Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Sep 27 2024 20:22:50 FOUS30 KWBC 272022 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 422 PM EDT Fri Sep 27 2024 Day 1 Valid 16Z Fri Sep 27 2024 - 12Z Sat Sep 28 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHWEST VIRGINIA AND SOUTHEAST WEST VIRGINIA... ....2030Z Special Update... With much of the rainfall from Helene largely ended in the hardest hit areas of the Carolinas, Tennessee, and far western Virginia, the High Risk has been downgraded with this update. A Moderate Risk remains in effect where heavy showers and storms continue over the harder hit regions of southwest Virginia near Roanoke. Expect these too to continue weakening and becoming more widely scattered, ultimately dissipating altogether with the loss of diurnal heating. The Slight Risk remains largely intact over the Piedmont of western and southern Virginia, was well as into the Ohio, Mississippi, and Tennessee Valleys. Rainfall rates here have been very low, struggling to get much above even a half inch per hour in most instances. Going forward steady light to briefly moderate rain will continue to pivot around the low across the mid-Mississippi Valley, but not to any rate considered capable of producing more than very widely scattered instances of flash flooding. Thus, the Moderate Risk in Tennessee was also dropped with this issuance. In the Southern Appalachians, Marginal and Slight risk areas were maintained primarily for the concern that very late tonight in the predawn hours, the rainfall, even if light and stratiform, that is over western Tennessee and Mississippi as of the time of this writing can get back to the hard hit regions of north Georgia and the far western Carolinas. No major impacts are expected, but any rainfall could at least slow any recessions of the area rivers. Wegman=20 Day 2 Valid 12Z Sat Sep 28 2024 - 12Z Sun Sep 29 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE OHIO AND TENNESSEE VALLEYS AS WELL AS FOR CENTRAL FLORIDA... ....2030Z Update... ....Ohio/Tennessee Valleys... The remnant combined upper low and Helene will continue spinning down as the low stalls over the Ohio and Tennessee Valleys Saturday. Even today the instability in that region has been nil, despite a fully saturated column and near record PWATs. The result of that today has been widespread stratiform rain with little in the way of convective enhancement anywhere. For this reason rainfall rates across the area have generally been at a half inch per hour or less. That's today. By Day 2/Saturday, the broad moisture plume will have largely rained itself out, and while there will still be tremendous forcing around the nearly stationary low center, without instability there's little reason tomorrow's rainfall will be any different than today's. Thus, despite abundant, but diminishing amounts of atmospheric moisture, and despite today's rainfall moistening the soils, Saturday's rain is unlikely to be heavy enough to produce any more than isolated flash flooding. In coordination with ILN/Wilmington, OH and LMK/Louisville, KY forecast offices, the Slight risk was downgraded to a Marginal with this update. ....Central Florida... The lingering cold front trailing south of Helene/trough's circulation will be stalled over Florida with little to move it one way or the other. Thus, with abundant instability and moisture but a strong southwesterly flow, strong storms may move over portions of the state both brushed by Helene and have seen heavy rain in recent days. The Marginal was largely left intact, with a very small northward nudge to it based on the latest guidance. Wegman ....Previous Discussion... Lingering rain associated with the combined upper low and Helene will continue mainly across the Ohio Valley into Saturday morning. Any flooding impacts may be realized to their greatest extent along this portion of the Ohio River on Saturday. A Slight Risk remains in effect for portions of the Ohio River Valley although with an eastward and southward adjustment to reflect the latest trends and observations. Additionally, southern portions of the Northeast/northern Mid-Atlantic were trimmed out of the Marginal Risk area due to drier antecedent conditions and not as much rainfall from Helene and the frontal system as once expected. Campbell Day 3 Valid 12Z Sun Sep 29 2024 - 12Z Mon Sep 30 2024 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE MID-ATLANTIC/CENTRAL APPALACHIANS AS WELL AS THE NORTHERN FLORIDA PENINSULA... ....2030Z Update... ....Mid-Atlantic/Central Appalachians... The remnant low of Helene will get picked up by the prevailing jet stream flow and eject eastward into the Mid-Atlantic and Central Appalachians. Just as on Day 2, moisture will be limited as much of it will have rained out, but in contrast there may be a bit more instability. Thus, portions of the Shenandoah Valley and west to the Allegheny Front in MD, WV, and VA may locally have enough heavy rain to result in isolated to widely scattered instances of flash flooding. A Slight was considered and may be needed with future updates but for now the rainfall footprint was low enough that a Marginal should suffice. ....North Florida... A stationary front, once associated with Helene will still be stuck over north Florida having drifted north from central Florida from Day 2/Saturday. Once again it may serve as a focus for fast-moving convection, and the storms may move over some areas hard-hit by Helene, but because each storm will be fast moving towards the northeast only a Marginal Risk is needed. Wegman ....Previous Discussion... Remnant energy and moisture from Helene will still be present over the region with lingering rain over extra sensitive soils. Some locations will likely have ongoing riverine flooding and additional rain amounts may aggravate the situation. A Marginal Risk for excessive rainfall remains in effect from Tennessee and Kentucky to points east to Maryland. Campbell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8-VJdujPYrwDvRVPaBW2tkkkSHawpiWz2L0JudGuCgzh= myLM4w8MFlL8J18OMSFRgV0uXFedTX0AOjJoxdFD2pKMuK8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8-VJdujPYrwDvRVPaBW2tkkkSHawpiWz2L0JudGuCgzh= myLM4w8MFlL8J18OMSFRgV0uXFedTX0AOjJoxdFD222kBmg$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8-VJdujPYrwDvRVPaBW2tkkkSHawpiWz2L0JudGuCgzh= myLM4w8MFlL8J18OMSFRgV0uXFedTX0AOjJoxdFDUarHWtQ$=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) .