Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Jan 12 2023 08:30:46 FOUS30 KWBC 120830 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 329 AM EST Thu Jan 12 2023 Day 1 Valid 12Z Thu Jan 12 2023 - 12Z Fri Jan 13 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE NORTHERN CALIFORNIA COAST... Models have have continued to shift slightly farther north and west with the plume of heaviest rainfall through today and tonight, so the Slight and Marginal Risk areas in northern CA were shifted and trimmed a little bit more with this update. That said, all rain that falls in the Slight and Marginal Risk areas of CA will be falling on very saturated soils, so nearly all of it is expected to run off, contributing to continued elevated flash flooding concerns, especially over burn scars and poor drainage areas. Amounts of 1-2" are generally expected, but with the gradient so close to the coast amounts of 2-4" could easily in the Slight Risk area with a small shift to the east (best represented by the 00z ARW). The combination of the atmospheric river (AR) aligning more parallel to the coast and being farther offshore should mean fewer extremely high measurements in the coastal mountains, as those totals are more likely when the AR is aligned more orthogonal to the coast. The Olympic Mountains, however, are aligned to soak up more rainfall on a nearly due southerly flow, so multiple inches of rain are possible. Fortunately it's been less wet than areas further south into CA, so widespread flash flooding issues are not expected at this time (especially given that rates here should generally remain below 0.5"/hr). Churchill/Wegman Day 2 Valid 12Z Fri Jan 13 2023 - 12Z Sat Jan 14 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE NORTHERN CALIFORNIA COAST AND ADJACENT MOUNTAINS... The AR (atmospheric river) that is expected to remain largely off the coast of CA on Thursday will move back into northern CA on Friday. The 24-36 hour break (shorter the further north you go) is not expected to be enough time for there to be much drainage of prior days rains before this next round moves in. Most of the heaviest rain (1-3" of additional rainfall across the Slight Risk area and over portions of the Olympic Peninsula) will be in the early morning hours as the main AR moves in, after which the front will weaken as it moves south down the Central Valley. Behind the AR, the usual off-and-on shower activity moving directly into the coast is expected, so coastal Northern California is not expected to be completely dry Friday afternoon and overnight, but for the most part the individual showers are not expected to produce widespread flooding activity. Churchill/Wegman Day 3 Valid 12Z Sat Jan 14 2023 - 12Z Sun Jan 15 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NORTHERN AND CENTRAL CALIFORNIA... Unfortunately, yet another AR is expected to come ashore during the early morning hours of Saturday. This will bring an additional 2-3" of rainfall to much of the Northern CA coastline, in addition to the foothills of the Sierra Nevada. Elsewhere along much of the CA coast, 1-2" of additional rainfall is expected.. classifying as a fairly weak AR, overall. However, given the very wet antecedent conditions, much of this rainfall will translate to immediate runoff and potential flooding concerns (particularly for burn scars and poor drainage areas). The weakening trend of the AR overall should help to alleviate widespread flooding concerns, but locally upslope totals may reach as high as 3-5".. which may necessitate targeted upgrades to Moderate Risk for future updates. Churchill Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-UgjtKgHT_sMD2NhU-2Y2kiQvQzMXVUZHHwErhA7NT_T= _w86pxKifGhe3VdRTcP6oAlpz5Q690_jelVqzYZTaUw4-6Q$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-UgjtKgHT_sMD2NhU-2Y2kiQvQzMXVUZHHwErhA7NT_T= _w86pxKifGhe3VdRTcP6oAlpz5Q690_jelVqzYZT-9Z4N3Q$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-UgjtKgHT_sMD2NhU-2Y2kiQvQzMXVUZHHwErhA7NT_T= _w86pxKifGhe3VdRTcP6oAlpz5Q690_jelVqzYZT0pC1bMs$=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 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .