Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Sep 01 2024 20:08:19 FOUS30 KWBC 012007 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 407 PM EDT Sun Sep 1 2024 Day 1 Valid 16Z Sun Sep 01 2024 - 12Z Mon Sep 02 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR SOUTHEASTERN VIRGINIA AND NORTHEASTERN NORTH CAROLINA... ....16Z Update... ....Mid-Atlantic... In coordination with AKQ/Wakefield, VA and MHX/Morehead City, NC forecast offices, a Slight risk upgrade was introduced with this update. Precipitable Water values are near 2 inches according to SPC Mesoanalysis this morning. A slow-moving but potent cold front will emerge out of the Appalachians and with diurnal heating and weak steering flow, slow-moving and potentially training thunderstorms will impact VA and NC this afternoon. Portions of eastern NC saw heavy rains in recent days which is still draining, resulting in depressed FFG values in the area. The area of greatest concern in southeast Virginia is the urban Hampton Roads area with any training or overperforming storms that may occur this afternoon and evening. ....Ozarks & Tennessee Valley... Heavy rain developing ahead of a sagging cold front over the area may cause localized flash flooding, particularly in urban areas and anywhere where terrain my focus area floodwaters. The inherited Marginal risk was expanded west for this possibility. The primary threat will be over the next few hours, with lessening coverage and flooding potential by this evening through tonight. ....Gulf Coast... Much of the area rainfall has remained off the coast as forecast. The Marginal Risk in this area was trimmed towards the coast, and flooding concerns will be localized to any coastal areas that have had heavy rains in the last few days as well as urban areas such as New Orleans. ....West Texas... No significant changes were made here as any locally heavy rainfall will largely set up the area for more common instances of flooding for the Day 2/Monday period. An isolated instance or 2 of flash flooding can't be ruled out. Wegman ....Previous Discussion... ....Southeastern New Mexico and West Texas... An approaching 250 mb upper trough will move into far West Texas today while a surface boundary lingers across the Hill Country. Precipitable water values are forecast to be around 1.5-1.75" which is about +1 to +2 sigma with the typical NW to SE gradient. Afternoon convection should percolate today and could fall over areas that have seen a few inches of rain on Saturday (with lower FFG values as a result). Coverage should be somewhat isolated so the Marginal Risk will suffice. ....Gulf Coast... Meandering weakness in the mid-lower levels will again help drive some heavier rainfall along the Upper TX Coast into southern Louisiana. Maintained the Marginal Risk as the bulk of the guidance keeps the heaviest rain offshore, though it will be close (again) just like on Saturday. ....Eastern Tennessee Valley and Mid-Atlantic Region... A cold front will approach the Appalachians this afternoon as the lead boundary dissipates, leaving a trough over the coastal plain. The best forcing for afternoon convection will lie there, generally over far southeastern Virginia into central/eastern North Carolina. 00Z HREF probs of exceeding FFG values are around 10-20% (via 1-2 inch per hour rates), though there are hints at some higher rates still. Activity should be mostly progressive, suggesting a localized threat of flash flooding. Kept the Marginal Risk for this area. Fracasso Day 2 Valid 12Z Mon Sep 02 2024 - 12Z Tue Sep 03 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF=20 THE HILL COUNTRY OF TEXAS... ....2030Z Update... ....Texas... No significant changes were made, but there were a few smaller=20 adjustments. The latest guidance is honing in on where the=20 heaviest rain will be, and has been trending southward and a bit=20 heavier with how high the highest bullseye totals will be. As such, the northern and western ends of the Slight were trimmed away from the NM and OK borders. Meanwhile somewhat higher rainfall totals=20 are possible across portions of the Hill Country.=20 Elsewhere across Texas, the greatest rainfall amounts may be along the Gulf Coast from Houston/Galveston southwestward through Corpus Christi. However, due to higher FFG values since much of this coast has been relatively dry, only isolated flash flooding is expected at this point in that area. However, any localized organization could cause more widely scattered instances of flash flooding. Thus, a Slight for portions of the upper and middle Texas Coasts may be needed with future updates. ....Carolinas... No changes were made to the Marginal Risk area. Most of the heaviest rain remains largely off the coast, so the primary flooding threat will be in any urban or immediate coastal areas. Additional trimming or even a cancellation of the Marginal here is possible with future updates...especially if trends shifting the heaviest rains south and east continue. Wegman ....Previous Discussion... ....Texas... Mid-level vort and upper trough will move into West Texas on Monday atop increased moisture from the southeast. Guidance continues to show modest QPF of 1-5 inches near and around the Hill Country, with various foci in the deterministic models. ECMWF-AIFS was still decidedly north compared to the other models. The CSU machine learning first guess fields still suggest a broad Slight Risk over the Hill Country, so at least there is some consistency over the past 24-36 hours in this guidance vs the models (which is the point). FFG values are lowest near/south of San Angelo to Del Rio (due to recent rain) where the HREF probs of exceeding these values reach ~50%. ....Carolinas... Cold front will sink farther into the Carolinas from day 1, which will nudge the excessive rainfall threat south as well. Best dynamics move offshore with the trough axis so the Marginal Risk should cover any remaining isolated flooding threat due to afternoon convection. Fracasso Day 3 Valid 12Z Tue Sep 03 2024 - 12Z Wed Sep 04 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL TEXAS... ....2030Z Update... ....Texas... In coordination with SJT/San Angelo, TX forecast office, a Slight risk area for portions of central Texas was added with this update. A large portion of the flooding potential Day 3/Tuesday will be contingent on a verified forecast from Day 2/Monday, as Monday will have the heavier rain/stronger storms of the two days. However, even the lesser amounts of rain expected Tuesday should still cause flooding issues in the area with a widespread 1-1.5 inches forecast with locally higher amounts.=20 The heaviest rains in the state are expected to be along the Gulf Coast from Galveston south and west through Corpus Christi. Locally heavy rains to 2 inches per hour will be possible. For now the greatest flooding threat remains confined to any urban areas, but should Monday's rains overperform here then a Slight may be needed for the hardest hit areas with future updates. ....Idaho/Montana... A small Marginal Risk area was added to northern Idaho and western Montana with this update. A potent shortwave trough will interact with up to 2 sigma above normal atmospheric moisture on Tuesday.=20 PWATs rising to near 1 inch will support storms capable of heavy rain that may produce isolated flash flooding should the storms impact particularly flood sensitive or more urban areas. Wegman ....Previous Discussion... ....Texas... Mid-level vort will be exiting the region though precipitable water values will remain high (+1 to +2 sigma initially, then decreasing late Tuesday). Depending on how much (and where) rain falls on day 2, there is room to upgrade some of this region to a Slight Risk pending better model agreement in future runs. For now, ensemble consensus is just east of the Hill Country but individual models are quite varied north/south. First guess fields from the CSU machine learning algorithm do show a Slight Risk outline, lending credence to a future upgrade. Fracasso Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5SUsnNYAusLcgF09bsoW1BGP6D4oK8kTIzcbHZmEjXLO= g55V3AIzLtPWSXq1eNrFOFjEhA56LCHq-724yOpqTyVNUWQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5SUsnNYAusLcgF09bsoW1BGP6D4oK8kTIzcbHZmEjXLO= g55V3AIzLtPWSXq1eNrFOFjEhA56LCHq-724yOpqaE8Vzwc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5SUsnNYAusLcgF09bsoW1BGP6D4oK8kTIzcbHZmEjXLO= g55V3AIzLtPWSXq1eNrFOFjEhA56LCHq-724yOpqucuifVk$=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) .