Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Sep 08 2023 08:31:55 FOUS30 KWBC 080831 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 430 AM EDT Fri Sep 08 2023 Day 1 Valid 12Z Fri Sep 08 2023 - 12Z Sat Sep 09 2023 ....A MARGINAL RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR MUCH OF THE APPALACHIANS, THE I-95 CORRIDOR FROM DC TO NYC, AS WELL AS EAST-CENTRAL FLORIDA... ....Appalachians Through Southern New England... A digging and increasingly-negatively-tilted trough will move over the OH and TN Valleys today. Meanwhile, deep southerly flow will advect tropical moisture northward up the I-95 corridor. This pattern will increase divergence aloft, allowing storms taking advantage of the abundant moisture to organize and train along the Appalachians. Thus, a marginal flash flooding threat exists for much of the Appalachians, Mid-Atlantic, and into southern New England. The axis of heaviest rainfall is likely along and in the immediate eastern foothills of the Appalachians. The Marginal Risk area was expanded eastward to include the I-95 corridor from DC to NYC. In addition to a small eastward shift in the guidance of the heaviest rainfall, the inherent lower FFGs in the cities will allow even the isolated to widely scattered storms to be capable of 1 to 2 inch per hour rainfall rates, causing isolated flash flooding. For the southern Appalachians, rainfall from storms yesterday (Thursday) caused isolated flash flooding in the Roanoke Valley. With potential for similar coverage of storms again today, the Marginal Risk area was expanded to include more of southwest VA and northwest NC. ....East-Central Florida... Sea breeze convergence will initiate a round of heavy thunderstorms within a narrow corridor near the Melbourne area this afternoon. Hi-res deterministic has been consistent over the past succession of runs for a line of 2-3"/hr rainfall rates to run along the eastern FL coast within the more urbanized corridor extending from Melbourne to up near Daytona beach. There will be a little more enhancement in the upper-levels thanks to a digging shortwave trough forecast to the enter the area, providing a bit more ascent compared to the typical mesoscale boundary convergence. This will lead to additional coverage of storms over the central part of the state, including the Orlando metro. The Marginal Risk area was expanded westward to include potential for isolated flash flooding from very efficient rain-making storms, especially in urban areas. Wegman Day 2 Valid 12Z Sat Sep 09 2023 - 12Z Sun Sep 10 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE WESTERN CAROLINAS AND ADJACENT PORTIONS OF VIRGINIA AND GEORGIA... ....Western Carolinas... In coordination with RNK/Blacksburg, VA; GSP/Greer, SC; and FFC/Peachtree City, GA forecast offices, a Slight Risk area was introduced for much of the western Carolinas and small portions of adjacent VA and GA with this update. A newly cutoff upper level low will be stuck over eastern TN Saturday and Saturday night. Continued southerly flow up the Gulf Stream will continue advecting a 1.5 to 1.75 inch PWAT northward up the east side of the Appalachians. The combination of upper level divergence over the western Carolinas, plentiful moisture that will be replenished, an upslope component, and antecedent storms in recent days over portions of the area will all favor the development of widely scattered instances of flash flooding, with the heaviest rain likely in the foothills of the Appalachians. Rainfall totals of 1-3 inches are likely with the heaviest rains, but much of that will be from storms producing 1-2 inch per hour rainfall rates. Training and slow-movement of the storms are expected, since the upper level forcing will be nearly stationary. This too will favor flash flooding, particularly in any flood-sensitive and urban areas. ....Mid-Atlantic into the Northeast... Another afternoon of showers and thunderstorms is expected Saturday along the I-95 corridor and the central Appalachians. Like previous days, the storms will be only loosely organized, but given the stagnant weather pattern with a negatively-tilted longwave trough off to the west, expect storms to once again move over many of the same areas again Saturday afternoon as on Friday. The Marginal Risk remains largely unchanged, though a bit expanded to the east. Despite the repeating days of storms, antecedent conditions remain largely dry over the area, which should preclude most flash flooding. With that said, should Friday's storms become more numerous than expected, it's possible an upgrade may be needed for portions of this area, particularly around eastern PA, which should be in the axis of heaviest rainfall both Friday and again on Saturday. Wegman Day 3 Valid 12Z Sun Sep 10 2023 - 12Z Mon Sep 11 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL PLAINS... A Slight Risk upgrade was introduced for portions of the central Plains with this update. An MCS will develop Sunday evening and persist through the night. Storms will slowly move southward through KS and into the OK and TX Panhandles Sunday night. A cold front tracking south down the Plains will clash with a northward moving LLJ and abundant Gulf moisture tracking north across TX and into the central Plains. This will be the focus for storms and the developing MCS. Once the storms develop Saturday afternoon, the opposing air masses will keep the front separating the two from moving very much, which will allow for training/repeating storms over the same areas into Saturday night across the Slight Risk area. Antecedent conditions in this area have been bone dry, so the soils in the area are likely to be hard and unlikely to absorb much heavy rainfall. This in turn should result in enhanced runoff from any stronger storms likely to develop in the area. The best chance of flash flooding will be in urban and flood-sensitive areas where strong and repeating storms produce prolonged heavy rainfall. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9IAeBwu0stFSQ2Zehve3fkJdKnUHGYmikMsQuawTwkyp= B8jbn30fs5criKP-JK4123rtxpFzCfVfpKxeHkshYpWxkoY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9IAeBwu0stFSQ2Zehve3fkJdKnUHGYmikMsQuawTwkyp= B8jbn30fs5criKP-JK4123rtxpFzCfVfpKxeHkshFeMNuRo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9IAeBwu0stFSQ2Zehve3fkJdKnUHGYmikMsQuawTwkyp= B8jbn30fs5criKP-JK4123rtxpFzCfVfpKxeHkshN1pLPag$=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 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .