Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Apr 21 2025 15:46:34 FOUS30 KWBC 211546 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1146 AM EDT Mon Apr 21 2025 Day 1 Valid 16Z Mon Apr 21 2025 - 12Z Tue Apr 22 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS SOUTHEAST TEXAS AND PORTIONS OF THE DEEP SOUTH... Texas Coast... On-going locally heavy rainfall over the TX coast will continue=20 for the next few hours in response to persistent low-level=20 confluence near a slowly-moving/stationary surface front. See MPD=20 #154 for more information in the near term. Several inches of rain=20 has already fallen, and even with high FFG values to start,=20 sensitive areas will see some flooding where rain is heaviest and=20 continues to fall (also in urban areas). PW values are high (>1.6=20 from CRP and near 1.5 at LCH per 12Z RAOBs which is near the 95th=20 percentile for this time of year), winds are light from the sfc to=20 700mb, and rain rates >2-3"/hr have occurred and are forecast for=20 this afternoon (per 12Z HREF) before subsiding this=20 evening/overnight. Will continue with the Marginal Risk here given=20 the high tolerance for rainfall and generally sandy soils. MS/AL... To the north, similar moisture exists and the cold front has been=20 a little more progressive but will slow and perhaps stall this=20 afternoon as it nudges a bit more to the southeast over northern=20 portions of MS/AL. Isolated showers/storms this morning will become more scattered during the afternoon/evening - supported both by=20 surface heating and low- level warm advection. Focus for higher=20 rainfall and rain rates will be near/north of the LA/MS border to=20 northern AL where FFG values are around 2-3.5"/hr. 12Z HREF=20 neighborhood probabilities for >2"hr rates are around 30%, but some CAMs were showing a few areas of >3-5" rainfall this afternoon.=20 Slowing front may allow for some training and a locally higher=20 threat for flash flooding (especially in urban areas like=20 Birmingham) so we have maintained the Marginal Risk outline.=20 Fracasso Day 2 Valid 12Z Tue Apr 22 2025 - 12Z Wed Apr 23 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS NORTHWEST TEXAS AND WESTERN OKLAHOMA... A return-flow regime begins in earnest during the forecast period across the southern Plains and results in areas of 60s dewpoints across the OK/TX Red River Valley area. Meanwhile, weak west- southwesterly mid-level flow and weak perturbations within that flow should spark initially higher-based thunderstorm activity across the Panhandles that should spread east-southeastward toward areas of greater moisture content after 00Z. The outflow dominant nature of initial storms should limit flash flood potential initially, with a conditional risk materializing later in the evening and overnight hours if 1) any upscale growth into one or two mesoscale convective complex can promote training and boost rain rates and 2) heavier rainfall can materialize across portions of north-central Texas/southern Oklahoma that received 3-8 inches of rain in the past 72 hours or so. These factors reflect a flash flood threat that is too conditional for anything beyond Marginal/5% probabilities at this time, and are highly dependent on a mesoscale evolution that remains uncertain at this time. Across North Carolina, models suggest that surface heating along and south of a cold front near the NC/VA state line could provide a focus for scattered thunderstorm activity during the afternoon hours. Zonal flow aloft (parallel to this boundary) would support some potential for convective training, although the coverage of storms is a bit uncertain given modest ascent/mid-level support. A Marginal Risk/5% area may be needed in later outlooks pending greater certainly on convective evolution. Cook Day 3 Valid 12Z Wed Apr 23 2025 - 12Z Thu Apr 24 2025 Substantial uncertainty exists for this outlook. Broad, western U.S. troughing aloft should result in southwesterly flow aloft across the High Plains for much of the forecast period. Meanwhile, southerly low-level flow should gradually moisten low-levels across Kansas and vicinity - especially in the latter half of the forecast period. Subtle waves within the flow aloft should promote scattered, initially high-based thunderstorm activity during the afternoon and early evening, though coverage and location is in question. Thereafter, there are hints that upscale growth of storms could result in gradually increasing heavy rain potential as low-level warm advection results in higher moisture content/PW values (1-1.5 inches) across central Kansas into central Oklahoma. Again, this scenario is uncertain given pre-existing moisture quality concerns. Given the uncertainty and dry antecedent soil conditions, probabilities for excessive rainfall are removed for this outlook across the Great Plains. Cook Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5f-xqIapLF4d9RRJQp6a9Ie8Fk8-LYXCxcVywMPpUrCY= HLSPcmzX9Li98v3GE5RP9_kSlq-U1RoCVjJet9DGgOewvig$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5f-xqIapLF4d9RRJQp6a9Ie8Fk8-LYXCxcVywMPpUrCY= HLSPcmzX9Li98v3GE5RP9_kSlq-U1RoCVjJet9DGfQfXdP0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5f-xqIapLF4d9RRJQp6a9Ie8Fk8-LYXCxcVywMPpUrCY= HLSPcmzX9Li98v3GE5RP9_kSlq-U1RoCVjJet9DGH_rnPu4$=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) .