Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Apr 12 2023 08:12:35 FOUS30 KWBC 120812 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 411 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 Valid 12Z Thu Apr 13 2023 - 12Z Fri Apr 14 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHEAST... The closed deep layered low near the central Gulf Coast will lift northeastward into the Southeast Thursday into Thursday night. It still looks like it will be gradually transitioning into a negatively tilted trough, although this process is taking a bit longer to occur on recent model runs. Thus will have decent large scale ascent over the region for most of the period. PWs increase into the 1.25-1.5" range, which is sufficient for heavy rainfall and getting towards or exceeding the 75th percentile in spots...but still not anything overly impressive. Guidance is indicating that ~1000 J/KG CAPE should be able to push into a good chunk of southern/central AL/GA and southern SC, so enough to support some intense deep convective cells. Mean layer flow is pretty fast off to the north, so cells should be quick moving...however unidirectional flow and 20-30 kts of 850mb inflow will likely support some brief backbuilding/training of activity as it moves off to the north. All this to say that some pockets of heavy rainfall appear likely, with localized amounts of 2-4" seeming probable within the Marginal risk (best chances from south central GA into eastern FL and coastal SC). Most of this area is hard to flood with high FFG, so still tend to think any flooding will remain localized in nature. So while some isolated instances of flash flooding are quite possible with this setup, the coverage of such flooding should stay low enough to justify just a Marginal risk in the ERO for now. Chenard Day 3 Valid 12Z Fri Apr 14 2023 - 12Z Sat Apr 15 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF EASTERN GEORGIA INTO THE CAROLINAS... We did go ahead and introduce a Marginal risk across portions of far eastern GA into the Carolinas for Friday into Friday night. This is a progression of the day 2 risk off to the northeast. The dynamics of the system will be weakening by this time, so tend to think the areal coverage of heavier QPF amounts will be lower compared to day 2. However we'll still have a closed low opening into a negatively tilted trough and over 1000 J/kg of CAPE to work with over the risk area. Some of these areas could see two rounds of convection with ongoing activity to start the day associated with an initial low level moisture surge, and then another round closer to peak heating as the shortwave moves across. Like the day before, PWs are not on the extreme end of the scale, but high enough (over 75th percentile) that when combined with the expected instability we should be able to get some heavy rainfall rates. The deterministic QPF from WPC is not all that high, likely indicative of the fact that areal averaged rainfall by this time will be on a downward trend. However, the ERO is a probabilistic product, and do think there is certainly a chance of locally heavy rainfall up towards 2-3" in spots. This part of the country is a bit more susceptible to flooding due to recent rainfall...with both soil moisture and streamflows running 75-90%+ above average for mid April. Borderline whether this risk area was needed, but these antecedent conditions tipped the scales in favor of outlining the area. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5iKisPjYjJ87BluIzs7GNqNnFNnAFUHdJ8tDog8CD19x= gzyz3Wyua4IJpI_DKx9x918EiAT-pDQ9tloioKEhlGQ1eeM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5iKisPjYjJ87BluIzs7GNqNnFNnAFUHdJ8tDog8CD19x= gzyz3Wyua4IJpI_DKx9x918EiAT-pDQ9tloioKEh_R_BFwc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5iKisPjYjJ87BluIzs7GNqNnFNnAFUHdJ8tDog8CD19x= gzyz3Wyua4IJpI_DKx9x918EiAT-pDQ9tloioKEhywe52uo$=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) .