Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Sep 24 2023 08:02:54 FOUS30 KWBC 240802 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 401 AM EDT Sun Sep 24 2023 Day 1 Valid 12Z Sun Sep 24 2023 - 12Z Mon Sep 25 2023 ....THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL FOR PORTIONS OF THE NORTHEAST AND ARKLATEX... ....Northeast... Post-T.C. Ophelia's remnant 850mb circulation will continue to gradually weaken but remain intact as it tracks over the DC/Baltimore metro areas Sunday afternoon and slowly tracks east towards the northern DelMarVa Peninsula Sunday night. There remains no shortage of moisture with PWs of 1.75" likely to be measured from Northern MD and the Lower Susquehanna Valley of PA on east across the Delaware Valley and into southern NJ. These areas in particular reside within a narrow ribbon of easterly 925-850mb theta-e advection with MUCAPE up to 500 J/kg and upslope enhancement into higher terrain (particularly eastern PA and northern NJ). Sampled soundings already showed warm cloud layers up to 13,000ft deep, but given the >90% RH values and lingering buoyancy from both available instability and forced-topographic ascent, hourly rainfall rates could top out as high as 0.75"/hr. The 850mb low will be in no rush to exit off the coast, so a prolonged period of moderate-to-heavy rainfall will occur for much of the day. Given these factors, as well as soils becoming a little more saturated following Saturday's rainfall, have maintained the Slight Risk from the Mason-Dixon line on east to the southern coast of Long Island. There are still likely to be efficient warm rain processes into southern New England, but given the lack of instability north of the coast, have trimmed down the northern extent of the Slight while maintaining most of the inherited Marginal Risk area. ....Southern Plains to the Lower Mississippi Valley... A similar setup that resulted in widespread thunderstorms on Saturday will continue to make its way south into central TX and as far east as the Lower MS Valley. Latest guidance still shows a good signal for Excessive Rainfall rates across the ArkLaTex, but more recent CAMs have put the axis a little farther south into eastern Texas. An 850mb low bordering the Red River will direct a continuous fetch of 850mb moisture flux at a stalled front positioned somewhere near the ArkLaTex, resulting in PWs topping 2" and as much as 2,000-3,000 J/kg of MLCAPE. Similar to Saturday, the steady stream of low level moisture becomes oriented quasi-parallel to the stationary front, providing a suitable setup for potential backbuilding and training storms. Storms will be more than capable of producing 2-3"/hr rainfall rates, and given the potential training of thunderstorms, localized amounts could reach 4-6". This is supported by the 00Z HREF showing 20-25% probabilities of 24-hr QPF > 5". Did increase the footprint of the Slight Risk area to include more of eastern TX and northern LA where recent CAMs showed the potential for storms to inch more to the south. Have held off on a Moderate Risk upgrade given the bulk of the Slight Risk area features higher FFGs compared to their neighbors farther north across eastern OK and southwest MO. Should confidence grow in a swath of 5+" amounts in new guidance over the next 12 hours, there is a chance a Moderate Risk may be needed for parts of the ArkLaTex or east Texas in future forecast cycles. ....Southwest Oregon... The leading edge of one of the upcoming cold season's first atmospheric rivers arrives Sunday afternoon and continues to deliver periods of rain to the Pacific Northwest into Sunday night. A narrow swath of heavy rainfall will be directed at the Oregon coast Sunday night thanks to a robust 1000 kg/m/s IVT that peaks at just over 8 standard deviations above normal for late September. Rainfall totals could range between 1-2" with locally higher amounts closer to 3", but the bulk of this is falling within a 6-12 hour span. Due to these possible rainfall rates, have kept the Marginal Risk that was in place with any nearby burn scars most at-risk for possible flash flooding. Mullinax 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!94b4blGyWZoYyPrpyDWOEd4nNW5LgWFo5DdJRw3HcwpT= 7yQrgsILWJjIo6uS9W3L9nY3DeG2n3IOM9pYepbaJOy5-aU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!94b4blGyWZoYyPrpyDWOEd4nNW5LgWFo5DdJRw3HcwpT= 7yQrgsILWJjIo6uS9W3L9nY3DeG2n3IOM9pYepbazQP2duc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!94b4blGyWZoYyPrpyDWOEd4nNW5LgWFo5DdJRw3HcwpT= 7yQrgsILWJjIo6uS9W3L9nY3DeG2n3IOM9pYepbaR_cOkog$=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) .