Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun May 11 2025 20:29:14 FOUS30 KWBC 112029 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 429 PM EDT Sun May 11 2025 Day 1 Valid 16Z Sun May 11 2025 - 12Z Mon May 12 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER MUCH OF THE SOUTHEAST... 16z Update: The best signal for isolated to scattered flash flooding this afternoon/evening appears to be over portions of southeast GA into northeast FL where backbuilding cells will result in rainfall locally over 3 inches. In fact both 12z HREF and 06z REFS neighborhood probabilities of exceeding 5" are over 50%, and if/where this occurs would expect at least some minor flood concerns to develop. By this afternoon convection should develop over portions of central AL into MS closer to the deep layer low center. This activity will also likely exhibit some training/backbuilding characteristics resulting in an isolated to scattered flash flood risk. Some localized exceedance of 3" appear probable here, and both the HREF and REFS indicate some FFG exceedance probs as well. The Slight risk was expanded a bit to the west to account for this activity over MS. The northern edge of the Slight risk over the Carolinas was tightened a bit as the main flash flood risk here will likely be on day 2 (after 12z Monday). The Marginal risk was expanded southward to include the rest of the FL peninsula as slow moving convection may try to move into south FL overnight. Chenard ....Previous Discussion... A stationary upper low will remain positioned over the Central Gulf Coast today while waves of upper level energy pivot around it over the Southeast. The low is expected to become slightly elongated north to south, resulting in strong southerly mean flow that will pump moisture from the Gulf directly into the Southeast. Additionally, a southern stream jet is forecast to strengthen over the northern Gulf, creating favorable upper level divergence over North Florida and Georgia to support widespread shower and thunderstorm development. At the surface, an occluded low pressure system will be slowly moving north across the region, with a cold front extending south across the Florida Panhandle and a stationary front extending east across Georgia and South Carolina. The best conditions for heavy rain will be in the warm sector over North Florida and southern Georgia, but heavy rainfall will also be possible along and north of the stationary boundary up into portions of the Carolinas. With plentiful moisture (PWATs 1.5-2+ inches) and instability (MUCAPE 1500-2000+ J/kg), storms in the warm sector will be capable of producing 2-3+ inch per hour rain rates from Florida to South Carolina, which could lead to some flash flooding concerns given 2-3 inch per hour FFGs. Flash flooding may also be a concern back into portions of Alabama and northern Mississippi where there will likely be enough moisture and instability in the vicinity of the surface low to produce showers and storms with 2+ inch per hour rain rates. Previous heavy rains have already saturated soils from Mississippi to Georgia, which will make these areas more sensitive to flooding with additional heavy rain today. To account for this flood potential there is a Slight Risk of excessive rainfall in place for much of the Southeast, including areas from North Florida to southern North Carolina and much of Alabama and northern Mississippi. The Slight Risk area is encompassed by a broader Marginal Risk that includes much of the Tennessee Valley, eastern Arkansas, upslope regions of the southern Appalachians, and Central Florida. Dolan Day 2 Valid 12Z Mon May 12 2025 - 12Z Tue May 13 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHEAST AND SOUTHERN MID-ATLANTIC... 20z Update: Continue to be very close to needing a MDT risk=20 upgrade across the upslope areas of the southern Appalachians, but we will hold at a higher end Slight risk for now. Orographically enhanced rainfall and embedded convective elements periodically=20 pushing rainfall rates over 1"/hr will likely drive at least a=20 minor flash flood risk across this portion of the western=20 Carolinas and southwest VA. However confidence on seeing more=20 significant higher end impacts remains lower. With easterly low=20 level flow in place over this corridor the better instability may tend to stay farther south over GA/SC and over the eastern=20 Carolinas. Probabilities of exceeding 1"/hr in the HREF peak=20 around 40-50% in the 12z HREF and REFS, likely indicative of some=20 exceedance, but probably not widespread. It will likely take these=20 higher rates of 1-2"/hr or greater to drive more significant=20 impacts.=20 The currently forecast 3-7" of rainfall, combined with=20 embedded rates of 0.5"-1"/hr should be enough to drive at least=20 scattered instances of minor flash flooding, with localized more=20 significant impacts still possible. For now the expectation of=20 scattered minor flash flood and localized higher end impacts falls=20 more into the high end Slight risk category. If confidence in=20 greater coverage of higher rainfall rates increases, that would in=20 turn increase the potential for higher end impacts and justify a=20 MDT risk upgrade. Will thus continue to closely monitor. The southeastern FL urban corridor is another area of concern for flash flooding Monday. Neighborhood probability of exceeding 8"=20 are over 50% in both the 12z HREF and REFS and some deterministic=20 members are over 10". Overall seems like a favorable synoptic=20 setup for training convection and thus higher end urban impacts=20 are possible. Not confident enough yet to go with a MDT risk=20 upgrade, however do