Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon May 12 2025 00:55:41 FOUS30 KWBC 120055 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 855 PM EDT Sun May 11 2025 Day 1 Valid 01Z Mon May 12 2025 - 12Z Mon May 12 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF=20 THE SOUTHEAST... Isolated to scattered flash flooding will remain a concern=20 overnight across portions of southeast GA and southern SC. Some of this area has already seen training convection and some flash=20 flooding earlier today, and additional convection is streaming=20 northward back into the region. The latest SPC mesoanalysis depicts an area of instability offshore, and with easterly low level flow,=20 this will likely advect inland and help sustain convection across=20 these areas. Expect periodic backbuilding/training to persist, and some additional flash flooding is likely. The REFS has been=20 favoring this corridor for heavy rainfall with both its 12z and=20 18z run, but the HREF and recent HRRR runs have not been as=20 aggressive. However the 23z HRRR run did make a pretty significant jump towards a wetter scenario...and based on observational=20 trends...it does appear like something closer to the wetter=20 guidance may indeed pan out. Opted against a MDT risk given some=20 lingering uncertainty on the location and coverage of higher=20 amounts, along with much of the coastal plain having high FFG...but do now consider this a higher end Slight risk and localized higher end impacts are possible. We also maintained a Slight risk across portions of central AL=20 into northern MS. Convection to the east of the deep layered low=20 will continue to pose an isolated to scattered flash flood risk.=20 Some recent indications that an uptick in activity may be starting to occur across portions of central AL. Guidance indicates some=20 maintenance of this convection through the evening hours, so do=20 anticipate some increase in the flash flood potential. A Marginal risk covers much of FL. Convective evolution is less=20 certain here...but do expect to see scattered to numerous showers=20 and thunderstorms. The combination of low confidence in model=20 solutions over FL and the high FFG keeps the risk at Marginal. It=20 is possible that none of the convection organizes enough or trains=20 long enough to cause flash flooding overnight...but can not rule=20 out some training segments hanging up and resulting in isolated=20 flash flood concerns. Chenard 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 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 pushing rainfall rates over 1"/hr will likely drive at least a minor flash flood risk across this portion of the western Carolinas and southwest VA. However confidence on seeing more significant higher end impacts remains lower. With easterly low level flow in place over this corridor the better instability may tend to stay farther south over GA/SC and over the eastern Carolinas. Probabilities of exceeding 1"/hr in the HREF peak around 40-50% in the 12z HREF and REFS, likely indicative of some exceedance, but probably not widespread. It will likely take these higher rates of 1-2"/hr or greater to drive more significant impacts. The currently forecast 3-7" of rainfall, combined with embedded rates of 0.5"-1"/hr should be enough to drive at least scattered instances of minor flash flooding, with localized more significant impacts still possible. For now the expectation of scattered minor flash flood and localized higher end impacts falls more into the high end Slight risk category. If confidence in greater coverage of higher rainfall rates increases, that would in turn increase the potential for higher end impacts and justify a 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" are over 50% in both the 12z HREF and REFS and some deterministic members are over 10". Overall seems like a favorable synoptic setup for training convection and thus higher end urban impacts are possible. Not confident enough yet to go with a MDT risk upgrade, however do consider this a higher end Slight risk and the potential is certainly there for significant urban impacts. We 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 low could result in localized flash flooding. The Slight risk is expanded into more of the central Appalachians. Overall still looks like a solid Slight risk from northeast NC into the portions of eastern WV. Convection near a stalled boundary should result in an isolated to scattered flash flood risk. Will need to closely monitor, as the slow moving closed low and a stationary boundary does suggests some higher end impact potential Tuesday within the 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!7GjBfxrS-_dhEbkP78K_nfRbIZkT2oEhVVpxIJaxv8cb= hHJTOoM74fC6AP60KVkKDPaiJAk9pbE8cap_ZrTvBj5Ig_A$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7GjBfxrS-_dhEbkP78K_nfRbIZkT2oEhVVpxIJaxv8cb= hHJTOoM74fC6AP60KVkKDPaiJAk9pbE8cap_ZrTvwEK0ryY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7GjBfxrS-_dhEbkP78K_nfRbIZkT2oEhVVpxIJaxv8cb= hHJTOoM74fC6AP60KVkKDPaiJAk9pbE8cap_ZrTvA9RMqGM$=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) .