Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Mar 28 2023 08:53:48 FOUS30 KWBC 280853 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 453 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 Valid 12Z Wed Mar 29 2023 - 12Z Thu Mar 30 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PARTS OF CENTRAL AND SOUTHERN CALIFORNIA... The atmospheric river impacting California will slide southward into Southern California early on Wednesday and orient the axis of highest atmospheric moisture content (400-500 kg/m*s) orthogonal to the mountain ranges of SoCal. Snow levels are expected to be up to 6000 ft across SoCal as well, until crashing Wednesday night and remaining below 4000 ft for much of the state during the Day 2 period. Rainfall rates may briefly peak at 0.25-0.5"/hr while relatively quick moving across the southern part of the Marginal Risk, but saturated soils could support a localized flooding/rapid runoff threat. The second phase of the Marginal Risk (mainly across central CA, but eventually sliding into the Transverse Ranges) involves the vertically stacked strong polar upper-level low sliding across the region. A secondary convergence axis wrapped around this low along with steeping lapse rates and a subtle layer of instability may contain scattered convective showers capable of containing localized heavy rainfall rates. Per the global guidance and GEM-regional, this convergence axis and areas of scattered showers starts near the central CA coast and Sacramento Valley midday until sliding down the central CA coast and into the Transverse Ranges after about 00z Thurs. Total QPF amounts during this entire time period are not that impressive alone, but when combined with the potential 2-3" amounts just prior and the already overly saturated ground, these convective showers and periods of modest rainfall could produce isolated flooding concerns. Hurley/Snell Day 3 Valid 12Z Thu Mar 30 2023 - 12Z Fri Mar 31 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PARTS OF THE UPPER MIDWEST... Vigorous deep-layer trough traversing the Great Basin and Southwest on Thursday will lift across the central-southern Rockies Thursday night. Increasingly difluent upper levels across the Upper Midwest during day 3 will make for a more favorable environment for deep-layer ascent, particularly Thu night as the the global models hint at enhanced right-entrance region forcing from the departing northern stream upper jet streak. A rapid increase in low-mid layer moisture transport from the western GOMEX is evidenced by both the GEFS and SREF, as 850-700 mb moisture flux anomalies climb quickly between 3 and 4 standard deviations above normal Thursday night on the heels of robust SSW 850 mb flow of 50-60 kts. At this point, the guidance is fairly transient with the areas of favorable dynamic and thermodynamic parameters, indicative of a fairly progressive warm front, transition into the warm sector (at least across southern portions of IA, where the 00Z ECMWF shows surface-based CAPEs climbing above 1000 J/Kg), and the cold frontal passage west to east on Friday. Because of this, at least for now, will maintain the Marginal Risk inherited from yesterday's Day 4 ERO. However, the ingredients are certainly present for a more enhanced risk, perhaps farther south, depending on the evolution of the system and upper-level pattern, and whether or not any further uptick and/or added duration of low-level frontogenesis can retard the frontal progression, thus prolonging favorable deep-layer forcing and potentially leading to more cell training. Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6u_UK8jqVJYgLTlmbeDjNFX87tQEJoGnhskG5omOG8op= F0HYM5HIvG-XWdhuLLu1ZTuXYkm1OFXxWkihCVomPr6wOwo$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6u_UK8jqVJYgLTlmbeDjNFX87tQEJoGnhskG5omOG8op= F0HYM5HIvG-XWdhuLLu1ZTuXYkm1OFXxWkihCVomqMtw9xo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6u_UK8jqVJYgLTlmbeDjNFX87tQEJoGnhskG5omOG8op= F0HYM5HIvG-XWdhuLLu1ZTuXYkm1OFXxWkihCVomDGPzD3s$=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) .