Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Nov 23 2024 07:48:22 FOUS30 KWBC 230748 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 248 AM EST Sat Nov 23 2024 Day 1 Valid 12Z Sat Nov 23 2024 - 12Z Sun Nov 24 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SIERRA NEVADA... A cold front and the remnants of the long duration, multi-day atmospheric river that has been impacting the West Coast will be=20 dropping south across the central Sierra Nevada early this morning. As the deeper layer moisture flux parameters/IVT magnitudes=20 continue to weaken and thin out, the rainfall rates will be=20 continuing to drop off. However, there still may be a sufficient level of upslope flow/orographic ascent in the foothills of the=20 central Sierra Nevada for a few more hours early this morning of spotty 0.25" to 0.50"/hour rainfall rates. Some weak post-frontal instability is also still forecast which could allow for some=20 scattered pockets of heavy shower activity going through midday to=20 impact some of the western slopes of the central Sierra Nevada.=20 This could result in some isolated quick- hitting 0.25" to 0.50"=20 rainfall amounts, but these convective cells are forecast to be=20 progressive. Overall, the additional storm total amounts based off=20 the 00Z HREF guidance may reach as high as 1 to 2 inches. Given the very wet antecedent conditions, the additional rains may foster some additional localized runoff/flooding concerns. Thus,=20 the Marginal Risk area over portions of the Sierra Nevada will be=20 maintained. Orrison Day 2 Valid 12Z Sun Nov 24 2024 - 12Z Mon Nov 25 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF NORTHWEST CALIFORNIA INTO FAR SOUTHWEST OREGON... Locally heavy rain will be possible Sunday into Sunday night over=20 northwest CA into far southwest OR. A front slowing near the coast=20 and a weak low to mid level wave riding along it will support a=20 modest uptick in IVT and plentiful low level convergence. Moisture=20 is a limiting factor for heavy rainfall with the better moisture=20 plume situated well south and not really getting drawn into this=20 system. Thus this is not expected to be a significant atmospheric=20 river, and normally would not expect a system of this nature to=20 pose a flood risk. However given the significant rainfall over the=20 past few days, this additional rainfall may be enough to cause some localized additional flooding as the area will be more=20 hydrologically sensitive than normal.=20 We are still seeing some spread in model QPF output over this=20 period. If some of the more aggressive solutions pan out then we=20 could see 1-3" of rainfall is spots with hourly totals peaking=20 around 0.3-0.4" in the typically more favored upslope areas.=20 Chenard Day 3 Valid 12Z Mon Nov 25 2024 - 12Z Tue Nov 26 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF=20 NORTHERN CALIFORNIA INTO FAR SOUTHWEST OREGON... Showery conditions will likely continue across portions of northern CA and southwest OR Monday into Monday night as a deep layered low and shortwave energy moves inland. Certainly not a classic setup=20 for flooding with limited moisture and dropping IVT=20 values...however this area remains more hydrologically sensitive=20 than normal following the significant atmospheric river event the=20 past few days. Additional rainfall over far northwest CA into=20 southwest OR is generally 1-3" in the guidance, with isolated=20 totals of this magnitude also possible over the western slopes of=20 the northern Sierra Nevada. A more significant plume of moisture/IVT will actually move into=20 portions of southern/central CA...generally from portions of San=20 Luis Obispo and Monterey counties into the western slopes of the=20 southern Sierra Nevada. QPF has been trending up with this system,=20 although still seeing some model spread. The latest model consensus=20 suggests a possible 2-3" of rain along the coastal ranges and the=20 southern Sierra below snow levels. Not seeing any instability with=20 this system and large scale forcing is only modest, so hourly=20 rainfall rates are generally not expected to exceed 0.5". Given=20 this, and the fact that most of this area has not seen significant=20 rainfall recently, will continue with no areas in the excessive=20 rainfall outlook for now. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6yeO_D40cNklhUiMlxIM4a7rsIHA0yVsXyWBCuurEtH4= 85DTi92n3jEwspeJtV6NBSTQjrc5sQyL8zRYWuHrkmJbmcI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6yeO_D40cNklhUiMlxIM4a7rsIHA0yVsXyWBCuurEtH4= 85DTi92n3jEwspeJtV6NBSTQjrc5sQyL8zRYWuHrzrOP1S8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6yeO_D40cNklhUiMlxIM4a7rsIHA0yVsXyWBCuurEtH4= 85DTi92n3jEwspeJtV6NBSTQjrc5sQyL8zRYWuHrXKMA4zM$=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) .