Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Mar 31 2025 00:47:03 FOUS30 KWBC 310046 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 846 PM EDT Sun Mar 30 2025 Day 1 Valid 01Z Mon Mar 31 2025 - 12Z Mon Mar 31 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE LOWER MISSISSIPPI, TENNESSEE, AND OHIO VALLEYS AND CENTRAL GULF COAST... ....Central Gulf Coast through the Ohio Valley... 01Z Update... With diminishing convection along the immediate Upper Gulf coast...removed the Slight risk area introduced earlier in the day. Was not inclined to remove the Marginal risk area which surrounded the Slight risk given how hydrologically sensitive the area became as a result of prolific rain-makers earlier today. The latest model guidance kept the bulk of QPF well to the north and west during the overnight hours. Even so...any convection still lingering near the coast of Alabama westward into southeast Louisiana could result in additional excessive rainfall concerns. Also trimmed the Marginal=20 risk area from the western Florida peninsula based on trends in=20 radar imagery. The remainder of the previously-issued outlook still covered the higher model QPF and any associated potential for=20 excessive rainfall. Bann Previous Excessive Rainfall Discussion... A pair of shortwave troughs emerge from the Intermountain West into the Central U.S. today, eventually partially phasing with an associated longwave trough by later tonight. This complex interaction is expected to result in a broad area of organized convective activity across much of the eastern half of the CONUS, including portions of the MS, TN, and OH Valleys. Despite the continued complexity of the synoptic and mesoscale details, models remain in fairly good agreement in indicating convection breaking out across the broad warm sector today, initially mostly discrete (and supportive of splitting supercells and multicell clusters) and evolving into a QLCS/LEWP ahead of the associated cold front by this evening. Broad inflow from the Gulf will result in precipitable water values of 1.25-1.75" (at or above the 90th percentile, per CFSR climatology) and SB CAPE of 2000-4000 J/kg, as deep layer (0-6 km) shear increases to 40-60 kts. Hourly rainfall of up to 3"/hr are possible in this environment, initially highly localized in association with established supercells, then more numerous to widespread with the passage of the QLCS/LEWP. Localized totals of 2-4" are generally expected within the Slight for the whole of the event (with the majority falling in a 3-6 hour period in association with the QLCS passage), with totals near the upper-end of the range (~4") being possible where storms train and/or where two or more mesocyclones align. While there were some changes per the new guidance, in general, continuity was maintained. Ongoing organized convection midday in the general vicinity of Mobile hadn't been disrupted by diurnal convection in its vicinity, which threatened mergers as disorganized convection moves more northward while organized convection moves more eastward. Cold cloud tops continued to show expansion with the convective area midday, which led to the introduction of a midday Slight Risk from southern MS across southern AL into the FL Panhandle. Hourly rain totals to 3" with additional local amounts to 6" remain possible for as long as this convective mass can persist. ....Florida Peninsula/Southeasternmost Georgia... A frontal boundary at 850 hPa slowly lifts through the state, bringing precipitable water values to 1.5-1.75". ML CAPE should maximize in the 2000-3000 J/kg, particularly in the southern=20 Florida peninsula. There appears to be enough effective bulk shear=20 for some degree of thunderstorm organization as well. The model=20 guidance --- mostly mesoscale -- indicates the potential for local=20 amounts in the 5-6" range. The Marginal risk was maintained with=20 minimal change. Roth/Churchill Day 2 Valid 12Z Mon Mar 31 2025 - 12Z Tue Apr 01 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL GULF COAST... ....Central Gulf Coast/Southeast... T A pair of shortwave troughs are progged to fully separate, as the northern stream shortwave continues to phase with the parent longwave trough into the Mid-Atlantic and Northeast, while the southern shortwave likely hangs back into the Mid-South and Lower MS Valley. The best moisture and instability lies over portions of the Southeast (where forcing from the secondary shortwave should still be sufficient for organized convective activity). The Marginal risk was generally maintained from continuity. However, due to recent convective events along the Central Gulf Coast, upgraded to a Slight Risk for portions of southernmost MS, AL, and the western FL Panhandle which should have increased sensitivity/ soil saturation. Localized totals of 2-4" are generally expected. ....Northeast... Thunderstorms with heavy rainfall are expected during the afternoon and evening hours ahead of a northern stream shortwave as MU CAPE peaks around 1000 J/kg and precipitable water values peak in the 1.25-1.5" range. Sufficient inflow and effective bulk shear is available for organized convection for much of the period, with activity expected to be in decline past Tuesday 06z. Hourly rain totals up to 2" are possible where convection trains, which could challenge the flash flood guidance across urban portions of the Mid-Atlantic States. The inherited Marginal risk showed minimal change. Roth/Churchill Day 3 Valid 12Z Tue Apr 01 2025 - 12Z Wed Apr 02 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CENTRAL PLAINS & MIDWEST... A blocking pattern begins to emerge by Tuesday, as the aforementioned shortwaves lift northeastward into Canada with large-scale ridging quickly builds across the eastern United States from the North Atlantic. Meanwhile, large-scale troughing becomes amplified out West, with a potent upper-low expected to develop over the Northern Rockies and Plains (translating across the Rockies from the Pacific Northwest). While 12z global model suites are in overall good agreement with the evolution of the synoptic scale, the GFS/NAM are drier than the rest of the guidance with the associated surface low (which is depicted as a sub 990 mb low from the GFS, ECMWF, and CMC). Within the GFS/NAM guidance, the idea of light QPF is internally inconsistent as the forecast precipitable water values (1-1.25") combined with the 1000-500 hPa thickness values near 5610 meters suggest a nearly saturated atmosphere, and there appears to be sufficient instability in the vicinity and effective bulk shear for convective organization, so the wetter guidance was more believable. However, nearly as soon as the necessary ingredients are in place, they take off to the east due to system progression. Most deterministic solutions follow that idea and depict localized totals of 2"+ (including the downscaled ECMWF, the UKMET, and the CMC- reg) with remarkably good spatial agreement for this range. The ingredients above suggest hourly rainfall up to 1.75" with local amounts in the 3-4" where/when cells have the opportunity train or backbuild, so the inherited Marginal risk remains, though the area showed some expansion. Roth/Churchill Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7FvVhN_Zi3kSrAApoHVfw4cT23o-VFAP-yoODJhaeXki= Z2eR4kcCzHkDeRWZTb0X8EbaXIAA2j6t02ykCV-wktI6fKI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7FvVhN_Zi3kSrAApoHVfw4cT23o-VFAP-yoODJhaeXki= Z2eR4kcCzHkDeRWZTb0X8EbaXIAA2j6t02ykCV-waHrYPDc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7FvVhN_Zi3kSrAApoHVfw4cT23o-VFAP-yoODJhaeXki= Z2eR4kcCzHkDeRWZTb0X8EbaXIAA2j6t02ykCV-wG577xqU$=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) .