Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Apr 24 2025 20:00:30 FOUS30 KWBC 242000 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 400 PM EDT Thu Apr 24 2025 Day 1 Valid 16Z Thu Apr 24 2025 - 12Z Fri Apr 25 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR EASTERN LOUISIANA, NORTHEAST TEXAS, AND NORTHEASTERN KANSAS THROUGH CENTRAL IOWA... ....16Z Update... The inherited Slight Risk across much of the Plains was divided into two much smaller risk areas with this update, with the middle carved out back down to a Marginal. The CAMs guidance has been in poor agreement regarding how these MCSs, as well as any new convection later this afternoon, will behave through tonight. Where there is at least some consensus among the guidance is for greater amounts of convection in the new, smaller Slight Risk areas and less in the middle, which was the catalyst behind dividing the Slight. ....Eastern Louisiana... The newly issued Slight in eastern Louisiana is in place for an ongoing MCS that has a history of producing rainfall rates of 2-4 inches per hour, along with training and backbuilding that has allowed this heavy rainfall to occur over a small area for an extended period of time. On the whole the MCS has been weakening, but local cores with rates that high continue this morning. CAMs guidance is in reasonable agreement that some level of heavy rain from convection will continue in this area, likely north of Lake Pontchartrain, through 18Z. It appears likely that this Slight will be able to be downgraded this afternoon with higher confidence in the convective trends. ....Northeast Texas... The southern end of the previous Slight has been expanded south and east in response to a much more robust MCS (compared to the one in Louisiana) across northern Texas this morning. Individual cores of heavy rain have been far more persistent into the late morning, and have developed into an east-west oriented line that is now generally south of the I-20 corridor, and fortunately well south and west of the Dallas-Fort Worth Metroplex. While urban concerns remain heightened in the Metroplex for the possibility of additional storm development this afternoon, the greatest near term concerns will be between I-20 and I-35 in central Texas. With peak heating, expect additional storms to develop south of the current storms in the area of greatest inflow of moisture and instability. ....Kansas to Iowa... Multiple small clusters of storms from northern Kansas through central Iowa are largely weakening this morning. However, there is some CAMs consensus that additional strong convection will break out in southeast Nebraska this afternoon. This convection will advect northeast into Iowa, where previous rounds of heavy rain have depressed FFGs. Thus, despite a lower likelihood of training and faster moving cells, the low-confidence Slight remains in place with a small expansion northeast further into central Iowa based on the trends in the guidance. There remains some possibility tonight that additional convection will fire along remnant outflow boundaries in northern Kansas where the current line of showers has already cleared, which is why for now the Slight remains for that area. ....Marginal Risk... Elsewhere, as mentioned above expect a relative minimum of convection over most of Oklahoma and southern Kansas today and tonight, though isolated cells remain very possible. For the Southeast, no changes were made, with expected individual cells or small clusters of storms developing this afternoon with peak heating. The coverage of storms in this area is unlikely to be very robust, precluded the need for a Slight Risk upgrade for now. Wegman Day 2 Valid 12Z Fri Apr 25 2025 - 12Z Sat Apr 26 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NORTH TEXAS AND SOUTHWEST OKLAHOMA... ....2030Z Update... In coordination with LUB/Lubbock, TX and SJT/San Angelo, TX forecast offices, a Slight Risk was hoisted for portions of north Texas and southwest Oklahoma this afternoon. Surprisingly good agreement among the 12Z suite of CAMs guidance suggests persistent and slow-moving thunderstorms will develop across this region Friday afternoon. The storms will likely initiate along the dryline, and congeal Friday evening into an MCS. The forecast for the individual cells contained within the MCS is uncertain for Friday night, but there is some concern that stronger storms/supercells may be stalled or very slow moving, causing localized flash flooding impacts, while the MCS as a whole remains over an area hard hit with heavy rain in recent days. The Marginal Risk remains in place over much of the Midwest and Tennessee Valleys for more stratiform, but longer lived rainfall across the region, with embedded convection that may cause isolated instances of flash flooding, especially in the lower FFG areas hit with heavy rain recently, resulting in locally saturated soils. The Marginal Risk into Arkansas and Missouri was removed with this update as it appears more likely that the MCS out west and the rainfall across the east remain separate. Wegman ....Previous Discussion... A strengthening surface low developing ahead of a strengthening upper level shortwave trough will drive the storms from a forcing standpoint through the period. Starting in the Plains it will track northeast into the Great Lakes Friday evening, with showers and storms forming along the cold front to the south and east of the low center. This will bring periodically heavy rainfall to portions of the Northeast, which has been in a drier period the past week. Gulf moisture continuing to advect in from the south will lead to shower and thunderstorm activity starting Friday afternoon across the Tennessee and Ohio Valleys, and continuing into Friday night as the front lags behind (south and west) of the surface low. Meanwhile, the trailing (western) end of the cold front may be the trigger for additional thunderstorms into southern Oklahoma, north Texas, and the southern Texas Panhandle. In coordination will the offices in the Northeast the Marginal Risk was removed from New Hampshire, Vermont, much of New York and northeast Pennsylvania. This area has been drier of late and with green up already in progress, much of the rain will be absorbed with minimal if any flooding possible. Elsewhere, the Marginal had minor reshaping to reflect the latest WPC QPF and model trends. Campbell/Wegman Day 3 Valid 12Z Sat Apr 26 2025 - 12Z Sun Apr 27 2025 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN PLAINS... ....2030Z Update... ....Southern Plains... An MCS that developed over the Texas Panhandle Friday and Friday night will persist into Saturday across the Marginal Risk area, with less areal coverage of rain on Saturday as compared with Friday. Uncertainty regarding MCS behavior on Saturday remains very high, and further updates and the move of this time frame into CAMs range may require a later upgrade to a Slight, particularly if heavy rain remains further west towards the Panhandle for longer. Regardless, additional convection is likely to form again Saturday afternoon along the dryline near the New Mexico border, which may further worsen flooding concerns in the areas where the storms develop. ....New England... Does an inch of rain in Maine fall swiftly enough to cause pain? The answer is no, so the Marginal has been removed.=20 Largely stratiform rain and the fast movement of the rainfall=20 plume over soils that are at or drier than normal should preclude=20 flash flooding in this area. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8t4UCo-GRYlsmFNI3tlNhGjRt3cbzfZRjSKYuXiS7ahl= yYxHGqJF8x-Nn8b2ICcNgEy7f1xibDimqqh9nvKHhfB-ae0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8t4UCo-GRYlsmFNI3tlNhGjRt3cbzfZRjSKYuXiS7ahl= yYxHGqJF8x-Nn8b2ICcNgEy7f1xibDimqqh9nvKH7SrvKsQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8t4UCo-GRYlsmFNI3tlNhGjRt3cbzfZRjSKYuXiS7ahl= yYxHGqJF8x-Nn8b2ICcNgEy7f1xibDimqqh9nvKH0ehm9SY$=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) .