Subj : D2 5/5 RISK Excess Rainfa To : All From : Mike Powell Date : Thu Apr 03 2025 14:31:00 FOUS30 KWBC 031909 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 309 PM EDT Thu Apr 3 2025 Day 2 Valid 12Z Fri Apr 04 2025 - 12Z Sat Apr 05 2025 ....THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE OZARKS AND MID-SOUTH... ....LIFE-THREATENING FLASH FLOODING EXPECTED FRIDAY AFTERNOON THROUGH EARLY SATURDAY... ....21Z Outlook Update... Both global models and CAMs have come into agreement with development of at least one north-northeast to south-southwest oriented band of convection located generally from the ArkLaTex into southeastern Missouri. Convection should be oriented generally parallel to fast, meridionally oriented flow aloft, with abundant moisture, low- level shear, and instability profiles all favoring abundant rainfall (potentially exceeding 10 inches on a localized basis). These totals will fall on areas that will have experienced 2-6 inches of rainfall leading up to the forecast period, with wet soils/terrain variations all suggestive of a potentially widespread and significant flash flood event. Expansion of Moderate Risk (and attendant probabilities) was considered across more of Illinois and central Indiana, but pre- existing dry conditions and uncertainty about duration of precipitation were mitigating factors for a higher flash flood threat. This region will be evaluated for a possible upgrade in later outlooks. Outside of these changes, the overall outlook was on track with an eventual broad axis of heavy precipitation expected from north/central Texas northeastward to Ohio/Pennsylvania. See the previous forecast discussion below for more details. Cook ....Previous Discussion... The previous synoptic scale forecast was relatively unchanged within the run to run assessment over the course of Friday into Saturday morning. The final surface wave within the atmospheric parade across the Southern Plains to Mississippi Valley will show itself by late- Friday morning through the remainder of the period. This wave will likely be the strongest of the 3 allowing a much more amplified surface evolution that will shift the focus of heavier precip further northwest while also providing some vigor in the overall QPF distribution. The trend the past 48 hrs is for a zone of significant (3-6+") totals to be recognized across portions of the Southern Plains up through the Ozarks and Mid-Mississippi Valley of Missouri, spanning as far north as south- central Illinois. When assessing the theta_e environment, there is a noticeable push north of more modestly unstable air with the most pertinent area of available instability situated across southern MO down through AR, southeastern OK, and even northeastern TX. A sharp uptick in regional PWATs with deviations between +3 to +4 anomaly- wise are forecast within that zone correlating with a substantial rise in area convection blossoming late Friday afternoon, onward. This setup is fortunately further west compared to the previous periods, so the overlap of the heaviest rainfall is not anticipated to be as problematic as the previous period. Having said that, the pattern will yield high precip totals due to widespread convective activity, falling over some areas that will have seen appreciable rain, and over other areas that are more prone to flooding due to complex terrain situated over the Ozarks. The pattern is such that several areas from east of DFW up through IL will see rainfall amounts of 3-6" with locally higher beginning Friday afternoon with the heaviest rainfall likely to occur by the evening with the implementation of yet another LLJ structure thanks to the surface wave mentioned previously. Further to the west, another closed upper reflection across the Southwest U.S. will only entice greater upper forcing within a broad axis of difluence situated over the Plains. This setup is more privy to convection forming as far west as the Western Rolling Plains of TX to points east and northeast. Even there, modest 1-2" with locally higher totals are forecast in-of those zones. The heaviest precipitation axis based off the latest NBM probabilities still has a significant footprint of at least 3" located near the Dallas metro and points just east (40-60%) to as high as 60-80% in a large area spanning the Red River of northeast TX up through much of western and northern AR, through the Ozarks of MO, finally reaching the Mississippi River south of St.Louis. Ensemble mean QPF output signals a massive area of at least 2" with urban centers like Dallas, Little Rock, St. Louis, and Springfield, MO all within a core of the heavy precip footprint. Considering the probabilistic output of the NBM, the consistency within guidance for a significant corridor of very heavy rainfall, and a well-defined axis of training precip potential, a broad Moderate Risk was maintained and even expanded to account for the expanse of likely flash flooding. A high-end Moderate is a likely tag for places within southeast OK up through western and northern AR through the Ozarks of Missouri as these areas will have the greatest threat of enhanced rates over top of either complex terrain or primed soils. An upgrade is plausible in future updates pending additional convective probability details. Kleebauer $$ --- SBBSecho 3.20-Linux * Origin: ILink: CCO - capitolcityonline.net (454:3/105) .