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   FOUS30 KWBC 221929   
   QPFERD   
      
   Excessive Rainfall Discussion   
   NWS Weather Prediction Center College Park MD   
   229 PM 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   
   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   
   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   
   flooding in smaller creeks that have had some time to drain a bit   
   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...   
      
   2100 UTC update: No changes made to the previous=20   
      
   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=20   
   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=20   
   Range region of Southern California after collaboration with WFO=20   
   LOX. Anomalous 850-700 mb moisture flux...5+ standard deviations=20   
   above the mean...will impact Southern CA late day 2 into day 3=20   
   ahead of the amplifying mid to upper level trof off the CA coast.=20   
   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=20   
   in this anomalous moisture flux axis with hourly rainfall rates of=20   
   .50-1.5"+ likely. The potential for high hourly rainfall totals=20   
   will accentuate runoff over soil that is relatively dry and or over   
   recent burn scar areas. This will pose a significant threat of=20   
   flash flooding, landslides, rock falls and mud slides. The greater=20   
   than normal travel during the Christmas Holiday will likely expose=20   
   a potentially larger number of people to these lift threatening=20   
   hazards and was taken into consideration with the upgrade to high=20   
   risk.   
      
   Oravec=20   
      
      
   Previous discussion...Widespread heavy rainfall will ramp up even=20   
   further across the Transverse Range and surrounding locations of=20   
   southern California. Areal average of 3 to 5 inches are expected=20   
   but isolated local maximums may reach as much as 9 inches. A=20   
   Moderate Risk is in effect for a majority of Santa Barbara,=20   
   Ventura, Los Angeles, range, San Bernardino, Riverside and San=20   
   Diego Counties. Although snow levels will be lowering across the=20   
   Sierra Nevada and adjacent locations and much of the highest=20   
   elevations will have significant snow, some of the lower elevations   
   could receive additional rainfall of 1 to 4 inches, which is=20   
   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=20   
   1 to 2+ are forecast. A new Slight Risk area was raised for much of   
   southern Nevada, along the California/Nevada border and for=20   
   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!4GJzdPFfhSqIc6MvwQcc3NoHN-m1qkvITl-c1J9_i1Rm=   
   Ex9fE4PUyETE83bkzU3qOn7GgeyGlzEcAGzjpGO2UFY885I$=20   
   Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/98epoints.txt__;!!DZ3fjg!4GJzdPFfhSqIc6MvwQcc3NoHN-m1qkvITl-c1J9_i1Rm=   
   Ex9fE4PUyETE83bkzU3qOn7GgeyGlzEcAGzjpGO2vJYVl20$=20   
   Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/99epoints.txt__;!!DZ3fjg!4GJzdPFfhSqIc6MvwQcc3NoHN-m1qkvITl-c1J9_i1Rm=   
   Ex9fE4PUyETE83bkzU3qOn7GgeyGlzEcAGzjpGO2yQZr8jM$=20   
      
   =3D =3D =3D   
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