Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Dec 26 2022 08:31:50 FOUS30 KWBC 260831 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 331 AM EST Mon Dec 26 2022 Day 1 Valid 12Z Mon Dec 26 2022 - 12Z Tue Dec 27 2022 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR COASTAL PORTIONS OF SOUTHERN OREGON AND NORTHERN CALIFORNIA... A shortwave trough will weaken as it drifts into British Columbia today. A potent upper-level low will move into the Pacific Northwest region tonight, kicking off a relatively brief but impactful atmospheric river event. A dying cold front with weak onshore flow will spread rain showers across the coastal ranges of the Pacific Northwest this morning before mostly diminishing this afternoon. A stronger low pressure system will move into the region early this evening, bringing with it a deeper plume of subtropical moisture. PWATs of around 1.25 inches are likely with this moisture regime. Rain rates as high as 0.75in/hr and a 50kt low level jet will contribute to the slight risk of excessive rainfall over the coastal ranges of southern Oregon and northern California. Relatively moist/susceptible soils will provide favorable conditions for flash flooding of burn scars and urban areas. High snow levels will certainly allow for heavy rainfall/flash flooding potential along the windward side of the Cascades. Kebede Day 2 Valid 12Z Tue Dec 27 2022 - 12Z Wed Dec 28 2022 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER COASTAL PORTIONS OF CALIFORNIA AND SOUTHERN OREGON... Anomalous moisture associated with a fast moving atmospheric river will glide down the California coast and Sierra on Tuesday. Moderate to heavy rainfall (rates of 0.1-0.25in/hr) will spread down the state of California where a slight risk of excessive rainfall is in effect. Orographic enhancement and forcing from a cold front will allow for higher amounts over the foothills of the Sierra and heavy snowfall at higher elevations, but a marginal risk will suffice in these areas for now. PWATs near 1.5 inches (4-5 standard deviations from normal) will lead to instances of flash flooding from the Bay area early Tuesday morning to Los Angeles County later in the afternoon. Remnant moisture will spread across the Pacific Northwest, generating heavy snow in the Cascades and modest rainfall over the coastal ranges and Cascades foothills Tuesday morning, where a marginal risk is in effect. Kebede Day 3 Valid 12Z Tue Dec 27 2022 - 12Z Wed Dec 28 2022 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE COASTAL RANGES OF SOUTHERN OREGON DOWN TO SOUTHERN/CENTRAL CALIFORNIA... ....2030Z Update... No major changes made. The front that moves into Northern CA continues to sag southward on Tuesday. It will weaken as it does so=C2but additional rainfall in many of the same areas as on Monday will keep the flash flood threat elevated for much of the CA coast north of L.A. Most of the heaviest precipitation will be in the Sierras, and in the coastal mountains between Point Conception and Monterey Bay. The greatest point of uncertainty will be the snow level in the Sierras, as most of the highest 24-hour precipitation totals will be in this area. Depending on how much of that precipitation falls as snow may require a localized upgrade to a Slight Risk in future updates. As the main forcing shifts east into AZ by Wednesday=C2the mountains from L.A. southward appear unlikely to be under threat for heavy rain with this particular rainfall event. Wegman ....Previous Discussion... A potent low pressure system will continue to advect anomalous moisture into the West Coast on Tuesday. Pacific moisture associated with an atmospheric river will work its way down the California coast on Tuesday. The upper-level shortwave energy from Monday will open up into a longwave trough as it moves into the Western half of the country. The heaviest rainfall with rates of around 0.1-0.25in/hr will glide down the central California coast Tuesday morning into the afternoon carrying PWATs of around 1-1.5 inches. The greatest flash flooding threat will be over burn scars and urban areas. Freezing temperatures will make their way into the Pacific Northwest and western mountain ranges which will support snowfall in the northern Cascades and Sierra. A 40-50kt low level jet will weaken as the day goes on and the axis of precip shifts southward. Kebede Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-92t64oM_rV5hMT_t3TqaaFNlbfYHe3Y-kUeygH7I3BE= Hsv5RnOAlX4pSE07FSJAkdDA2F2OezNi0PGXdb7yn20LJYw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-92t64oM_rV5hMT_t3TqaaFNlbfYHe3Y-kUeygH7I3BE= Hsv5RnOAlX4pSE07FSJAkdDA2F2OezNi0PGXdb7ypbBOgu4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-92t64oM_rV5hMT_t3TqaaFNlbfYHe3Y-kUeygH7I3BE= Hsv5RnOAlX4pSE07FSJAkdDA2F2OezNi0PGXdb7yO6Nf1O0$=20 $$ --- SBBSecho 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .