Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Sep 05 2025 15:56:42 FOUS30 KWBC 051556 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1156 AM EDT Fri Sep 5 2025 Day 1 Valid 16Z Fri Sep 05 2025 - 12Z Sat Sep 06 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE=20 SOUTHWEST, SOUTH FLORIDA, AND PORTIONS OF THE TENNESSEE VALLEY/CENTRAL APPALACHIANS... ....Southwest... Another active day of convection expected as monsoonal moisture overlaps with weak impulses lifting within broad ridging across the Southwest to produce widespread ascent within favorable thermodynamics today. Additionally, although the remnants of Lorena have dissipated over Mexico, residual elevated moisture remains as it gets funneled northeast downstream of a trough over CA. This is reflected by PWs that were measured on morning U/A soundings of 1.76" at PHX and 1.41" at VEF. While some cloud cover is noted in the GOES-E visible imagery, in general sky conditions should=20 support rapid destabilization today, with MUCAPE potentially=20 reaching as high as 3000 J/kg, especially in CA/NV/AZ. Weak=20 impulses in the flow will help organize what should be widespread=20 convection this aftn, with rain rates potentially producing 0.75"+=20 rainfall in less than 1 hour as supported by the HRRR and HREF=20 neighborhood probabilities.=20 In general, convection is expected to be scattered enough and moving fast enough (after initiation and once moving off the higher terrain) to preclude an upgrade to a SLGT risk today so the inherited MRGL risk appears to be still supported. However, there is some potential for some more widespread impacts across NV where the CAMs indicate a greater chance for some storm organization beneath a shortwave trough, and HREF FFG exceedance probabilities peak above 20% despite a subtle moderation since 00Z. After coordination with WFO VEF opted to maintain the MRGL risk at this time. ....Tennessee Valley into the Central Appalachians...=20 A cold front draped from Illinois to Oklahoma this morning will=20 steadily progress eastward through D1 as a shortwave impulse=20 dropping across the Upper Midwest drives increasing height falls=20 downstream of the parent trough. The most impressive synoptic=20 ascent today is progged to stretch from northern TN through eastern KY and into southeast OH/western WV, and this is where weak=20 impulses aloft will interact with the front to produce at least a=20 weak wave of low pressure. The accompanying ascent into favorable=20 thermodynamics (PWs approaching 2" this evening combined with=20 MUCAPE of 2000-3000 J/kg) will likely result in widespread=20 thunderstorms along and ahead of the front. Although storms are=20 likely to move progressively as reflected by 0-6km mean winds of=20 20-25 kts, this flow will generally be parallel to the front to=20 help incur training of echoes this evening. With the HREF=20 neighborhood probabilities for 1"/hr rain rates reaching above 40%, any training could produce local flash flood impacts, and the MRGL risk was adjusted a bit farther northeast for faster model=20 progressions into more sensitive soils with lower FFG. ....South Florida... Morning soundings at KEY and TBW indicate an impressive PW=20 gradient from 2.34" (KEY) to 1.89" (TBW) reflective of a stalled=20 out stationary front draped across the southern Florida Peninsula.=20 This boundary has been plaguing the region for several days now,=20 leading to 7-day rainfall that has been more than 300% of normal in some areas of the southern peninsula. As this front wavers one=20 again today, and weak mid0-level impulses move along it to drive=20 weak surface low development just to the east, the overlap of this=20 ascent with the anomalous moisture will once again result in=20 scattered to widespread thunderstorms, primarily during peak=20 aftn/eve heating. Storms should generally drift east to west, but=20 with weak bulk shear suggesting pulse-type convection, storm=20 mergers and collisions are likely which will result in chaotic=20 motion at times. As the environment will support 2-3+"/hr rain=20 rates, locally more than 3" of rain is possible which could produce isolated instances of flash flooding, primarily should these=20 amounts fall atop urban areas. Weiss Day 2 Valid 12Z Sat Sep 06 2025 - 12Z Sun Sep 07 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE INTERMOUNTAIN WEST, FLORIDA, AND THE NORTHEAST... ....Intermountain West... Monsoonal moisture will continue to steam northward across portions of the West on Saturday. While some of that moisture can be directly attributed to the remnants of Lorena, Lorena will have long since ceased existing as a tropical entity by this period