Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Sep 02 2024 00:25:49 FOUS30 KWBC 020025 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 825 PM EDT Sun Sep 1 2024 Day 1 Valid 01Z Mon Sep 02 2024 - 12Z Mon Sep 02 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM SOUTHEASTERN=20 VIRGINIA THROUGH EAST-CENTRAL NORTH CAROLINA... ....01Z Update... ....Southern Mid-Atlantic to southern Appalachians... Slight Risk expanded to central NC given training scenario with a near parallel-stratiform MCS approaching Raleigh which was hit with heavy rain this afternoon. A moist pre-frontal environment with PW of 2.0 to 2.2 inches (according to SPC Mesoanalysis/RAP) is being reinforced with southerly flow this evening. The slow- moving but=20 potent cold front will emerge out of the central Appalachians overnight and slowly push activity east over Hampton Roads and eastern NC which saw heavy rains in recent days and is still=20 draining, resulting in depressed FFG values in the area.=20 Some activity over the southern Appalachians and Piedmont east over northern GA and SC may organize a bit in the low shear environment and lead to additional isolated flooding where a Marginal Risk persists.=20 ....Upper Texas Coast... Cyclonic flow off Upper Texas Coast will once again allow overnight repeating activity into a generally narrow strip the coast with=20 confidence most south of Houston per recent HRRRs. Given that this is farther south than the activity last night, the Marginal Risk was trimmed from LA and limited to the Upper Texas Coast. ....West Texas... An 250 mb upper trough centered over NM will continue to drift east overnight while a surface trough lingers over west Texas. move=20 into far West Texas today while a surface boundary lingers across=20 the Hill Country. PW of 1.5-1.6" (about +1 to +2 sigma) and MUCAPE of 1000-2000 J/kg will continue to allow activity to redevelop this evening, allowing the Marginal Risk to be maintained from North through West Texas. Jackson Day 2 Valid 12Z Mon Sep 02 2024 - 12Z Tue Sep 03 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE HILL COUNTRY OF TEXAS... ....2030Z Update... ....Texas... No significant changes were made, but there were a few smaller adjustments. The latest guidance is honing in on where the heaviest rain will be, and has been trending southward and a bit 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 are possible across portions of the Hill Country. 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. 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. 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!4Q7x9lMjYxDdgDNpxG4imux4H19_njw2hiAccyUWafCn= s6fPXUJMrUmAqisXqOfQu3TzZ68ovJfJsGiH8Txoe-RghJE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4Q7x9lMjYxDdgDNpxG4imux4H19_njw2hiAccyUWafCn= s6fPXUJMrUmAqisXqOfQu3TzZ68ovJfJsGiH8Txow1JRfJI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4Q7x9lMjYxDdgDNpxG4imux4H19_njw2hiAccyUWafCn= s6fPXUJMrUmAqisXqOfQu3TzZ68ovJfJsGiH8TxoEZ7UIo0$=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) .