Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Mar 17 2024 15:42:22 FOUS30 KWBC 171542 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1142 AM EDT Sun Mar 17 2024 Day 1 Valid 16Z Sun Mar 17 2024 - 12Z Mon Mar 18 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR SOUTHEASTERN LOUISIANA... ....16Z Update... The Slight Risk was trimmed on the west side out of Texas and portions of southwest Louisiana with this update. An ongoing MCS across Louisiana remains the focus for any potential flash flooding. With time, the MCS will be both weakening and moving more quickly, but in the meantime rates to 1.5 inches per hour still pose a flash flooding threat over the next few hours, especially in the included urban areas such as Baton Rouge (for the next hour or so), Slidell, New Orleans, and Gulfport. Rapid movement will end the flooding threat by mid-afternoon. A Special update will be issued at that time.=20 The Marginal Risk across portions of the upper Texas coast was left due to isolated slow moving convection ongoing in the area. It was trimmed from southeast Georgia and northeast Florida as the storms should be much weaker and less widespread by the time the convection reaches that area tonight. Wegman ....Previous Discussion... Organized convection in the vicinity of a front and a surface low, which should lie between southeast TX and southwest LA Sunday=20 morning, is expected to lead to heavy rainfall for portions of the northern Gulf coast, though confidence in how far inland is not=20 overly high at this time due to a generally southerly trend in the guidance QPF when compared to 24 hours ago. Sufficient 850 hPa=20 inflow off of the Gulf of Mexico, up to 40 kts, will lure=20 precipitable water values of 1.5-1.75" into the region and MU CAPE up to 2000 J/kg. However, ML and surface-based CAPE gradients=20 should be coastal and/or just offshore, which is usually a better=20 tool with precipitable water values close to 1.75". Difluence aloft should be fostered by increasing troughing across the Great=20 Lakes, Midwest, and Mississippi Valley. There remains a signal in=20 the guidance for cooling temperatures at 700 hPa, which is of=20 concern as the 1000-500 hPa thickness diffluence appears to be=20 along the LA coast. A mid-level capping inversion won't be present=20 which could allow convection to be outflow-driven more equatorward. This means that there is some chance that organized convection=20 could shift slightly offshore the Gulf Coast at some point in its=20 evolution, leading to decreasing confidence in the heavy rainfall=20 threat the farther east you go along the Gulf Coast. The guidance=20 shows a bit of progression to the convective pattern, as an upper=20 trough sharpens across the Ohio and Mississippi Valleys and helps=20 act to shift the moisture plume/best 850 hPa inflow east with time. Hourly rain totals to 2.5" with local amounts to 5" are possible=20 where limited cell training occurs and/or mesocyclones happen to=20 develop. Because of the heavy rainfall that has occurred recently,=20 the Slight Risk accounts for both the previous couple days of=20 rainfall and the expected rainfall from early Sunday morning=20 onward. The northern portion has been shifted a little south of=20 continuity across portions of southeast LA and southern MS, which=20 was coordinated with the LIX/Slidell LA forecast office. Roth Day 2 Valid 12Z Mon Mar 18 2024 - 12Z Tue Mar 19 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Roth Day 3 Valid 12Z Tue Mar 19 2024 - 12Z Wed Mar 20 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Roth Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6Aj8SbM4DQ9GLCU2vO4Gy-ngTJ_dxUL72ANeBHcG45yA= QBdg2WFNN7jICfDoEvmwdoOnnxL2zdmbkzXfEGm8ZX034RM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6Aj8SbM4DQ9GLCU2vO4Gy-ngTJ_dxUL72ANeBHcG45yA= QBdg2WFNN7jICfDoEvmwdoOnnxL2zdmbkzXfEGm8qxRpek0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6Aj8SbM4DQ9GLCU2vO4Gy-ngTJ_dxUL72ANeBHcG45yA= QBdg2WFNN7jICfDoEvmwdoOnnxL2zdmbkzXfEGm8OaUib9w$=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) .