisture   
   into the Sierra Nevada, resulting in diminishing upslope=20   
   enhancement of the rain. That said, the plume of moisture moving up   
   I-80 from the Bay Area into the central Sierra Nevada range will   
   trend north as a digging trough in the upper levels realigns the   
   flow to more out of the SSW. This too will gradually diminish the   
   rainfall rates in the hard-hit upslope regions.   
      
   As the plume of rain drifts north this afternoon, expect a renewed   
   round of rain into the hardest hit portions of the Sierra Nevada   
   from Oroville east. This is likely to cause additional or renewed   
   flooding...however the diminishing rainfall rates should keep   
   impacts below Moderate Risk criteria. The rain will continue north   
   into the northern Sacramento Valley by tonight due to the   
   increasingly southerly flow. While soils are also saturated from   
   recent rainfall in this area, since the predominant flow prior to   
   today was westerly, it should be different slopes of the northern   
   coast ranges and the Klamath Mountains that get the heaviest rain   
   today, and even then, due to the diminishing pressure gradient,   
   should not be anywhere near as heavy as prior days overnight   
   tonight.   
      
   Given the transient nature of the rain plume today, the Moderate   
   Risk has been downgraded to a Slight, with no changes to that   
   Slight or the surrounding Marginal. Impacts from the rain that does   
   fall today could still include land slides, mud slides, and flash=20   
   flooding in smaller creeks that have had some time to drain a bit=20   
   this morning.   
      
   Wegman   
      
      
   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...   
      
   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 MODERATE RISK FOR EXCESSIVE RAINFALL ACROSS SOUTHERN   
   CALIFORNIA...   
      
   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!5FEEMgEHYuQjzag0MPa2J6VPeF4oMA4BDhUWgl3ELArv=   
   o00F0DzY2pQOdloZyEI7ou_P4JPUJSLUfvxwJEdCgn_mC7I$=20   
   Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/98epoints.txt__;!!DZ3fjg!5FEEMgEHYuQjzag0MPa2J6VPeF4oMA4BDhUWgl3ELArv=   
   o00F0DzY2pQOdloZyEI7ou_P4JPUJSLUfvxwJEdCHCRaXeI$=20   
   Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/99epoints.txt__;!!DZ3fjg!5FEEMgEHYuQjzag0MPa2J6VPeF4oMA4BDhUWgl3ELArv=   
   o00F0DzY2pQOdloZyEI7ou_P4JPUJSLUfvxwJEdCQnXTZ1c$=20   
      
   =3D =3D =3D   
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