Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Mar 04 2025 15:52:34 FOUS30 KWBC 041550 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1050 AM EST Tue Mar 4 2025 Day 1 Valid 12Z Tue Mar 04 2025 - 12Z Wed Mar 05 2025 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM PORTIONS OF THE CORN BELT INTO THE ARKLATEX... ....16z update... ....Lower MS Valley/Deep South... RADAR mosaic and GOES-E imagery suite show bowing squall line across eastern Texas is starting to sever/interrupt southerly LLJ and moisture/instability flux as pivot/low level cyclogenesis increases across E OK toward the Ozarks. Solid confluence/convergence with pre-squall line cells appears to have fully occurred reducing the duration of heavy rainfall at any given location. Sub-hourly totals of 1-2" can be expected as the squall- line/cold front advance through the lower MS valley into the Deep South this evening and should remain at or below higher FFG values. HREF 1"/3hr show broad areas of 80-99% probability, though 2"/3hr are generally below 20%, cementing the quick but strong burst nature of the rainfall/flooding risk. This still may result in=20 highly localized rapid inundation especially in/near urban centers and traditionally prone areas. As such, the broad Marginal Risk=20 remains in place from current activity in AR/LA to GA and the=20 upslope southeastern portions of the Southern Appalachians.=20 ....Lower Missouri to Mid-MS valley... Showers and thunderstorms have pre-soaked the grounds with 1.5-2"=20 across the NE OK/SE KS/NW AR/SW MO and into the Ozarks where=20 naturally lower FFG values exists. Even further north, grounds=20 remain cold/harder due to lack of green-up resulting in increased=20 potential for run-off from showers thunderstorms. However, the strong dynamics and cold air aloft dropping into the deep layer cyclone over the Plains will help to steepen lapse rates this afternoon into evening as the cyclone pivots across KS. As such, scattered convective showers with capability of .5-1"/hr rates are expected to occur again this evening and maintain the potential for an additional 1-2" and may push longer term FFG exceedance. Additionally, stronger convergence along the downstream deformation zone across N MO into S IA, W IL will be orthogonal to the warm conveyor and provide modest isentropic ascent with potential for moderate showers with widely scattered embedded narrow convective elements. Slower northward advancement of the deformation zone should allow for increased duration for spotty 2-3" totals by 05.12z. Given the colder ground, lower FFG values; maintained the lower end Slight Risk across this area with Marginal Risk extending downstream across S WI/N IL where totals will be more in the 1-2" range.=20 Gallina ---Prior Discussion--- A deep layer cyclone progresses across the=20 middle portion of the country, drawing in moisture and instability from the south. QPF maxima stretch between two general regions,=20 in and around the ArkLaTex (primarily with a squall line which=20 could have a LEWP/QLCS character at times and preceding isolated=20 to scattered convection which could merge into the line) and much=20 of IA into northern MO (mainly from the pivoting low pressure=20 center as it occludes). While flash flood impact coverage should=20 remain somewhat limited in both areas (a lower end Slight risk=20 probability) with relatively dry antecedent conditions and a=20 rather progressive storm system motion, the dynamic nature of the=20 system is likely to bring periods of strong convection with hourly rain totals to 2" and localized storm totals nearing 3". While a=20 portion of the QPF over IA is likely to be in the form of snow on=20 the back-end of the low, the expected 3" total will also be=20 relatively long duration, as rates will be limited to 0.5"/hr with a relative lack of instability for northern areas of the Slight=20 Risk. The main change this update was to link together the Slight=20 Risk areas. Over the Southeast, the bigger problem spot appears the be the southern Appalachians as Atlantic and Gulf fetches maintain moisture/stronger low-level inflow longer than other areas of the region. The 00z HREF probabilities of 3"+ are above 50%, with non- zero probabilities of 5"+ amounts. The area has been dry as of late, so antecedent conditions suggest that some degree of this rainfall would be beneficial. Capped the risk as a Marginal for now, after coordination with GSP/the Greer SC forecast office, though it is a high end Marginal Risk for the southern Appalachians. Roth/Churchill Day 2 Valid 12Z Wed Mar 05 2025 - 12Z Thu Mar 06 2025 ....A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF SOUTHERN CALIFORNIA AND THE NORTHEAST... ....Southern California... A cyclone moving towards the region at the base of an amplifying and progressive West Coast upper level trough brings precipitable water values near the coastal ranges above 1", with 0.75"+ extending into the deserts. The strength of the low-level inflow has weakened from what was seen in the guidance from 24 hours ago, except in eastern sections of the Transverse Ranges, the Peninsular Ranges, and upslope areas of the Sierra Nevada. The system has some instability to work with which could allow for hourly rain totals to reach 0.5", which would be most problematic in burn scars. Overall totals appear to be capped near 3". The inherited Marginal Risk was pared back a bit across the western Transverse Ranges and coastal ranges north of Lompoc from continuity. ....Northern Mid-Atlantic States/Southern New England... A progressive deep layer cyclone moves through the Great Lakes into Southeast Canada, bringing of shot of 60 kt inflow at 850 hPa into the region with precipitable water values rising to 1-1.25". Portions of PA, NY, and the southern coast of New England appear to have some instability to work with, as MU CAPE of 250-500 J/kg is forecast to be available. Along with low- to mid-level frontogenesis, hourly rain totals to 1.25" should be possible. While most of this area has had below average precipitation this past week, a good portion of New England north of CT/RI/southeast MA have 1-2" of snow water equivalent on the ground, so melting snow should be occurring as well, considering the strength of the warm air advection which could raise both temperatures and dew points well into the 40s. The inherited Marginal Risk both fits the most recent QPF guidance and the ideas above, so changes to the area depicted were minimal. Roth Day 3 Valid 12Z Thu Mar 06 2025 - 12Z Fri Mar 07 2025 ....A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF SOUTHERN CALIFORNIA... ....Southern California... Sufficient low-level inflow from the Pacific, as high as 40 kts or so, combined with small amounts of instability and precipitable water values of 0.5-0.75" appear sufficient for hourly rain totals to rise towards 0.5" an hour, so left the Marginal Risk from continuity reasonably intact. It is expected to be the relatively cold atmosphere, so heavy snow is possible within the risk area in the higher peaks. The main flash flood risk would be to areas with burn scars. Roth Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_DioOz0e0I2_MCWCOpOh7z5hW5O2LyW_31hkt2QYJzav= BghB49wF-siYBSDZN932PpW_cJr79O1V1k-FPycmONdS2_E$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_DioOz0e0I2_MCWCOpOh7z5hW5O2LyW_31hkt2QYJzav= BghB49wF-siYBSDZN932PpW_cJr79O1V1k-FPycmSbJaC9Q$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_DioOz0e0I2_MCWCOpOh7z5hW5O2LyW_31hkt2QYJzav= BghB49wF-siYBSDZN932PpW_cJr79O1V1k-FPycmI_fk9nk$=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) .