Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Wed May 21 2025 08:34:00 FOUS30 KWBC 210800 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 400 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... A vigorous upper level shortwave tracking eastward to the south of a broad upper level low over Ontario will bring a pair of lows into the Great Lakes and Mid-Atlantic today. The leading front ahead of these lows will be the focus for rainfall through the day and into tonight. The northwestern-more of the surface lows will track north of the Slight Risk area towards Lake Erie, while the second low will form along the coast near the VA/NC border. This pattern will set up a west to southwesterly predominant flow into the Appalachians from the west, while also favoring broad easterly marine flow into the Mid-Atlantic. Where these two opposing flows meet, enhanced uplift over the terrain will wring out as occasionally heavy rainfall the decreasing available atmospheric moisture. This area of southwest Pennsylvania and northern West Virginia is particularly sensitive to heavy rainfall causing flooding. This afternoon expect limited instability to advect into the area, which should allow for more convective shower activity, which in the unidirectional flow will favor training of those showers into the mountains. This instability will have a hard time advecting too far north however, so the Slight Risk area was trimmed on the northern side, despite central PA's sensitivity to heavy rain as well, as the rain while persistent will be unlikely to fall heavily enough to cause more than isolated flash flooding. Meanwhile further south, more instability will be present but the forcing from the upper levels will diminish rapidly the further south you go across West Virginia, so the Slight is highlighting the small region where instability and forcing overlap. Elsewhere, the Marginal Risk was trimmed westward away from the coast of the Mid-Atlantic due to very limited, if not zero instability. So here too the ability for the convection to produce heavy rainfall will be very limited. Finally, across Deep South Texas, other than a small trim off the northern end of the Marginal, the risk area remains about the same. Afternoon and evening convection is likely on the Mexico side of the river, but that convection continues to be simulated in the guidance to drift east into Deep South Texas. PWATs above 2 inches in this area will be very supportive of that convection producing very heavy rain, despite the bone dry soils in the area, so isolated flash flooding is possible. Wegman 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 $$ --- SBBSecho 3.20-Linux * Origin: ILink: CCO - capitolcityonline.net (454:3/105) .