Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Apr 12 2023 20:22:15 FOUS30 KWBC 122022 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 420 PM EDT Wed Apr 12 2023 Day 1 Valid 16Z Wed Apr 12 2023 - 12Z Thu Apr 13 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR ATLANTIC COASTAL SOUTHEAST FLORIDA... ....16Z Update... ....South Florida... Very few changes were needed to the ERO risk areas for today's midday update. The under-performance of the CAMs overnight continues into this morning, which has improved the flooding outlook a bit. Current radar presentation shows convection developing offshore over the Gulf Stream and the Bahamas, then slowly pushing east into the urban Miami-Ft. Lauderdale area. While hourly precipitation rates are only occasionally reaching 0.75 inches, the very low FFGs in that area may allow even those relatively low rainfall rates to still suffice to cause localized flash flooding. As the day goes on, instability may continue to support cellular convection moving into the area until the whole area of moisture begins moving north by this evening. There is surprising inconsistency between the CAMs as to how quickly the northward move begins, along with differences in how heavy the convection will be in the area even over the next few hours. The HRRR and ARW are both very much on the heavy side, which has not materialized as of this writing...and these models (especially the HRRR) are likely heavily impacting the HREF probabilities...with over 90% chance of 3" noted over the Miami area in the 40 km neighborhood probabilities, and over 50% of 8" or more. These probabilities are entirely too high given that a good amount of that precipitation was supposed to have already happened over the past few hours. In short, the HREF probabilities were largely disregarded for this issuance. Even with those overly wet solutions, all of the CAMs suggest that the bulk of the precipitation will have moved north of the Miami-Ft. Lauderdale area by this evening, so it's likely the Slight Risk will be able to be dropped at that time. The only change of note in the risk areas was to expand the Marginal Risk area along the Florida Gulf Coast a bit further north to include the Ft. Myers area. A larger area of light to moderate rain is ongoing across that area, and the more sensitive urbanized area may see localized flooding as the rain is expected to continue for much of the day. Some instability may be enough to allow heavier embedded elements to continue moving into the southwest FL Gulf coast. ....Central Gulf Coast... No changes were made to the Marginal Risk area that hugs the coast from the western FL Panhandle to the central LA Gulf Coast. As with yesterday, the most likely area to see excessive rain within the Marginal Risk is right along the Mississippi River Delta south of New Orleans, but as the vertically stacked low very slowly moves east, the added instability from the upper level low may allow for pockets of heavy rain to move ashore from south to north across the Gulf coast in the Marginal Risk area through the day today, though the greatest threat slowly moves east with time. FFGs remain very high, so only multiple rounds of training thunderstorms, which while unlikely are possible will have enough total rainfall to cause any localized flash flooding, especially if such a scenario can align with a more urbanized portion of the coast. Despite the low confidence as to which portion of the coast will see the most rain today, there's higher confidence that any such convection struggles mightily to move very far inland, and as such the Marginal Risk remains confined to coastal areas. Wegman ....Previous Discussion... ....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... ....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!4EkTz3y2n8ehP730r4sAvyoiYQ5dSKSHTP7WqdN66vHa= nLKk7z54rNO70YQ9MOY_DHAn9ujrm2B6Cud42cOW3g7pTf0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4EkTz3y2n8ehP730r4sAvyoiYQ5dSKSHTP7WqdN66vHa= nLKk7z54rNO70YQ9MOY_DHAn9ujrm2B6Cud42cOWZP5_1Xs$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4EkTz3y2n8ehP730r4sAvyoiYQ5dSKSHTP7WqdN66vHa= nLKk7z54rNO70YQ9MOY_DHAn9ujrm2B6Cud42cOWkfojIrY$=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) .