Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat May 13 2023 08:42:33 FOUS30 KWBC 130842 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 441 AM EDT Sat May 13 2023 Day 1 Valid 12Z Sat May 13 2023 - 12Z Sun May 14 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR SOUTH TEXAS... ....Texas and Oklahoma... Tricky excessive rainfall forecast for today into tonight. The inherited Moderate risk looks to be in some trouble, as convection overnight into early this morning has been forward propagating at a greater clip than anticipated. Thus the greatest flash flood risk may be past by the time the new day 1 starts at 12z. It will be close, but still think coastal areas of south TX should still be getting heavy rainfall at 12z, and the eastern portion of the line should still be onshore and producing some very heavy rainfall rates over 2" in an hour. Although even this portion of the line is forecast to begin a weakening trend by mid morning. So while heavy rainfall rates will probably be ongoing at 12z, the forward progression of convection and gradual weakening trend should limit the magnitude/extent of the flash flood risk. With that said, the heavy rates and short duration backbuilding could still pose a locally significant flash flood risk into the morning hours, especially over any urban areas that are impacted. Once this activity clears, the question becomes do we get another round of convection tonight. Initially instability will be eroded, so we'll have to see if the continued southeasterly inflow can advect instability back into the area. Additional weak shortwave energy may move into the area out of Mexico, and so could envision convection forming in Mexico and moving back into the area tonight. Several of the 00z HREF members depict a solution like this, which results in 3"+ neighborhood probabilities of 40+ percent between 00z-12z Sunday. However it is not unanimous, and the 06z HRRR notably does not develop additional convection. This is a plausible outcome as well, and so will just need to monitor observational and model trends through the day. Given some potential lingering convection at 12z this morning, and at least some potential for redevelopment overnight, did not want to completely remove the MDT risk at this time (although certainly some chance this is eventually dropped). Instead opted to just reduce the size and trim back the western portion of the risk area. A second area of convective focus should be across portions of northern TX into OK. This is related to the retrograding shortwave/MCV that is expected to lift north northwest across the region today into tonight. Even here there remains uncertainty with regards to the degree of instability and location of boundaries helping focus convection. The overall setup does appear to favor some swaths of training convection, so just a matter of whether we get enough instability to drive rainfall rates high enough, and exactly where this better convective concentration will end up. For now left a pretty broad Slight risk over the area. A bit broader on the southern extent to cover where some overlap of additional convection and heavy antecedent rainfall may occur, and a bit sharper with the TX Panhandle cutoff as this areas has been drier and instability should generally be lower the further north and west you go. Oklahoma is interesting and could very well be the focus for heaviest rainfall today over the Southern Plains within training banded features east of the MCV/low. However ongoing convection this morning likely complicates boundary locations and instability later today, so even here confidence is low...but still within Slight risk levels. ....Northern Plains into Upper MS Valley... Training convection this morning into the afternoon should pose an isolated to scattered flash flood risk over portions of eastern SD, southern MN and central/eastern IA. Convection is focusing along the 925/850mb boundary east of slow moving deep layered low, and convergence along this boundary should remain pretty persistent through the morning hours. Low level southeasterly flow should maintain instability into the region as well...with 1000-2000 J/KG of CAPE. Expect that we will continue to see periodic backbuilding/training convective segments along this corridor, which should be enough to drive some flash flood risk. HREF guidance supports a broad area of 1-2" and localized amounts over 3". In fact trends support some 3-5" potential within swaths across this corridor. Portions of southern MN are saturated from recent heavy rainfall which would allow for a quicker development of potential flash flooding. The heaviest rain today may end up just south of this saturated area, but it will be close. Other areas of the Slight risk have been drier and so it will likely take longer training for issues to develop...but even here there is enough of a signal to support at least an isolated to scattered flash flood risk. ....OH/TN valley into the Mid Atlantic... Above average moisture and instability will cover portions of this region on Saturday. A weak shortwave currently over OH will move into the Mid Atlantic and help drive a convective threat. Individual cells will be quick moving, but broad forcing is supportive of multiple convective rounds. On top of that these cells should be capable of dropping a quick 1-2" in localized swaths. So do think at least an isolated, mainly urban, flash flood risk could exist. Chenard Day 2 The Day 2 outlook will be updated by 0830Z. Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!76dRfA7vOa8D46lob9yE-ijW0W9hU8UPTLnV8MPgPNVW= zCB4_EMmIhyQcWyUAfa7xJ-o6BCV9XLBgNvjThAd9n8Xz_k$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!76dRfA7vOa8D46lob9yE-ijW0W9hU8UPTLnV8MPgPNVW= zCB4_EMmIhyQcWyUAfa7xJ-o6BCV9XLBgNvjThAdu_98z5o$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!76dRfA7vOa8D46lob9yE-ijW0W9hU8UPTLnV8MPgPNVW= zCB4_EMmIhyQcWyUAfa7xJ-o6BCV9XLBgNvjThAdciQqe8M$=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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