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 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. 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://www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt