Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Oct 01 2023 15:47:05 FOUS30 KWBC 011546 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1146 AM EDT Sun Oct 01 2023 Day 1 Valid 12Z Sun Oct 01 2023 - 12Z Mon Oct 02 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE EAST COAST OF THE FLORIDA PENINSULA... ....16Z Update... ....Florida... Only a few minor tweaks were made to the ongoing Slight Risk area across the east coast of Florida this afternoon. A very slow moving front is finally allowing the associated convection near Cape Canaveral to begin to drift southward...a trend that is expected to continue through the day. Meanwhile, convection just off the coast of Miami should begin to redevelop over the mainland as diurnal heating locally increases surface-based instability. Once again due to lack of forcing, any storms that form will be nearly stationary, resulting in localized high rainfall totals. Much of the east coast of FL has been at or above normal for rainfall over the last couple weeks, so soils are generally at or near saturation, especially when adding the urban factor. Thus, the Slight Risk remains in place. The Slight was nudged northward to account for ongoing heavy rainfall near Cape Canaveral, and was also stretched southward to include more of Miami and its southern suburbs as well. The southward expansion is due to the aforementioned expectation for afternoon and evening convection to develop over urbanized areas. There remains considerable uncertainty as to how far inland/west the rainfall will extend from the coast, but there is good agreement that it will not be as far west as the west coast, so the Marginal Risk was trimmed out of the Ft. Myers/Naples area, as any heavy rain in most of the guidance is likely to form southeast of there over the Everglades, and thus not pose a flash flood threat. ....Eastern NM/Portions of the TX and OK Panhandles... No significant changes were made other than a small northeastward extension of the Marginal Risk area to include more of the TX/OK Panhandles based on the latest guidance trends. Otherwise the afternoon through overnight showers and storms are still expected in the Marginal Risk area, though only isolated flash flooding is expected as a result. Wegman ....Previous Discussion... ....Florida... We've upgraded the Florida Marginal to a Slight Risk. There's been a consistent signal for heavy rain to occur over central/southern Florida's Atlantic coast for a couple days now, but the signal has increased significantly over the last few runs. PWATs will be well over 2 inches with some shallow instability. The presence of a surface front will allow for thunderstorms to continuously develop and dissipate through the afternoon. HREF 24 hour exceedance probabilities of 5" are quite high as well. ....New Mexico/west Texas... The deep diffluent pattern over the Great Plains will continue to generate convection in the Southern Plains on Sunday. Instability around 1000J/Kg and PWATs of over 1" could support some instances of flash flooding this afternoon and evening. The global models appear to be well clustered over the Southern High Plains as well, however the GFS appear to suggest that the heaviest rainfall will be scattered across the marginal risk area, so an upgrade is not necessary at this time. Ensemble mean exceedance probabilities of 1" and 2" in 24 hours including the 00z GEFS are well clustered over the northeastern New Mexico and the Texas panhandle. Kebede Day 2 Valid 12Z Mon Oct 02 2023 - 12Z Tue Oct 03 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHERN HIGH PLAINS OF NEW MEXICO AND TEXAS... ....Southern High Plains of New Mexico and Texas... Confidence has increased enough to warrant an upgrade of the New Mexico/west Texas Marginal to a Slight Risk. The Southern High Plains will be under the influence of deep diffluent flow from a potent upper trough propagating through the West on Monday. This will be coupled with the arrival of a strong upper jet aloft and favorable low level jet dynamics. Cape values in the 1000-1500J/Kg range and moisture anomalies well over 2 standard deviations (1-1.5in") should support thunderstorm activity (some severe) with excessive rainfall potential. ....Florida... Albeit weaker than today's threat, Monday's heavy rainfall threat will shift to southern Florida as the surface front, which will continue to be the focus for storms, shifts southward. Instability will weaken on Monday so any excessive rainfall threats will likely be few and far between. The main threat from potential heavy rainfall will be over urban areas. Kebede Day 3 Valid 12Z Tue Oct 03 2023 - 12Z Wed Oct 04 2023 ,...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE GREAT PLAINS... A rapidly developing mid-latitude cyclone, supported by a deep upper-level trough will produce thunderstorms (some severe) and moderate to heavy rainfall across the Great Plains on Tuesday. Confidence has diminished a bit since the last issuance regarding excessive rainfall over parts of the Northern Plains, therefore the Slight Risk has been removed. The broad Marginal area remains due to the potential for excessive rainfall, particularly over parts of the Central Plains where favorable instability within potential severe storms could enhance rainfall rates. PWATs will be well over 1 inch across the Plains on Tuesday with impressive low level winds. Kebede Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6-c_00cGjkNc_MJbuMucx6XRcor7e4BkpVgAmuWzPURF= Ht6HhEgET8Gl6SZ_r3IV4k6zuJAdznEWkr165S-GD9sgFzM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6-c_00cGjkNc_MJbuMucx6XRcor7e4BkpVgAmuWzPURF= Ht6HhEgET8Gl6SZ_r3IV4k6zuJAdznEWkr165S-Gehb9kKg$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6-c_00cGjkNc_MJbuMucx6XRcor7e4BkpVgAmuWzPURF= Ht6HhEgET8Gl6SZ_r3IV4k6zuJAdznEWkr165S-Gk8embVs$=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) .