Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Feb 01 2025 00:23:58 FOUS30 KWBC 010023 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 723 PM EST Fri Jan 31 2025 Day 1 Valid 01Z Sat Feb 01 2025 - 12Z Sat Feb 01 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE APPALACHIANS AND PARTS OF THE WEST COAST... West Virginia... Main change from the previous outlook was to remove the Slight and to remove the Marginal Risk area from all but far eastern West=20 Virginia where rainfall was lingering in/near the terrain. Drier=20 air should continue to filter eastward during the evening=20 hours...with a corresponding decrease in rainfall amounts. With=20 some higher reflectivity echoes still located upstream...did not=20 want to completely remove the Marginal risk area but certainly=20 reduced the areal coverage. Latest MRMS depicts maximum hourly=20 rainfall rates barely above 0.25 inches with 3-hour rainfall totals generally remaining under 0.5 inches. This probably not enough to=20 generate flooding of a more flashy variety. However the right=20 combination of embedded convective elements over areas that have=20 already received rain and the amount of runoff that could occur=20 between the rainfall and snow melt...felt removing the risk areas=20 was a bit premature. West Coast...=20 Persistent on-shore flow of deep moisture will continue to support moderate to locally heavy rainfall from western Washington=20=20 southward into northern CA through tonight. The IVT plume is=20 generally progressive and weakening through the period, so not=20 expecting major rainfall totals or impacts. Generally forecasting=20 an additional 1 inch or less through 12Z Saturday with the higher=20 amounts in areas where the terrain is aligned perpendicular to the=20 wind direction. This would be the main driver for any isolated=20 minor flood threat overnight. Bann Day 2 Valid 12Z Sat Feb 01 2025 - 12Z Sun Feb 02 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CALIFORNIA... ....1930 UTC Update... Changes to the Day 2 ERO were mainly to expand the northern end of the Slight Risk area a bit farther north across northern CA, to include Redding and the Park Fire burn scar. This given the guidance trends (with the 12Z runs) along with uptick in 0.25-0.50"/hr rainfall rates per the HREF. Meanwhile, the latest CSU First-Guess ERO (UFVS verified version) supports the northern extent of the Slight as well. Hurley ....Discussion... The atmospheric river is forecast to stall over central/northern CA on Saturday. Most model solutions indicate an increase in IVT moving into the CA coast near San Francisco just after 12z Saturday, which should bring an uptick in rainfall rates onshore. PWs will be quite impressive for the time of year, with values forecast to exceed the 99th percentile. A lack of strong dynamics and frontal convergence, combined with only moderate IVT values, will cap the magnitude of rainfall rates. Nonetheless, expecting to see enough of an uptick in rainfall Saturday with the stalling AR to support an increase in flood impacts, warranting the Slight risk. While IVT values along coastal areas will be moderate, what this AR will have going for it is an impressive inland extent of IVT magnitudes...with values exceeding the 99th percentile (and around climatological max values) within the Sierra. This supports the inland extension of the Slight risk across the Sacramento Valley and into the mountains. Snow levels will be high, so the Slight risk extends pretty far into the terrain. We also made sure to extend the Marginal risk over the crest and into the eastern slopes of the Sierra, as given the IVT, we should see enough rain make it over the crest to support at least a minor flood risk. Two day rainfall totals are currently forecast upwards of 2-4" along coastal areas, 1-3" within Sacramento valley locations, and upwards of 4-7" within the Sierra Nevada (most of which will be rain given high snow levels). Chenard Day 3 Valid 12Z Sun Feb 02 2025 - 12Z Mon Feb 03 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CALIFORNIA... ....1930 UTC Update... Minimal changes were made to the Day 3 Slight and Marginal Risk areas across northern CA and far western NV, based on the latest (12Z) model guidance. Hurley ....Discussion... The slow moving atmospheric river will continue to bring a heavy rainfall and flood threat to portions of northern CA Sunday into Sunday night. The IVT core and max rainfall axis should shift a bit north compared to day 2 (Saturday). PWs drop off compared to earlier in the event, however the persistent IVT should support a prolonged moderate to locally heavy rain. Snow levels are quite high, and thus the Slight risk was expanded into the terrain, and even a bit to the east of the crest given the inland IVT anomalies. Rainfall rates within the higher terrain of the Sierra and just downwind of the crest will more be driven by the persistent IVT over the entire period, and thus flood impacts will be possible through Sunday and into the overnight hours. Along coastal and valley areas, rainfall rates should increase by later Sunday night as convergence increases along a front moving into the CA coast. Given what should be saturated conditions by this time, this uptick in rainfall rates along coastal areas into portions of the northern Sacramento Valley may result in an uptick in flood impacts by later Sunday night. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4UhI-1koFWIkqR_lGCTHIVgj00J18s8ElR2dZe86tpwT= S-S8ZreIqiuy6_Y9vr88qfpFsx6sqVztfUVmOlX5rqrwaCQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4UhI-1koFWIkqR_lGCTHIVgj00J18s8ElR2dZe86tpwT= S-S8ZreIqiuy6_Y9vr88qfpFsx6sqVztfUVmOlX58ldu1k0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4UhI-1koFWIkqR_lGCTHIVgj00J18s8ElR2dZe86tpwT= S-S8ZreIqiuy6_Y9vr88qfpFsx6sqVztfUVmOlX5anITpHM$=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) .