Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Sep 28 2024 00:56:54 FOUS30 KWBC 280056 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 856 PM EDT Fri Sep 27 2024 Day 1 Valid 01Z Sat Sep 28 2024 - 12Z Sat Sep 28 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF=20 THE WESTERN OHIO VALLEY... The good news to report is the rainfall intensity has decreased substantially over the past several hours from what is now post tropical cyclone Helene, and this has allowed for the removal of the existing Moderate Risk area from the previous ERO Day 1 issuance, and a big reduction in the overall size of the Slight Risk area. Much of the region encompassed by the previous Slight Risk area over Tennessee and northeastern Arkansas has received rainfall rates under half an inch per hour, and the latest CAM guidance suggests even lighter rainfall rates during the overnight hours, so many of these same areas are now just under a Marginal Risk area through 12Z. The remaining Slight Risk area extends from western Kentucky to southern Indiana and southwest Ohio, where current radar trends are indicating the heaviest rainfall that is north of the dry slot seen on water vapor imagery. The consensus of the CAM guidance shows the highest 12-hour QPF centered over west-central Kentucky, with some 1 to localized 2 inch rainfall totals possible.=20 Elsewhere, some lingering upslope flow across western Virginia through midnight will keep the potential for some scattered showers that might briefly train over the same areas, and a weakening cluster of showers and storms over the Delmarva Peninsula will likely track northward towards New Jersey overnight, with a low-end potential for nuisance level flooding with the heaviest cells. Hamrick 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!87IZiwEIRNG5pMzZA_Y1SkOUqT6f_vfwZHmTkLa13-oy= aDzsMaVwCuqjuG1g6TcnBGlQoaZ8IKaFuLvgPA_eiMDbPlw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!87IZiwEIRNG5pMzZA_Y1SkOUqT6f_vfwZHmTkLa13-oy= aDzsMaVwCuqjuG1g6TcnBGlQoaZ8IKaFuLvgPA_eLezxj4I$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!87IZiwEIRNG5pMzZA_Y1SkOUqT6f_vfwZHmTkLa13-oy= aDzsMaVwCuqjuG1g6TcnBGlQoaZ8IKaFuLvgPA_eriDSxN4$=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. 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