Subj : HVYRAIN: Moderate Risks P To : All From : Mike Powell Date : Mon Dec 22 2025 09:10:20 FOUS30 KWBC 220809 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 309 AM EST Mon Dec 22 2025 Day 1 Valid 12Z Mon Dec 22 2025 - 12Z Tue Dec 23 2025 ....THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL IN EFFECT FOR PORTIONS OF THE SIERRA NEVADA IN NORTHERN CALIFORNIA... Deep moisture continues to surge inland across portions of northern and central California dumping heavy rainfall across the northern Sierra Nevada Range. Much of the forecast QPF footprint of 2-3+ inches will be where several inches occurred and flooding and debris flows were observed yesterday. The Moderate Risk area was maintained since accumulations of this magnitude will likely lead to several instances of flooding, sloughing and debris flows, especially near recent burn scars. Campbell 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 $$ --- SBBSecho 3.28-Linux * Origin: ILink: CCO - capitolcityonline.net (454:3/105) .