Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Jan 01 2025 07:52:43 FOUS30 KWBC 010752 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 252 AM EST Wed Jan 1 2025 Day 1 Valid 12Z Wed Jan 01 2025 - 12Z Thu Jan 02 2025 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR SOUTHWEST=20 OREGON AND NORTHWEST CALIFORNIA... The 00Z HREF guidance has come in with a somewhat wetter footprint for today across areas of southwest Oregon and northwest California as a new round of offshore shortwave energy and proximity of a=20 front helps to bring in a new round of moderate to heavy rainfall.=20 The GFS and ECMWF solutions suggest IVT values increasing to near=20 500 kg/m/s along the immediate coast from Curry County, Oregon down through Del Norte and Humboldt Counties in northwest California. Offshore CMORPH2/microwave data shows heavier rainfall rates=20 associated with this next modest surge in atmospheric river activity that will be arriving, and the HREF guidance suggests=20 rainfall rates will generally peak in the 0.25" to 0.50"/hour range with perhaps an occasional instance or two of 0.50"+/hour rates.=20 Going through 12Z/Thursday, the HREF consensus suggests some=20 24-hour rainfall amounts of 2 to 4 inches with a couple of spotty=20 5+ inch amounts possible, and especially for northwest California where some of the higher IVT values will persist a bit longer. The antecedent conditions are wet across the region, and these rains=20 may cause some isolated runoff concerns. As a result, a Marginal=20 Risk of excessive rainfall has been depicted for this area. Orrison Day 2 Valid 12Z Thu Jan 02 2025 - 12Z Fri Jan 03 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF=20 SOUTHWESTERN OREGON AND NORTHWESTERN CALIFORNIA... Multiple impulses of upper level energy racing along a 150 kt WSW=20 jet will move into the Oregon and California coast near the state=20 line during this Day 2/Thursday period. The first is a front that=20 will have already moved into the area tonight, and will diminish in=20 intensity and slowly shift north through the early morning. A second=20 impulse moves in Thursday afternoon, resulting in a much more=20 concentrated area of rain about 50 miles or so to either direction=20 of the state line. Finally, a third and the strongest impulse will=20 move in late Thursday night and Friday morning. Each of these 3=20 impulses of energy will cause rainfall rates to pick up on the=20 Oregon and far northern California coast. None of them are expected=20 to advect in much instability, so upslope will likely be the primary=20 forcing mechanism for rainfall. With nearly continuous rainfall,=20 albeit to varying intensity with each impulse over already very=20 saturated ground, runoff and small stream flooding will be possible. Since the heavier/steadier rainfall will be ongoing at=20 the start of the period over northern California, and guidance has=20 been creeping up as far as total rainfall amounts go, the Marginal=20 Risk extends south into Humboldt County to account for continuing=20 rainfall and isolated runoff problems that may be ongoing from the Day 1/Wednesday period. Wegman Day 3 Valid 12Z Fri Jan 03 2025 - 12Z Sat Jan 04 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR FAR=20 SOUTHWESTERN OREGON AND PORTIONS OF NORTHERN CALIFORNIA... A strong shortwave and associated surface low will plow into the=20 coast of California and Oregon on Friday. Locally heavy rain with=20 rates around 1/2 inch per hour will be ongoing Friday morning along=20 the coast. The shortwave and low will move into the coast Friday=20 evening and through the overnight. This will shift the heaviest=20 rains both southward and inland, bringing northern portions of the=20 Sacramento Valley, which has been largely missed with the previous=20 days' rains into the potential for heavy rain and higher elevation=20 snow. The Marginal Risk inherited was left unchanged with this=20 issuance. The heaviest rains are likely to be Friday morning as the=20 aforementioned shortwave and surface low approach. Once they move=20 into the coast Friday evening and overnight, rainfall rates should=20 diminish quite a bit as the associated moisture moves inland. Any=20 flooding from the Day 2/Thursday period will likely continue into=20 Friday morning along the Oregon and California coasts...with the=20 flooding for the Sacramento Valley more likely Friday afternoon. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-WVL2toOY_f6DpCmDYn4q231pvomp3QYT5Cwb9KIKkZZ= tsY3DtJLC6pTvCvWBGomVZFy3FlLkIOfHQ260eZhIG5sCT8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-WVL2toOY_f6DpCmDYn4q231pvomp3QYT5Cwb9KIKkZZ= tsY3DtJLC6pTvCvWBGomVZFy3FlLkIOfHQ260eZhkujEBcM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-WVL2toOY_f6DpCmDYn4q231pvomp3QYT5Cwb9KIKkZZ= tsY3DtJLC6pTvCvWBGomVZFy3FlLkIOfHQ260eZhS3rTmnY$=20 =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .