Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Apr 06 2025 00:57:39 FOUS30 KWBC 060056 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 856 PM EDT Sat Apr 5 2025 Day 1 Valid 01Z Sun Apr 06 2025 - 12Z Sun Apr 06 2025 ....THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF ARKANSAS, THE MID-SOUTH, AND THE LOWER OHIO VALLEY... ....LIFE-THREATENING FLASH FLOODING CONTINUES TONIGHT... A High risk was maintained across eastern AR into northwest MS, western TN and western KY. The highest rainfall rates have generally moved east of this area, however a broader area of stratiform rain and embedded heavier convective cores will persist for several more hours. Even absent higher rates upwards of an additional 1-2 inches of rain falling on saturated and already flooded areas will continue to pose a significant and potentially life-threatening flood risk. A Moderate risk is maintained further northeast across KY and southern IN and OH. Rainfall rates have come down over this area and recent HRRR runs may be overdoing additional rainfall magnitudes=20 here...however even an additional 1-2" of rain will prolong and=20 locally exacerbate flash flood impacts given the saturated=20 conditions and ongoing flooding over these areas. The heaviest additional rainfall overnight will likely fall over central to northern MS into northwest AL and south central TN.=20 This is mostly over areas that have not been hit by the excessive=20 rainfall over the past few days. This will help delay the onset of flooding impacts and limit the coverage of higher end impacts.=20 With that said, slow moving convection with high rainfall rates=20 will likely surpass even the higher FFG over these areas, and so=20 do expect numerous instances of flash flooding overnight. The 18z=20 HREF neighborhood probabilities of exceeding 5" are in the 40-60%=20 range, and recent HRRR runs depict swaths of 3-5", locally as high=20 as 5-7". There is some model spread on the exact location of these=20 higher totals, but based on model data and recent radar trends it=20 appears like central to northern MS into northwest AL and south=20 central TN will be the area mostly likely to see slow moving=20 training convection tonight. Given this is just to the southeast of the axis of heaviest rains the past couple days...impacts will=20 likely not be as widespread or catastrophic as what has occurred=20 further northwest over AR and portions of KY and western TN.=20 Nonetheless, expecting high enough rates for flash flooding, some=20 of which could be locally significant. Chenard 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 AND SOUTHERN TO CENTRAL APPALACHIANS ON SUNDAY... ....21Z Outlook Update... Marginal and Slight Risk areas were modified slightly westward to account for somewhat slower timing of convection depicted in the 12Z suite of model guidance from western Kentucky through Mississippi and also across southern Georgia. Slight risk was also introduced across eastern Kentucky and portions of West Virginia. This threat is somewhat conditional on the extent of instability that develops in the wake of morning thunderstorms that should cross the Appalachians before midday. The most aggressive models spread another 1-2 inches of rainfall in an orientation favoring training/repeating during the afternoon. Additionally, soil moistures are wet from 3-5 inches of rainfall over the past 72 hours, and should support at least isolated instances of excessive runoff if the more aggressive QPFs verify. Lastly, a couple of CAMs appear to depict focused, training bands of convection from central Alabama into the Atlanta Metro area during the afternoon and evening hours, with local amounts exceeding 6-8 inches. While these impressive rainfall totals cannot be completely ruled out, they will depend on the ultimate evolution of prior-day convection across Mississippi/Alabama and any lingering convective outflows that can serve as a focus for renewed convection during the forecast period. If current trends hold, a Moderate Risk upgrade would be needed for portions of AL/GA in later outlooks. Cook ....Previous Discussion... 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. ....21Z Outlook Update... The ongoing forecast is on track. Some differences in timing of convection have appeared in the 12Z guidance, with slower solutions depicting heavier rainfall across Georgia and the Florida Panhandle early in the forecast period. The Marginal risk area has been expanded westward to account for this potential slower convective evolution, although the ongoing forecast philosophy remains the same as outlined the previous discussion. Cook ....Previous Discussion... 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!8pWaimRbx4BRwbD4cBGioww-vIz9Gg__n7E9YzOi6GNd= qqyd975hKF7CFk1AbOHNx6wpawW_v37iAjTWMb2gWqN9bmM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8pWaimRbx4BRwbD4cBGioww-vIz9Gg__n7E9YzOi6GNd= qqyd975hKF7CFk1AbOHNx6wpawW_v37iAjTWMb2gGSjx87Q$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8pWaimRbx4BRwbD4cBGioww-vIz9Gg__n7E9YzOi6GNd= qqyd975hKF7CFk1AbOHNx6wpawW_v37iAjTWMb2g-3ogQxA$=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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