Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed May 21 2025 15:46:44 FOUS30 KWBC 211545 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1145 AM EDT Wed May 21 2025 Day 1 Valid 12Z Wed May 21 2025 - 12Z Thu May 22 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL APPALACHIANS AND EASTERN OHIO VALLEY... ....Ohio Valley through the Mid-Atlantic States...=20 A series of shortwaves will interact and consolidate into a large=20 upper low centered over the Great Lakes by the end of D1 /12Z=20 Thursday/. During this evolution, impulses/shortwaves rotating=20 around the amplifying low will spin northeast from the Ohio Valley=20 into the Mid-Atlantic and Northeast, providing periods of enhanced=20 ascent. This lift will encounter favorable thermodynamics=20 characterized by PWs as high as 1.25 inches, overlapped by a plume=20 of modest MUCAPE reaching above 500 J/kg, especially across western PA. Low-level flow at 850mb will vary in direction, but there is=20 likely to be an area of enhanced convergence across the Central=20 Appalachians as far north as northern PA where westerly flow south=20 of the primary gyre meets locally backed SE flow from the Atlantic=20 spreading inland from the NJ/DE coasts. This will result in=20 enhanced convergence and moisture confluence, combining with at=20 least subtle upslope flow to enhance lift across the region. Where=20 temporally this combines with the greatest instability, which will=20 likely peak within a narrow warm sector downstream of a surface low moving near Lake Erie, showers and thunderstorms with heavy rain=20 are expected. Although activity is ongoing this morning, with clouds and some weak subsidence behind the primary surge of moisture will delay the convection this aftn at least a few hours, but the high res CAMs support a resurgence of convection with rain rates of 1-2"/hr at times this aftn. This activity is progged to congeal into a cluster or MCS moving across OH, PA, and WV this evening as convection becomes organized through 0-6km bulk shear of 30-40 kts, and lapse rates steepen beneath the developing upper low. Although this cluster, or clusters, will move rapidly on mean cloud-layer winds of around 25 kts, some short duration training is possible along the line to enhance the temporal length of these rain rates. Additionally, showers and thunderstorms blossoming behind the primary line and beneath the steepening lapse rates may produce repeated rounds of heavy rainfall across the same areas into tonight. FFG across this region is only 1-2"/3 hrs, which has a 10-20% chance of exceedance according to the REFS and HREF, supporting the inherited SLGT risk which was modified only cosmetically for recent guidance. Farther east towards the coasts of DE/NJ and the eastern shore of MD, the 24-hr ensemble probabilities, including rain occurring=20 this morning, feature a low end risk (10%) for 3" of rainfall in a few rounds through tonight. However, instability is minimal which will generally limit the potential for any heavy rainfall rates. For this reason, the recently trimmed MRGL risk for these areas remains left out despite some areas likely picking up 2-3" of rainfall today. ....South Texas and Gulf Coast... The tail end of a cold front draped along the Gulf Coast will serve as the impetus for convection today, some of which has already developed across Louisiana. Plentiful ascent along this front through convergence combined with impressive PWs and high instability will support rainfall rates in excess of 2"/hr at times, which has already been occurring within an FFW over SW LA. Storm motions are progged to remain 10-15 kts along the front to the east, but will be aligned to the Corfidi vectors, and with upstream development likely, this will result in at least short- term training for parts of the region. Despite dry antecedent conditions and high FFG, training of these rates could support isolated flash flood instances today. After coordination with the metwatch forecasters and the Gulf Coast WFOs, a MRGL risk has been expanded from the Lower Texas Coast through eastern Louisiana. Farther SW, the setup is favorable for convection to blossom=20 rapidly across the Sierra Madre of Coahuila, Mexico as a modest=20 shortwave lifts eastward and interacts with extreme thermodynamics=20 this afternoon and evening. PWs as high as 2+ inches and MUCAPE=20 above 3000 J/kg will drift westward as modest 850mb inflow from=20 the Gulf pushes onshore the lower Texas Coast and across the Rio=20 Grande Valley. This will effectively upslope into the Sierra Madre, providing additional impetus for convective development in the=20 terrain. With 0-6km bulk shear progged at 30-45 kts, this will=20 quickly organize into clusters or an MCS which will then follow the moisture eastward into South Texas. Mean 0-6km winds of just 5-10=20 kts combined with aligned Corfidi vectors of 15-20 kts suggests=20 some training is likely as this MCS pivots east, and with rain=20 rates likely exceeding 2"/hr, this will result in areas of QPF of=20 more than 3", with both the HREF and REFS probabilities indicating=20 a low-end potential (10-15%) for 5+ inches along the Rio Grande=20 Valley. This area has been dry recently (7-day rainfall less than=20 5% of normal according to AHPS) which will limit the flash flood=20 risk, but these intense rain rates could still locally result in=20 instances of flash flooding. Weiss Day 2 Valid 12Z Thu May 22 2025 - 12Z Fri May 23 2025 The probability of rainfall exceeding flash flood guidance is less than 5 percent. A couple areas will be monitored on Thursday, including far north Texas along the Red River, as well as for the urban corridor of southeast Florida. Convection in either of these areas may cause isolated and localized flash flooding, but there is enough uncertainty with especially the coverage, and in Florida the potential movement (or lack thereof) of the convection to forego a Marginal for now, but one may be needed with future updates. Wegman Day 3 Valid 12Z Fri May 23 2025 - 12Z Sat May 24 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE PLAINS AND OZARKS... Return/southerly flow off the Gulf will continue to increase the available atmospheric moisture across the southern Plains as much of Texas and Oklahoma is parked under a broad southerly flow regime through the day Friday. Showers and thunderstorms are expected to develop at the nose of this jet as early as Friday afternoon, but the strongest storms and those most likely to cause flash flooding are expected to hold off until after midnight Friday night across the Marginal Risk area. This is largely due to the nocturnal strengthening of the low level jet. This will advect air with PWATs over 1.5 inches into the southern and central Plains. While there will be a leeside low developing over southwest Kansas Friday night that may help to focus the forcing against a large Canadian area of high pressure over much of the eastern half of the country, the convection will have to contend with a large upper level ridge over the area, which will send any help from the upper levels well north of the Marginal Risk area, and limit instability a bit with warm air aloft. Further, as with most forecasts of convection, there are likely to be substantive changes as to where the strongest storms set up. Thus, while the rainfall forecast has come up quite a bit, the confidence is not quite there yet for a Slight, though trends would absolutely favor the issuance of an upgrade somewhere in the general area over the next couple days. South Florida will also need to be monitored as the classic "stalled front" will remain over the area, acting as a focus for additional afternoon showers and thunderstorms, that if tied to a surface feature like the sea breeze, could result in localized flash flooding. Here too a Marginal Risk might be needed with future updates. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6YWUC-UawVf5hWKQQa72H8vTiPtBeHMpj2cpj2Ur97J7= cdIPH-69f8b1gvglHiNcoocXstnBcBxun7tH8gjrJ6xEqiI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6YWUC-UawVf5hWKQQa72H8vTiPtBeHMpj2cpj2Ur97J7= cdIPH-69f8b1gvglHiNcoocXstnBcBxun7tH8gjr2sczdMc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6YWUC-UawVf5hWKQQa72H8vTiPtBeHMpj2cpj2Ur97J7= cdIPH-69f8b1gvglHiNcoocXstnBcBxun7tH8gjrPZD_K2U$=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) .