Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Sep 29 2024 20:19:07 FOUS30 KWBC 292018 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 418 PM EDT Sun Sep 29 2024 Day 1 Valid 16Z Sun Sep 29 2024 - 12Z Mon Sep 30 2024 ....THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL VIRGINIA INTO THE ALLEGHENY HIGHLANDS... ....16Z Update... The upper low that absorbed the circulation of Helene will begin to move from its stationary position in the Tennessee Valley today. For most areas around the low, any rain will be light. However ahead of the low where the predominant flow turns southerly to southeasterly, the interaction of the upper low with a surface trough in the area, additional Gulf/Atlantic moisture, and the support of a right-entrance-region of an upper level jet will greatly increase the coverage and intensity of heavy rain into portions of Virginia. CAMs guidance has come into rather remarkable agreement that from late this afternoon through much of tonight, a nearly stationary band of rain with frequent embedded convective elements will impact the Moderate Risk area, with the cells moving in the rather unusual direction from southeast to northwest. The heavy rain will impact several ranges within the broader Appalachians, including the Blue Ridge and the Allegheny Highlands further inland. These areas are still recovering from the 2-6 inches of rain from Helene's first run at the area, so FFGs throughout the Moderate Risk area, but especially along those ranges. Hourly FFGs are at less than a half inch, and 6 hourly FFGs are up to 1.5 inches. Meanwhile, the wettest CAM, the 12Z HRRR shows over 7 inches of rain in portions of the Moderate Risk area through tonight...where the broader CAMs average are between 3-5 inches in that same time frame. Given the aforementioned setup of training convective cells, the FFGs values are very likely to be exceeded in the Moderate Risk area. Elsewhere, the surrounding Slight Risk was expanded west to include much of West Virginia. The greatest rainfall amounts and therefore the greatest chances of flooding are into the eastern part of the state, closer to the Moderate. However, there are some signals near the WV/KY/OH tripoint, so the Marginal was expanded west in that region as well. For now, it appears the predominant westerly flow into the southern Appalachians will keep any rain there today very light, allowing Helene recovery efforts to continue. In central Florida expect another afternoon of occasional heavy showers and thunderstorms, but nothing organized is expected. The area remains a very low-end Marginal. Wegman ....Previous Discussion... ....Mid-Atlantic/Central Appalachians... Remnant moisture from Helene will remain caught up in the upper trough pattern across the Ohio Valley while the mean trough begins to slowly advance eastward through the period. During this period precipitable water anomalies will be on the order of 1 to 2 standard deviations above normal and will encounter increased forcing from the upper jet and modest lift through the column over much of the Mid-Atlantic. This setup will support the development of scattered to numerous thunderstorms that will likely have slower storm motions. Recent rains have lowered local FFGs and increased sensitivity across the region. Areal average QPF for the region will be in the 1 to 3 inch range with isolated maximums in excess of 3 inches will be possible. A Slight Risk for excessive rainfall is in effect for extreme western Maryland, eastern West Virginia, northern/central/south-central Virginia and north-central North Carolina. A Marginal Risk area spans from northeast Georgia/northwest South Carolina to southern Pennsylvania and southeast Ohio. ....Florida... An unstable airmass coupled with lingering deep layer moisture will promote isolated to widely scattered thunderstorm activity capable of localized flooding, especially within urbanized zones. Any of these storms will have the capability to produce 2 to 4 inches, especially along the coastline. A Marginal Risk for excessive rainfall is in effect for central portions of the peninsula. Campbell Day 2 Valid 12Z Mon Sep 30 2024 - 12Z Tue Oct 01 2024 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL AND SOUTHWESTERN VIRGINIA AND EASTERN WEST VIRGINIA... ....2030Z Update... There has been an overall shift south with much of the guidance as to where the heaviest rain is expected to occur Monday across the Virginias. While the GFS holds on to the northern Shenandoah Valley, the rest has shifted more into portions of southwestern Virginia. This is largely due to some interaction of the slow moving upper low transferring its energy to a coastal low off the VA/NC coast. This will likely keep mostly light rain ongoing across central Virginia, but also prolong the easterly upslope flow into the southern Blue Ridge. The Slight was expanded south to the VA/NC border, with some concern it may need to later include the northernmost tier of counties in NC.=20 A higher-end Slight is considered for where the Day 1 Moderate Risk is, but extends a bit further south to near Roanoke. This of course is largely contingent on the rainfall that occurs tonight, but should the rain tonight overperform in these already hard-hit areas, then another small, targeted Moderate Risk may be needed again Monday. However, given the likely use of most of the atmospheric instability available to the convection for tonight's rainfall, it seems unlikely such intensity will be able to persist into the Day 2 period. Thus, with somewhat lower forecasted rainfall totals across southwestern Virginia on Monday as compared to today, continuing the Slight, even with a southward expansion looks good. The Marginal Risk was extended to cover much of coastal NC in association with the developing coastal low potentially locally increasing rainfall totals due to slow-moving convection especially on the western side of Pamlico Sound.=20 Wegman ....Previous Discussion... High pressure to the north, and a developing surface reflection off the Mid-Atlantic coast thanks to the upstream trough will create a persistent northeasterly flow setup with greater upslope emphasis within the terrain across Western Virginia into the far eastern West Virginia. There will be an enhancement of forcing through the column of atmosphere near the Appalachian/Blue Ridge spine that may result in hourly rainfall rates of 1 to 1.5 inches/hour during this period. Soils near saturation from recent rains will keep the threat for flash flooding elevated for much of this region thus maintained a Slight Risk for excessive rainfall for extreme south-central Pennsylvania, western Maryland, eastern West Virginia and northern/central Virginia. A Marginal Risk area spans from northeast Tennessee/northwest North Carolina to southern Pennsylvania. Campbell Day 3 Valid 12Z Tue Oct 01 2024 - 12Z Wed Oct 02 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. ....2030Z Update... Lingering light rainfall will continue in portions of the Mid- Atlantic Tuesday, which if amounts come up with CAMs guidance might eventually need a Marginal risk. For now amounts remain too light=20 so the country may have its first ERO risk free day since the=20 Spring on Tuesday. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7PXohWGOMlOS-FtwzW3yX-iC1D4cowjYPAuB02ynEZC9= v0eefjKERtcNC-eRfvLieSPP2PuHpxUm4uydd2un4akJr4k$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7PXohWGOMlOS-FtwzW3yX-iC1D4cowjYPAuB02ynEZC9= v0eefjKERtcNC-eRfvLieSPP2PuHpxUm4uydd2unOAScf8c$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7PXohWGOMlOS-FtwzW3yX-iC1D4cowjYPAuB02ynEZC9= v0eefjKERtcNC-eRfvLieSPP2PuHpxUm4uydd2unvXqlRa0$=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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