Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri May 23 2025 15:55:03 FOUS30 KWBC 231554 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1154 AM EDT Fri May 23 2025 Day 1 Valid 16Z Fri May 23 2025 - 12Z Sat May 24 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM SOUTHEASTERN KANSAS AND NORTHEASTERN OKLAHOMA INTO THE OZARKS... ....Kansas, Oklahoma, Missouri, Arkansas... There remains a good model signal for the development of heavy rainfall later today, in what is expected to be the first of several rounds impacting the region over the next few days. The synoptic pattern will evolve today into one that will support heavy rain within periods of organized convection/MCS beginning tonight. This will be in response to the central part of the CONUS being sandwiched between an elongated trough to the northeast and a building ridge over the Four Corners, which leaves broad W/NW flow across the area. Within this flow, ascent will occur through periodic impulses moving through the flow combined with the RRQ of a jet streak aloft. The resultant frontogenesis from the RRQ of the jet streak will efficiently overlap a warm front wavering in the area, providing additional lift, which will then be enhanced by isentropic ascent of the LLJ atop this front. Forcing will be pronounced, and will act upon favorable thermodynamics, which will become increasingly robust the latter half of today and tonight. This will occur as the impressively broad 850mb LLJ emerges from the Gulf and surges to 40+ kts, angling orthogonally into the warm front. This will produce strong moisture convergence as reflected by the moisture transport vectors, and resupply elevated CAPE of 3000 J/kg northward into the Plains. As convection blossoms this evening, generally after around 04Z, it will expand rapidly and organize thanks to 40-50 kts of bulk shear. This will result in clusters of storms, potentially growing upscale into an MCS, and training of echoes is likely due to aligned flow to the warm front. There continues to be some uncertainty into how quickly the convection will grow into an MCS allowing it to push faster to the southeast towards daybreak Saturday, but until that happens, some upstream growth to the NW is expected which will prolong training of echoes. With a high probability for rain rates exceeding 2"/hr, any training could lead to rainfall of 2-5", with locally higher amounts possible (10-20% chance), especially near the MO/KS/AR/OK intersection. Another fly-in-the-ointment for the risk area is how elevated convection north of the warm front, especially early tonight, will impact the progression SW. There are a few high-res CAMs, and several global models, that suggest an outflow boundary will be pushed south more into Oklahoma tonight, which will be aided by residual boundaries from any dry line convection moving east, and the front itself. This could result in a bi-modal distribution to rainfall, and a secondary maxima of flash flood risk across eastern OK. While confidence is higher in the isentropic/elevated heavy rain risk farther north, the SLGT risk was tailored a bit SW from inherited to account for this potential. Otherwise, the inherited risk areas were modified only cosmetically for recent guidance, primarily being pulled NW (the MRGL risk area) more into KS for backbuilding convection during the development stage. ....South Florida... A weakening front across South Florida will serve as a focus for slow moving thunderstorms once again today, drifting east to the Gold Coast through the aftn/eve. This front and a lingering tail of an upper jet streak will provide synoptic lift into favorable thermodynamics characterized by PWs of 1.75 to 2.00 inches, and SBCAPE above 2000 J/kg. This overlap will support rainfall rates within thunderstorms for which the HREF indicates has a 40% chance of exceeding 2"/hr, and the 15-min HRRR suggests brief rainfall intensity of 4"/hr possible. Where these storms slow/stall, primarily along the southeast coast where they interact with the sea breeze, this could result in spots of 3-5" of rainfall. Should this rain occur atop urban areas or over locations that have received heavy rainfall the past 48-hrs, isolated flash flood instances could result, so the MRGL risk was maintained. Weiss Day 2 Valid 12Z Sat May 24 2025 - 12Z Sun May 25 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM SOUTHEASTERN KANSAS and NORTHEASTERN OKLAHOMA INTO THE OZARKS... Expect convection to remain ongoing into the early part of the period, however most of the CAMs show these storms weakening as they move progressively to the south and east Saturday morning. Redevelopment is expected back to the north and west beginning in the afternoon and continuing into the overnight as the low level jet reintensifies across Southern Plains, replenishing the deeper moisture pooling along a boundary as it lingers from eastern Oklahoma and southeastern Kansas through the Ozarks. Additional rounds of heavy precipitation are expected late Saturday into early Sunday as this moisture interacts with low-amplitude shortwave energy moving out of the Plains. Once again, training or backbuilding cells may contribute to heavy rainfall totals. Given the lingering uncertainty regarding how much overlap there will be in the heavy amounts that occur on days 1 and 2, opted to maintain just the Slight Risk for now. However, an upgrade to a Moderate this period may be forthcoming should the models start to show a broader consensus that provides greater confidence. Pereira Day 3 Valid 12Z Sun May 25 2025 - 12Z Mon May 26 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FROM SOUTHEASTERN KANSAS and NORTHEASTERN OKLAHOMA INTO THE OZARKS... With many of the detail differences typical of a day 3 period, there is general model agreement signaling a widespread moderate to heavy rainfall event that will extend from parts of the Southern Plains into the Tennessee Valley this period. This will likely include those areas impacted by heavy amounts during the previous periods. With some timing and amplitude differences, most models show a fairly well-defined shortwave moving out across the Southern Plains by Sunday night. Similar to the previous days, moisture along the lingering frontal boundary will provide ample fuel for heavy rain. In contrast the previous days, guidance shows moisture deepening a little further to the west, supporting heavy rainfall developing back across northwestern Texas and southwestern Oklahoma as this system moves across the Plains. Heavy rains will spread east late Sunday through the overnight, likely impacting parts of eastern Oklahoma, southeastern Kansas, southern Missouri, and northern Arkansas once again. With some models showing an additional 2-4 inches, pushing three day totals over 6 inches in some locations, the Moderate Risk, which was introduced and centered over the region in the previous Day 4, was maintained for the new Day 3. Also maintained was the Slight Risk extending back through southwestern Oklahoma into northwestern Texas. Part of the previously noted detail differences contributed to an uncertainty as to how far south to extend the Slight Risk across eastern Texas. Some of the models, most notably the ECMWF, show heavy amounts progressing well to the south over East Texas. For now, kept the Slight Risk closer to the broader consensus centered farther north. Pereira Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6CbR3KFLcudbNPh__asXj70D8QiiVSNtS6RRIr_nEA80= DR2sEUjKcJzZ7bM6hAQJg5gG14u2cIs7PFGALl6GXa4BzXs$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6CbR3KFLcudbNPh__asXj70D8QiiVSNtS6RRIr_nEA80= DR2sEUjKcJzZ7bM6hAQJg5gG14u2cIs7PFGALl6Gp8pQJQA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6CbR3KFLcudbNPh__asXj70D8QiiVSNtS6RRIr_nEA80= DR2sEUjKcJzZ7bM6hAQJg5gG14u2cIs7PFGALl6G3z9nuJs$=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) .