Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Apr 12 2023 08:09:04 FOUS30 KWBC 120809 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 407 AM EDT Wed Apr 12 2023 Day 1 Valid 12Z Wed Apr 12 2023 - 12Z Thu Apr 13 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR ATLANTIC COASTAL SOUTHEAST FLORIDA... ....Southeast Florida... Another day of heavy rainfall potential over southeast FL as a warm front lifts northward across the area. Given the lifting warm front, both higher instability and PWs will lift northward into more of southeast FL, with CAPE over 1000 J/kg and PWs potentially topping 1.75". Thus should be a pretty good combo of moisture and instability, along with slow cell motions given easterly low level flow and westerly mid/upper level flow. Generally it is a similar environment that was over the Keys on Tuesday, although large scale forcing may actually be a bit stronger today with the closed deep layer low over the central Gulf coast intensifying. While there were areas of heavy rainfall on Tuesday, QPE generally under performed compared to what some of the HREF guidance was suggesting yesterday. This does give some hesitation to fully buying into the HREF probabilities today, although do tend to think the increased upper support, northward lifting front and coastal convergence support pretty good convective coverage and areas of heavy rainfall. HREF probabilities are again impressive over southeast FL, with EAS probabilities supporting ~40-60% coverage of 2"+ rainfall amounts over the urban corridor...and neighborhood probabilities over 90% for 3", 70% for 5" and 45% for 8". Thus high end rainfall amounts on a localized scale are clearly possible today. These could be a bit overdone, but the overall ingredients do support the potential. Isolated to scattered flash flooding is expected across the urban corridor of southeast FL today into tonight with this activity, with a localized risk also possible on the southwest coast of FL and over the FL Keys. ....Central Gulf Coast... At least some heavy rainfall is expected to move onshore today into tonight across portions of the central Gulf Coast. The closed deep layer low will intensify, with a stronger surface low reflection also expected as the day progresses. The better instability and deeper convection should stay offshore for a good portion of the day, however some showers and embedded heavier convective cells may periodically move into far southeastern Louisiana. By later today into tonight the low will begin moving northward drawing some better instability and moisture along with it. Thus the best chance for more significant convection moving onshore is most likely later tonight, and thus, while non-zero during the day, the better flash flood risk is also later tonight. There remains some model spread with the track of the low and location of the heaviest rainfall. Most of the newer guidance has shown an eastward shift, which results in less QPF over LA (expect for far southeast LA), and more getting into coastal MS/AL and the western FL Panhandle. Given the westerly shear in the Gulf and the tendency for organized convection to orient downshear, the eastward shift does seem to make sense. So while the thought is that this eastward shift is most likely, can not rule out a further west QPF max over LA as shown by the 00z EC and Gem Reg. Either way it looks like any notable instability stays confined pretty close to the coast, so we were able to trim back some of the northern extent of the Marginal risk with this update. Still think the flash flood risk is only localized in nature, and so a Marginal risk should continue to suffice for now. The highest probabilities of 3-5"+ amounts is far southeast LA, and this area is hard to flood, with the risk of these higher amounts getting to New Orleans still lower. Given the eastward shift and better banding potential on the eastern side of the eventual low, Biloxi or Mobile may have a slightly higher flash flood risk tonight. Although still a question on rainfall rates/magnitudes onshore, with HREF 3" neighborhood probabilities 15-30% and 5" 10% or less. So from a probabilistic sense still think the risk falls within the Marginal level, but will continue to monitor trends through the day. 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!7CbTNFOi2o8DtRYSddM7G8byk66ZImejatTy8TN8A8vJ= vctAcQPQpfLU4gVLyxMOSLffOUjFiSkiFZ8JDQ7dz5USHyU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7CbTNFOi2o8DtRYSddM7G8byk66ZImejatTy8TN8A8vJ= vctAcQPQpfLU4gVLyxMOSLffOUjFiSkiFZ8JDQ7dJEvkxvk$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7CbTNFOi2o8DtRYSddM7G8byk66ZImejatTy8TN8A8vJ= vctAcQPQpfLU4gVLyxMOSLffOUjFiSkiFZ8JDQ7dyD7TzsE$=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 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .