Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Apr 05 2025 15:39:41 FOUS30 KWBC 051539 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1139 AM EDT Sat Apr 5 2025 Day 1 Valid 12Z Sat Apr 05 2025 - 12Z Sun Apr 06 2025 ....THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF=20 ARKANSAS, THE MID-SOUTH, AND THE LOWER OHIO VALLEY... ....LIFE-THREATENING FLASH FLOODING IS EXPECTED TODAY AND TONIGHT... ....16Z Outlook Update... The ongoing Moderate and High Risk areas are largely unchanged from the previous forecast. Models/observations continut to depict a potentially catastrophic flash flood scenario unfolding today as widespread areas of additional 3-5 inch rainfall amounts fall on water-logged areas of Arkansas, western Tennessee, and much of Kentucky that are already experiencing numerous, widespread flooding impacts. FFGs are near zero through a large part of this region, and the added rainfall will continue to exacerbate flooding problems through the day. Reference Mesoscale Precipitation Discussions 127 and 128 for additional details on short term flash flood potential. Some trimming of categorical/probabilistic areas was needed across eastern Oklahoma and northeast Texas for this outlook, with the bulk of deeper convection and heavier rainfall now east of those areas. With the upper low moving overhead later tonight, isolated excessive runoff could occur especially in low-lying spots that have not recovered from antecedent rainfall. See the prior discussion for additional details. Cook ....Previous Discussion... Today is expected to be the final day in the multi-day high-=20 impact flood event that has been unfolding across the Mid-South,=20 Lower and Mid- Mississippi Valleys, and Lower Ohio Valley. On-going convection was occurring in a corridor along and north of a quasi stationary front extending from Arkansas northward into the lower Mississippi Valley and the nearby portions of the Mid-South into the Lower Ohio Valley. The expectation is that the overnight convection will wane...followed by renewed convection upstream that moves over many of of the same waterlogged areas later this=20 afternoon and evening. Flow aloft will be highly diffluent to the east of a mid-level low pushing eastward from the southern Rockies. This should foster deep convection given favorable shear profile=20 in the lower part of a moisture-rich atmosphere with upwards of=20 3000 J per kg of CAPE by the time of maximum afternoon heating.=20 The combination of slow storm motion and efficient/high rainfall=20 rates is expected to result in additional widespread coverage of significant rainfall.=20 The High Risk of excessive rainfall largely remains in place from the previously issued outlook...namely areas from Arkansas through southeastern Missouri to western Tennessee and Kentucky and the northwest corner of Mississippi. All major deterministic models=20 depict an expansive footprint of 3-6 inches of rainfall, with=20 locally up to 7-9 inches. These areas have already received=20 significant rainfall since Wednesday, and any additional rainfall=20 could result in a very dangerous flash flooding scenario with=20 considerable to potentially catastrophic impacts. There are signals in the computer models that large-scale deep upper level trough makes its way eastward from the south Rocky mountains...which finally dislodges the front and starts to disrupt the flow of deep-layer moisture from the Gulf. During the last 6 to 12 hours of the outlook period...shower and thunderstorms begin to spread south- and eastward. With precipitable water values still lingering in the 1.8 to 2.0 inch range along the axis of the PW plume....some locally intense rainfall rates and high rainfall totals may still result in instances of flash flooding across the parts of the Tennessee Valley into parts of Alabama in the late night hours of Sunday morning (an area with considerably drier antecedent conditions). Bann Day 2 Valid 12Z Sun Apr 06 2025 - 12Z Mon Apr 07 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHEAST ON SUNDAY... On Sunday, upper level southern stream energy that had been supporting the heavy rainfall event over the past 4 days will begin to interact with a northern stream trough and accelerate northeastward. This will push the surface cold front across the southeast, with an axis of high moisture ahead of and along the front. There should be enough instability and shear to support thunderstorms with locally heavy rainfall from the Central Gulf Coast through Central Alabama and Northern Georgia. Rainfall totals are expected to reach 2-3+ inches in these areas, and rainfall rates could challenge FFGs of 2-2.5 inches per hour (although the 3+ inches per 3 hour FFG may be be less likely to be met/exceeded). Dolan/Bann Day 3 Valid 12Z Mon Apr 07 2025 - 12Z Tue Apr 08 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHEAST US ON MONDAY. The focus for any lingering moderate to heavy rainfall will continue to shift eastward on Monday as the large scale flow pattern remains progressive. Current model QPF keeps some 1.5 inch to 2.5 inch totals scattered from far northern Florida northward into the eastern portions of the Carolinas. The juxtaposition of the upper jet and the plume of 1.75+ inch precipitable water values becomes less favorable with time...but the moisture transport of deeper moisture remains favorable to support a low-end risk of excessive rainfall (especially in regions of poor drainage), Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4WETYEhVxWGxknI8Z0Yhym1WBrq4-JIDKuSnI1dW84c0= oNlX7ezQPsvdkgy3I2TdFWiJdvmqmoVihk_S5RODbIIwMuw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4WETYEhVxWGxknI8Z0Yhym1WBrq4-JIDKuSnI1dW84c0= oNlX7ezQPsvdkgy3I2TdFWiJdvmqmoVihk_S5RODuyBdUK0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4WETYEhVxWGxknI8Z0Yhym1WBrq4-JIDKuSnI1dW84c0= oNlX7ezQPsvdkgy3I2TdFWiJdvmqmoVihk_S5RODLQwHRFI$=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. 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