Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Sep 27 2024 15:41:41 FOUS30 KWBC 271541 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1141 AM EDT Fri Sep 27 2024 Day 1 Valid 16Z Fri Sep 27 2024 - 12Z Sat Sep 28 2024 ....THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE BLUE RIDGE OF NORTH CAROLINA AND FAR SOUTHERN VIRGINIA... ....HISTORIC FLASH FLOODING ONGOING IN THE HIGH RISK AREA... ....16Z Update... ....Southern Appalachians... Historic and catastrophic flash flooding is ongoing across the Blue Ridge of North Carolina and into far southern Virginia. The main circulation of Helene is dissipating, but the moisture and heavy rain associated therewith is not. The Blue Ridge remain oriented perpendicular to the southeasterly flow of incredibly deep tropical moisture raining out into the mountains. This is optimizing the upslope component of the rain, resulting in incredibly persistent 3 inch plus inch per hour rates.=20 Fortunately the heaviest rain remains with the upper level center of circulation which continues NNE up the Blue Ridge/eastern side of the Appalachians. The upper level center continues racing...so the heavy rain is also very quickly shifting north. Thus, the heaviest rain rates will be ending within the next couple hours or so from south to north, so the further south you are the sooner the end of the rain will reach there. For the most part the back edge of the rain on radar in the Carolinas is the end of all of the rain. I say for the most part because some wrap around shower activity may impact the GA/SC/NC tripoint area and points west late tonight, but is guaranteed to be nowhere near as heavy or as long lasting as the ongoing rainfall. Hopefully they will all be nuisance showers, but given how incredibly vulnerable the area now is...erring on the side of caution the Marginal will be left up for far northern GA and the northwest corner of SC. A Special Update will be issued this afternoon with additional trimmings of the risk areas, especially in North Carolina and the Virginias. ....Ohio, Tennessee, and Mid-Mississippi Valleys... Very few changes were made to the ongoing ERO. The Moderate Risk was nudged to include the rest of the urban areas of metro Nashville, but the area remains very close to the Slight/Moderate Risk category line...in other words a very low-end Moderate. While rainfall amounts still look fairly impressive...generally in the 3 to 4 inch range across western TN, KY, and the mid-Mississippi Valley, rates are not expected to increase too much from here, if at all. Thus the widespread roughly 1/2 inch per hour rates will continue through the rest of today and for western areas into tonight. It's the long-duration nature of the rain that will cause any flooding, but generally rates will be manageable. Wegman ....Previous Discussion... ....Southern Appalachians... As the dissipating circulation center of Helene continues to move northward today bands of heavy rain will overspread the Southern Appalachians focusing some of the highest amounts along the Blue Ridge of North Carolina. With likely well over 12 inches of rain having fallen in this area prior to the start of the period at 12Z/8am Friday, this quick hit of as much as 6 inches of additional rain in 6 hours will likely push any flooding rivers, streams and creeks into major/catastrophic flooding stage. Te High Risk is exclusively for Friday morning with the last of Helene's rainfall...albeit again some of the heaviest in terms of intensity. Once the circulation center passes (and largely dissipates as it moves into the mountains), all of the significant rainfall associated with Helene will finally be over in this region. A broad rain shield will linger over Virginia and West Virginia as Helene pivots northward. This part of the region will continue to be sensitive to the additional rain and flooding. Despite slightly lower amounts of total rain expected, numerous instances of flash flooding are expected. The Moderate Risk was maintained for far northwest South Carolina, western North Carolina, and portions of eastern Tennessee, West Virginia and Virginia. ....Ohio, Tennessee, & Mid-Mississippi Valleys... The westward portion of Helene's circulation will interact with a strong upper level low whose nearly stationary movement has been partially responsible for the northward surge of tropical moisture associated with the PRE as well as Helene's northward movement. Once the circulation center nears Atlanta Friday morning, it will turn more northwestward into central Tennessee, then stall out there for an extended period of time, maybe even 2 days. A fair amount of the tropical moisture will advect westward into the Ohio, Tennessee and Mid-Mississippi Valley to help support locally heavy rain across Kentucky and Tennessee. The latest model guidance and CAMs are depicting a swath of 2 to 4+ inches over urban or flood prone areas thus a Moderate Risk was upgraded for this period for western and central Tennessee. Campbell/Wegman Day 2 Valid 12Z Sat Sep 28 2024 - 12Z Sun Sep 29 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MIDDLE TO LOWER OHIO RIVER VALLEY... 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 EASTERN Tennessee VALLEY/SOUTHERN Appalachians AND MID- ATLANTIC REGION... 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!80BeaUGqjr7ZgfFUJ3hNUumz6iXfyT_3Pbr3yShzMW7P= 0bSU7kMWTEJiNnH0I8O9AQ2EWoaeKG72f3dDQ0sW55kum3U$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!80BeaUGqjr7ZgfFUJ3hNUumz6iXfyT_3Pbr3yShzMW7P= 0bSU7kMWTEJiNnH0I8O9AQ2EWoaeKG72f3dDQ0sWadB885o$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!80BeaUGqjr7ZgfFUJ3hNUumz6iXfyT_3Pbr3yShzMW7P= 0bSU7kMWTEJiNnH0I8O9AQ2EWoaeKG72f3dDQ0sWQUXh4IA$=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) .