on Saturday. Thus, the forcing mechanisms for heavy rain will turn to more typical monsoonal factors, such as upsloping and an impinging shortwave trough. For some areas, this will result in a higher threat for heavy rain and subsequent flooding, particularly into far southern Arizona and much of New Mexico. Nevertheless, no one forcing will be strong enough to promote organization and more robust heavy rains, so the Marginal Risk for the area remains in place. Further north, a digging shortwave trough will be the primary focus driving the heavy rain, as the storms tap into some of that monsoonal moisture. ....Northeast... The cold front that was moving into the Appalachians on Friday will continue across the urban corridor of the Northeast on Saturday. Diurnal heating and robust moisture transport up the I-95 corridor will support training storms, which may be locally enhanced with upslope flow from the DC Metro area northeast through Boston and into Maine. Corfidi Vectors parallel to the frontal interface suggest fast-moving training thunderstorms will be common. The widespread urbanization along I-95 also supports at least a Marginal/isolated flash flooding risk. It will be far from a rainy day for most, with most areas staying dry. However, when storms move through, they will likely be strong, with abundant moisture to tap into. Thus, the inherited Marginal Risk was expanded well south and west to include all of the I-95 corridor through the DC Metro area. Localized urban flooding will remain the primary threat from any repeating storms despite very dry antecedent conditions. The front will continue eastward into Saturday night, ushering in yet another cool and very comfortable air mass from Sunday through much of next week. The rainfall forecast has most notably increased across portions of New England, from NYC north. This is where the greatest risk of flash flooding will be on Saturday. ....Florida... The stalled out front and associated tropical wave will slowly begin to retrograde back towards the north on Saturday, resulting in a spreading of the potential for heavy rains well up the Florida Peninsula. Thus, the Marginal Risk was expanded north through central Florida with this update. High uncertainty as to convective coverage and behavior will keep a Marginal going for the time being, but there remains some potential a later Slight may be needed in portions of Florida. Wegman Day 3 Valid 12Z Sun Sep 07 2025 - 12Z Mon Sep 08 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN PLAINS AND THE FLORIDA PENINSULA... ....Southern Plains... While some monsoonal moisture will linger in portions of the West into Sunday, there will be far more of a Gulf component to the moisture as predominant southeasterly flow advects deep Gulf moisture into south Texas, which will combine with what moisture is left from the monsoon. While the moisture will be plentiful, the forcing will be anything but, as a strong upper level ridge builds across the area. The result will be storms that for many areas will be tied to the mountains, cold pools from other storms, or localized pockets of higher instability. Thus, storm organization across Texas on Sunday will be quite lacking. Given this, despite the atmospheric moisture, soils across south Texas are quite dry, which is average for this time of year. Thus, for most areas, rain from any storms will be largely beneficial and will soak into the ground, precluding much flooding threat. Thus, in coordination with EWX/San Antonio, TX, CRP/Corpus Christi, TX, and SJT/San Angelo, TX forecast offices, the inherited Slight for portions of the Hill Country was removed with this update. ....Florida Peninsula... It's the same old song and dance weather-wise across Florida once again on Sunday. The front that has been stuck there will still be there, taking advantage of plentiful Gulf/Atlantic moisture. Uncertainty with how the storms will form and organize remains high, so the Marginal Risk remains in place, with a northward expansion included to account for potentially heavy rains across central Florida from the day Saturday continuing into Sunday. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6PkZFZhIoq9YTvyzlYQHJZUaXp49CZhyA7AoFD6ROlq5= RUDt4wB02aHwwEv45V354MN5EnythO0LFVeyxyKxoFUBzmk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6PkZFZhIoq9YTvyzlYQHJZUaXp49CZhyA7AoFD6ROlq5= RUDt4wB02aHwwEv45V354MN5EnythO0LFVeyxyKxz1rpw4Q$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6PkZFZhIoq9YTvyzlYQHJZUaXp49CZhyA7AoFD6ROlq5= RUDt4wB02aHwwEv45V354MN5EnythO0LFVeyxyKxPcNY4AM$=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 3.28-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .