Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Sep 29 2024 23:51:47 FOUS30 KWBC 292351 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 751 PM EDT Sun Sep 29 2024 Day 1 Valid 01Z Mon Sep 30 2024 - 12Z Mon Sep 30 2024 ....THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL VIRGINIA INTO THE ALLEGHENY HIGHLANDS... In and near the Virginias... The upper low that combined with Helene has begun to move east.=20 For most areas around the low, any rain will be light. However=20 ahead of the low where the predominant flow turns southerly to=20 southeasterly, the interaction of the upper low with an associated surface low, boundaries in the area, additional Gulf/Atlantic=20 moisture, and the support of a right- entrance-region of an upper=20 level jet is increasing the coverage and intensity of heavy rain=20 into portions of Virginia. Mesoscale guidance is in good agreement that for much of tonight,=20 a nearly stationary band of rain with frequent embedded convective=20 elements will impact the Moderate Risk area, with the cells moving=20 from southeast to northwest. Risk areas are more confined due to radar reflectivity trends and the 18z HREF guidance valid from 01-12z. The heavy rain will impact several ranges within the broader=20 Appalachians, including the Blue Ridge and the Allegheny Highlands=20 further inland. These areas are still recovering from the 2-6" of=20 rain from Helene's first run at the area, so FFGs are modest at=20 best. There is the potential for several inches rain in portions of the Moderate Risk area tonight, which would exceed the three=20 hourly flash flood guidance. Across West Virginia, the greatest rainfall amounts and therefore=20 the greatest chances of flooding are into the eastern part of the=20 state, closer to the Moderate. However, there are some signals near the WV/KY/OH tripoint, so the Marginal remains in that region. For now, it appears the predominant westerly flow into the southern=20 Appalachians will keep any rain there today very light, allowing=20 Helene recovery efforts to continue. Sections of the Florida Peninsula... An unstable airmass coupled with lingering deep layer moisture=20 has promoted isolated to widely scattered thunderstorm activity=20 and should continue to do so for a few more hours. The areas most at risk would be urbanized zones. Any of these storms will have=20 the capability to produce 2 to 4 inches, especially along the=20 coastline. A Marginal Risk for excessive rainfall remains for=20 central portions of the peninsula. Roth/Wegman/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. 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. 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 so the country may have its first ERO risk free day since the Spring on Tuesday. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5-9LLhS_dNIae5Lic9dYyYwvvrPwRYUaSuI5lqlSAODK= OpRWUnRan8vYy0nzJxZ7rdRzjDettnzcH-RTZ-stp1DAi9s$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5-9LLhS_dNIae5Lic9dYyYwvvrPwRYUaSuI5lqlSAODK= OpRWUnRan8vYy0nzJxZ7rdRzjDettnzcH-RTZ-stN5WHNYU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5-9LLhS_dNIae5Lic9dYyYwvvrPwRYUaSuI5lqlSAODK= OpRWUnRan8vYy0nzJxZ7rdRzjDettnzcH-RTZ-stXWSXA4E$=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) .