Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Mar 28 2023 08:24:18 FOUS30 KWBC 280824 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 423 AM EDT Tue Mar 28 2023 Day 1 Valid 12Z Tue Mar 28 2023 - 12Z Wed Mar 29 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE CENTRAL CALIFORNIA COAST AND SAN FRANCISCO BAY AREA, AS WELL PARTS OF THE SOUTHEAST... ....California... The Day 1 ERO across California remains on track, with only slight adjustments compared to the previous forecast. An upper level polar cutoff low begins the day Tuesday morning centered off the Oregon coast, and tracks southeastward to be centered over the northern California coast by Wednesday morning. A roughly 110 kt westerly upper level jet streak wrapping around the low will direct a plume of Pacific moisture and precipitation into California as it tracks south. PWATs are still expected to peak between 0.75-1.00", which is about +1.5-2 sigma above the climatological normal. IVT values also peak at about 500-600 kg/m*s along central California between ~ 12-18Z Tuesday. This will bring with it a brief increase in snow levels as high as 7,000 ft along coastal ranges during the day Tuesday. The attendant surface low will follow a similar track as the upper level cutoff low through this period, making it a vertically stacked low, that will result in largely unidirectional flow throughout the column south of the low. Since the vertically stacked low has polar origins, it will have plenty of cold air and a lack of plentiful Pacific moisture with it. Thus, this rainfall event is not expected to be as intense as the atmospheric rivers impacting the state in recent weeks. Nevertheless, the residual impacts from those ARs linger across coastal central California, evidenced by the high soil moisture percentiles (>80%) within the top 100 cm per latest NASA SPoRT, and the injection of scattered 3-5 inch rainfall totals per the CAMs into the coastal mountains are expected to cause or worsen flooding impacts as that rainwater flows down the mountains and swells already high rivers. Thus, the Slight Risk was maintained due to steady forecasted rainfall through the Bay area and especially the mountains north of there, while also spanning south to include the westernmost Transverse Ranges of Santa Barbara County. Further inland, slightly lower snow levels, much higher mountains, and more recovery time from the last round of atmospheric rivers were factors that led to the continuance of the Marginal Risk, as much of the heaviest precipitation is likely to fall as snow. The highly favorable antecedent conditions for flash flooding remain a significant factor in the Slight Risk area. ....Central-Eastern Gulf Coast Region and Parts of the Southeast... Compared to yesterday's Day 2 ERO, the Slight Risk was expanded a little farther east across south-central GA and northern portions of the FL Big Bend, to include Tallahassee and Valdosta. Potentially heavy rainfall could overlap areas with recent and/or ongoing rainfall during the Day 1 period. By Tuesday evening, as the main shortwave to the north traverses the Mid Atlantic Region, the surface cold front will get more of a southerly push through North Florida. Beforehand however, pre-frontal convection will capitalize on decent deep-layer instability (mixed-layer CAPEs 1000-2000 J/Kg) along with PWs ~ 1.6" to 1.8" in supporting hourly rainfall rates of 1.5" to 2+". These more scattered cells will have the potential to grow into organized linear clusters ahead of the eventual line of storms associated with the progressing cold front. This cold front should push through the entire region by about 06Z and end the threat of excessive rainfall. 00Z CAMs all show localized rainfall totals of potentially 3-4+ inches, which would pose an isolated to scattered risk for flash flooding, especially over urban areas and areas where heavy rainfall occurred the prior 48 hours. Very low FFGs reside just to the north of the Slight Risk across AL, so this area will need to be monitored in case any heavier QPF amounts shift northward. Hurley/Snell Day 2 The Day 2 outlook will be updated by 0830Z. Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6kDesa3UlUITjW25stxsfrndpndYXocOQJYGlw3MhIMp= rwiXzXfJDMXZjhJyDYj7DSoAdyKpykVgvcuQnoSZ7D2StKI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6kDesa3UlUITjW25stxsfrndpndYXocOQJYGlw3MhIMp= rwiXzXfJDMXZjhJyDYj7DSoAdyKpykVgvcuQnoSZSLk0HYo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6kDesa3UlUITjW25stxsfrndpndYXocOQJYGlw3MhIMp= rwiXzXfJDMXZjhJyDYj7DSoAdyKpykVgvcuQnoSZjZK_uDs$=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) .