consider this a higher end Slight risk and the potential is certainly there for significant urban impacts. We=20 will continue to monitor observational and model trends. Chenard ....Previous Discussion... The nearly stationary upper low that impacted the Southeast over the weekend will finally gain some momentum and shift north towards the Tennessee Valley on Monday, pulling the surface low pressure system along with it. The trailing cold front will gradually push towards the Southeast Atlantic Coast while the warm front lifts into the southern Mid-Atlantic, and another round of widespread showers and storms are forecast Monday into Monday night in the warm sector of the system. Deep moisture will surge north ahead of the cold front, extending from Florida through the Carolinas to southern and central Virginia. PWATs are anticipated to reach 1.5-2+ inches in these areas with even higher values possible over the Florida Peninsula. Strong south-southeasterly mean flow will remain in place, which will likely result in two convergent zones, one along the Southeast Coast and one along upslope regions of the southern Appalachians where high QPF totals will be possible. Strong diurnal convection over the Florida Peninsula will also contribute to high QPF values over portions of Central and South Florida. The largest flooding concerns will come with repeat/training activity, which could result in high QPF totals over a relatively short period of time, potentially reaching or exceeding the 1 and 3 hour FFGs in these regions. To account for this flash flood risk, there is a Slight Risk area stretching from Florida up the Southeast Coast to North Carolina and southern Virginia and also for the upslope regions of the Appalachians from Georgia to Virginia. A Marginal Risk area extends even further into the Mid- Atlantic where moderate rain rates may contribute to localized flash flooding and into the Tennessee and Mid-Mississippi Valleys where enough moisture will likely wrap around the surface low to create locally heavy rainfall in showers and storms. Upslope regions of the southern Appalachians are particularly sensitive to heavy rain, especially in areas that were impacted by Hurricane Helene. Impacts from heavy rain on Monday will likely fall into the high-end Slight Risk category, and ERO upgrades may be needed in the future if high QPF trends hold,. However, instability may be a limiting factor as the better instability is expected to reside along the coast. After coordinating with local forecast offices, it was decided to hold off on any upgrades at this time. By this afternoon, the CAMs will go out far enough to cover the entire Day 2 period and will likely give us a better idea of the magnitude of potential flooding impacts. Dolan Day 3 Valid 12Z Tue May 13 2025 - 12Z Wed May 14 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE MID-ATLANTIC... 20z Update: The Slight and Marginal risk areas were expanded a bit to the west with this update. The Marginal risk covers more of the OH valley where slow moving convection ahead of the deep layered=20 low could result in localized flash flooding. The Slight risk is=20 expanded into more of the central Appalachians. Overall still=20 looks like a solid Slight risk from northeast NC into the portions=20 of eastern WV. Convection near a stalled boundary should result in an isolated to scattered flash flood risk. Will need to closely=20 monitor, as the slow moving closed low and a stationary boundary=20 does suggests some higher end impact potential Tuesday within the=20 Slight risk area. Chenard ....Previous Discussion... The slow-moving upper low and occluded surface low will continue to move north, triggering showers and storms across the Mid- Atlantic on Tuesday. A deep stream of moisture ahead of the system will push PWAT values above 1.5 inches, and there will be enough instability (MUCAPE > 1500 J/kg) across portions of North Carolina and Virginia to support high rain rates in showers and storms. It seems likely that rain rates could reach or exceed the 1-2 inch per hour FFGs in Central Virginia (especially in upslope and urban areas), and possibly the 2.5-3 inch per hour FFGs in southern Virginia and northeastern North Carolina as well. Models have been picking up on a faster trend with this system, but there is some uncertainty in how fast or slow it will move. This will affect the location of the heaviest rainfall in the warm sector and along/ahead of the cold front. The general consensus is for the highest totals to fall over Virginia and northern North Carolina, but some models are advancing heavy rain even further north. To account for the faster trend in guidance, the inherited Slight and Marginal Risk areas from the previous Day 4 ERO have been shifted north, with the Marginal extending up into portions of Pennsylvania and New Jersey. The southern edge of the Slight Risk area has been trimmed up to match QPF trends, and it was extended east to coastal Virginia where urban areas tend to be more sensitive to heavy rain and flooding. It's possible that the location of the Slight Risk will shift in future updates as the speed of the system becomes more certain. Dolan Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-qWvfywJDyz3btDSdyo-ZVQjPScOqp0_O9qCDNZblQ2B= Jh6iRXJYu3d2ZKkHV56zlbxsAdFtCIBPq4VYs9kFOEjPsTM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-qWvfywJDyz3btDSdyo-ZVQjPScOqp0_O9qCDNZblQ2B= Jh6iRXJYu3d2ZKkHV56zlbxsAdFtCIBPq4VYs9kFnJpm8yc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-qWvfywJDyz3btDSdyo-ZVQjPScOqp0_O9qCDNZblQ2B= Jh6iRXJYu3d2ZKkHV56zlbxsAdFtCIBPq4VYs9kFm9D9QL8$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .