Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Apr 12 2023 22:31:45 FOUS30 KWBC 122231 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 631 PM EDT Wed Apr 12 2023 Day 1 Valid 2146Z Wed Apr 12 2023 - 12Z Thu Apr 13 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR SOUTH FLORIDA... ....Southeast Florida... Heavy rain has fallen across South FL as a warm front lifts northward across the area, with South Florida Management District rain gauges measuring up to 5.85" in portions of eastern Broward County, with rainfall estimates per radar near 12" between Dania Beach, Fort Lauderdale, and Sunrise. Given the lifting warm front, both higher instability and PWs will lift northward through South FL, with CAPE over 1000 J/kg and PWs topping 1.75". Thus should be a pretty good combo of moisture and instability, along with continued slow cell motions and backbuilding given southeasterly low level flow and west-southwesterly mid/upper level flow. Heavy rainfall should move up the coast with time as the warm front lifts northward around the eastern periphery of the Gulf low, which itself is currently drifting east-northeast well to the west of Fort Myers and well south of Biloxi. The 18z HREF indicates that another 5" of rainfall could fall upon portions of eastern Miami-Dade, Broward and Palm Beach counties, so highly localized Moderate (or higher) Risk impacts cannot be ruled out, especially in eastern Broward county.=20 ....Central Gulf Coast... Some heavy rainfall is expected to move onshore today into tonight across portions of the central Gulf Coast, though chances across southeast LA are dwindling by the hour per radar trends and satellite trends with regards of the recent east-northeast movement of the Gulf low. 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/the Mouth of the Mississippi River. By tonight the low is expected to track more 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. The recent track of the low supported the eastward shift noted earlier, a hallmark of convective lows in the Gulf of Mexico, which results in less QPF over LA (expect for far southeast LA), and more getting into coastal MS/AL and the FL Panhandle. Given the westerly shear in the Gulf and the tendency for organized convection to orient downshear, it is possible that the guidance could continue shifting east. Used the 18z HREF to make changes to the Marginal Risk area for the special update; left New Orleans inside the risk area as a course of least regret. Given the eastward shift in the low track and better banding potential on the eastern side of the low, Biloxi, Mobile, or Pensacola appears to have the slightly higher flash flood risk tonight. Roth 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... ....2030Z Update... No significant changes were made with the afternoon update today. On Thursday the Gulf low will continue tracking slowly northeast into the Southeast, which in turn will draw the moisture, instability, and forcing along with it. The guidance is a hair faster this afternoon as compared with 24 hours ago, so the only change of note to the D2 ERO map is to shrink the Marginal in FL a bit further away from the Gulf Coast. No changes were made to the northern side of the Marginal risk area.=20 Wegman 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... ....2030Z Update... No changes were made to the Marginal Risk area across the Carolinas with this afternoon's update. The guidance has not changed much from the overnight, and the points discussed below largely remain valid. This remains a very low end Marginal risk as much of the area is unlikely to pick up much more than an inch of rain, however due to the factors described below, including the 1000 J/kg of CAPE, potential for multiple rounds of storms, localized spots with lower FFGs, and similar PWs, have left the Marginal risk area as is with this update. Some of the guidance shows far eastern NC getting a bit more rain, but the higher FFGs in that area more than make up for that. Wegman ....Previous Discussion 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!9DBBm0WtBCfV2pqkhvwG-T1JAK0KhHo_EtaDQFg2G4KC= PU80sA-E3n8iCq8D3CPDw3s-EszdudRL-m2neZAcQnHPqDg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9DBBm0WtBCfV2pqkhvwG-T1JAK0KhHo_EtaDQFg2G4KC= PU80sA-E3n8iCq8D3CPDw3s-EszdudRL-m2neZAcaHjXmP4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9DBBm0WtBCfV2pqkhvwG-T1JAK0KhHo_EtaDQFg2G4KC= PU80sA-E3n8iCq8D3CPDw3s-EszdudRL-m2neZAcOIbKE9M$=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) .