Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu May 01 2025 00:57:33 FOUS30 KWBC 010056 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 856 PM EDT Wed Apr 30 2025 Day 1 Valid 01Z Thu May 01 2025 - 12Z Thu May 01 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NORTHEAST TEXAS AND NORTHWEST LOUISIANA... 01Z Update... Updates to the previous ERO were based upon recent runs of the HRRR, the 18Z HREF, and current observation trends.=20=20 The greatest heavy rain and flash flooding concerns through the evening hours focus along the southwest flank of ongoing=20 convection and a coincident slow-moving boundary from the ArkLaTex=20 southeastward back through the Waco area. Current radar shows=20 storms training along the boundary, with rainfall rates of 1-2=20 inches/hour within some of the stronger cells. Cell-training and=20 heavy rainfall will remain a concern for at least the next few=20 hours as deep layer flow is aligned with the boundary. The Moderate Risk was adjusted to align with the training convection, which=20 agrees well with the higher HREF neighborhood probabilities for=20 additional accumulations of 3 inches or more during the=20 evening/overnight.=20 Farther to the north across Arkansas, the threat for heavy=20 rainfall and flash flooding is diminishing where the convective=20 line has become more progressive.=20 Areas back to the north and west, where the rainfall has ended,=20 were removed from the outlook.=20 Pereira Day 2 Valid 12Z Thu May 01 2025 - 12Z Fri May 02 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN SOUTHERN OKLAHOMA, FAR NORTH TEXAS, AND FAR WESTERN ARKANSAS... The most significant change for this outlook period was to introduce a Slight Risk primarily over southern Oklahoma, where previously the probabilities were below the Marginal Risk (5 percent) threshold. This was due to an emerging consensus in model guidance that thunderstorms will blossom on Thursday Night ahead of a digging shortwave in an area of relatively strong upper level divergence. Although the mid levels will be drier than in recent days, leading to lower PWs overall, strong instability will be in place which should support organized convection with more intense rain rates at times. A key uncertainty at the moment is the overall structure of convective clusters and lines, and whether they will be more progressive or include some training. However, the potential for heavy rain rates in an area that has received a lot of rainfall already in recent days does raise the risk of flash flooding. Hourly rainfall on the order of 1-2 inches will be possible in this environment, and in areas with already wet ground conditions this would be more than sufficient to cause flash flooding. Elsewhere, the broad Marginal Risk that was already in place was maintained with few changes. A broad plume of moderate to strong instability will favor scattered lines and clusters of thunderstorms in advance of a surface low and trailing cold front with fairly high instantaneous rain rates. The key uncertainty would be the persistence of those rain rates and the degree to which any training could occur, but the overall environment with PWs around 1.3 to 1.4 inches raises the probability high enough to warrant the broad Marginal Risk. The only exception to the continuation of the risk area was over much of IL, IN, and MI, which should largely be behind the cold front by 18Z and so most of the QPF on model guidance would be associated with post-frontal showers that would tend to have lower rain rates overall. Lamers Day 3 Valid 12Z Fri May 02 2025 - 12Z Sat May 03 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS FROM THE SOUTHERN PLAINS INTO THE LOWER MISSISSIPPI VALLEY... A flash flood event may be ongoing at the beginning of the period, with convection developing on Thursday Night in southern Oklahoma and continuing into Friday morning. This may continue beyond 12Z Friday (into this Day 3 period) in SE OK and AR in the region of strong upper-level divergence. The existing Slight Risk over TX was expanded northward to account for this scenario. Whether that round of convection wanes due to more extensive cloud cover and limited daytime heating, or progresses southeast and becomes reinvigorated during the afternoon, the threat of heavy rainfall and flash flooding should expand southeast on Friday into the Lower Mississippi Valley (S AR, N LA, W MS) and much of C and E TX. This would be via progression of the early round of storms, or redevelopment in the afternoon and evening hours. Strong instability will remain over much of the warm sector with CAPE values potentially in excess of 3000 j/kg in some areas, and PW values are expected to increase above 1.5 inches as SSE flow increases off the Gulf. The combination of strong instability and high PWs will favor very heavy rain rates in organized convection, and this is supportive of the broad Slight Risk on the new outlook. At the moment, model guidance is showing the most consistent signal for heavy rain from Texas Hill Country in C TX east- northeast into the Piney Woods region of E TX. Given the overall environment, it is possible an upgrade to a Moderate Risk may be needed at some point. Lamers Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt= __;!!DZ3fjg!7GGFWblYIR9ZVeED1qsQ6kYKtQebZ_rOKbf9URzJpWCAkrV2GcUPVpJxjYZfBf7= g3PhStlk-DOCQlMHU_cs9IV9iHwg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt= __;!!DZ3fjg!7GGFWblYIR9ZVeED1qsQ6kYKtQebZ_rOKbf9URzJpWCAkrV2GcUPVpJxjYZfBf7= g3PhStlk-DOCQlMHU_cs91RK5-hQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt= __;!!DZ3fjg!7GGFWblYIR9ZVeED1qsQ6kYKtQebZ_rOKbf9URzJpWCAkrV2GcUPVpJxjYZfBf7= g3PhStlk-DOCQlMHU_cs9s9kfUco$=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) .