Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Jan 01 2025 18:47:10 FOUS30 KWBC 011846 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 146 PM EST Wed Jan 1 2025 Day 1 Valid 16Z Wed Jan 01 2025 - 12Z Thu Jan 02 2025 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR SOUTHWEST OREGON AND NORTHWEST CALIFORNIA... ...16Z Outlook Update... Current observational/model trends support continued, yet spotty areas of ~0.25 inch/hr rain rates at times within the Marginal Risk. These rain rates are falling on wet soils from recent rainfall events, and could prompt minor/localized excessive runoff and flood issues. The bulk of the heavier rainfall should persist through about 03Z today before tapering off thereafter. Rainfall totals of 1-2 inches (locally higher) are possible. These trends continue to support a Marginal Risk, and no changes are made to the previous outlook. See the discussion below for more information. Cook ....Previous Discussion... 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 front helps to bring in a new round of moderate to heavy rainfall. The GFS and ECMWF solutions suggest IVT values increasing to near 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 associated with this next modest surge in atmospheric river activity that will be arriving, and the HREF guidance suggests 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. Going through 12Z/Thursday, the HREF consensus suggests some 24-hour rainfall amounts of 2 to 4 inches with a couple of spotty 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 may cause some isolated runoff concerns. As a result, a Marginal 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 SOUTHWESTERN OREGON AND NORTHWESTERN CALIFORNIA... ....21Z Outlook Update... The ongoing forecast is on track and no changes are needed. A few areas of 0.25-0.5 inch/hr rain rates are expected throughout the forecast period, with the greatest focus for relatively heavier rainfall expected to occur across southwestern Oregon both at the start of the forecast period (around 12-18Z) and peaking again in the 06-12Z Friday period. 2-4 inch rainfall totals are expected to fall on saturated grounds, likely prompting a few areas of excess runoff/minor flooding. See the previous discussion for more details. Cook ....Previous Discussion... Multiple impulses of upper level energy racing along a 150 kt WSW jet will move into the Oregon and California coast near the state line during this Day 2/Thursday period. The first is a front that will have already moved into the area tonight, and will diminish in intensity and slowly shift north through the early morning. A second impulse moves in Thursday afternoon, resulting in a much more concentrated area of rain about 50 miles or so to either direction of the state line. Finally, a third and the strongest impulse will move in late Thursday night and Friday morning. Each of these 3 impulses of energy will cause rainfall rates to pick up on the Oregon and far northern California coast. None of them are expected to advect in much instability, so upslope will likely be the primary forcing mechanism for rainfall. With nearly continuous rainfall, albeit to varying intensity with each impulse over already very saturated ground, runoff and small stream flooding will be possible. Since the heavier/steadier rainfall will be ongoing at the start of the period over northern California, and guidance has been creeping up as far as total rainfall amounts go, the Marginal Risk extends south into Humboldt County to account for continuing 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 SOUTHWESTERN OREGON AND PORTIONS OF NORTHERN CALIFORNIA... ....21Z Outlook Update... The ongoing forecast is on track and no changes are needed. A=20 strong cold front will move inland across northern California=20 through the day Friday. The front will aid in continued areas of moderate to heavy rain early in the forecast period, although eventually a focused area of convection/orographic lift will promote heavier rainfall across the windward side of the Sierra and adjacent Sacramento Valley regions. Isolated instance of flooding/flash flooding remain possible given the aforementioned scenario. See the previous discussion for more details. Cook ....Previous Discussion... A strong shortwave and associated surface low will plow into the coast of California and Oregon on Friday. Locally heavy rain with rates around 1/2 inch per hour will be ongoing Friday morning along the coast. The shortwave and low will move into the coast Friday evening and through the overnight. This will shift the heaviest rains both southward and inland, bringing northern portions of the Sacramento Valley, which has been largely missed with the previous days' rains into the potential for heavy rain and higher elevation snow. The Marginal Risk inherited was left unchanged with this issuance. The heaviest rains are likely to be Friday morning as the aforementioned shortwave and surface low approach. Once they move into the coast Friday evening and overnight, rainfall rates should diminish quite a bit as the associated moisture moves inland. Any flooding from the Day 2/Thursday period will likely continue into Friday morning along the Oregon and California coasts...with the 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!8ObqAC1d1I6fbkQ4arjIAmCJkm6g4gW6qRcSY8HW32dR= 2DyncHDZLbWUJpZoDgLtD-2AIqQWIgl2t23TQmj620mEirw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8ObqAC1d1I6fbkQ4arjIAmCJkm6g4gW6qRcSY8HW32dR= 2DyncHDZLbWUJpZoDgLtD-2AIqQWIgl2t23TQmj6M1fsc_4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8ObqAC1d1I6fbkQ4arjIAmCJkm6g4gW6qRcSY8HW32dR= 2DyncHDZLbWUJpZoDgLtD-2AIqQWIgl2t23TQmj640p6g9U$=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) .