Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat May 10 2025 19:53:13 FOUS30 KWBC 101953 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 353 PM EDT Sat May 10 2025 Day 1 Valid 16Z Sat May 10 2025 - 12Z Sun May 11 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE FLORIDA PANHANDLE AND SOUTHEASTERN ALABAMA... ....16Z Update... ....Southeast... No big changes were needed with this morning's update. The Slight and Marginal Risk areas are largely unchanged, short of a small carve out in the Marginal around the Big Bend region for higher FFGs there. The Slight remains in place for potential flash flooding from ongoing convection, but perhaps a bit more concern for potential additional convection in the area tonight. The jet stream has brought a plume of dry air into the area, which is greatly impacting convective coverage, even if locally increasing the potential instability. This dry air will for sure prevent much light rainfall outside of any convection, though that convection may have enough moisture from the lower levels to support a flash flooding threat. A large contributor to the flash flooding threat remains recent heavy rains, especially in the Florida Panhandle portion of the Slight, which have locally saturated the soils but it will still take a lot of heavy rain from any one or line of training storms to support the flash flooding threat there tonight since the soils are sandy and swampy in the area. The upper level low forcing the convection across the Southeast is still getting its act together, so the better forcing and more widespread convection is more likely into the Day 2/Sunday period. The broad Marginal remains in place for localized training convection, with little certainty on where exactly any associated flash flooding will occur. ....New England... In coordination with GYX/Gray, ME forecast office, the Marginal Risk was removed with this update. All of the rain in the area is of a stratiform nature, and is struggling to even reach 1/2 inch per hour rates based on radar estimates. While any additional rainfall especially into New Hampshire will contribute to any ongoing riverine flooding, the flash flooding threat from this system is very low. The rain will continue pivoting east with the back edge over central New Hampshire now crossing Maine and into New Brunswick by this evening. For much of Maine, soils are much drier due to missing recent heavy rainfall, so the flooding threat there is also very low. Wegman ....Previous Discussion... ....Southeast... A stationary upper low will remain parked over the Central Gulf Coast through the weekend. A potent shortwave trough will pivot around the upper low today, sparking convection across the Southeast. At the surface, a low pressure system slowly tracking north into the Central Gulf Coast states will become occluded, with nearly stationary frontal boundaries extending south into the Gulf and east to the Atlantic Coast. Thunderstorms are expected to form in the warm sector of the system today, with some overrunning precipitation on the northern side of the system as well. Heavy rain should initially focus on the Florida Panhandle this morning, then spread further into the Southeast and Florida peninsula throughout the day. Showers and storms tracking north along the frontal boundary that extends into the Gulf will result in areas of repeat convection in the Florida Panhandle and southeastern Alabama that will likely cause high rainfall totals over the span of a few hours. Storms will be capable of producing 2-3 inch per hour rain rates with ample moisture (PWAT>1.5 inches) and CAPE (>1500 J/kg) to tap into. FFGs in the Florida Panhandle and far southeastern Alabama have decreased to 3-5 inches in 3 hours, which will be in danger of being exceeded with areas of repeat convection today. Additionally, heavy rain fell last night/early this morning in the Florida Panhandle, which has helped to prime the ground and saturate soils ahead of more heavy rain today. To account for this heightened flood potential, a targeted Slight Risk area has been introduced for portions of the Florida Panhandle and far southeastern Alabama. Elsewhere in the Southeast, persistent areas of moderate rainfall are expected on the northern periphery, north of the west-east oriented stationary boundary. This should result in some decent rainfall accumulations today from central and northern Mississippi to South Carolina. The higher flood potential will be along and south of the frontal boundary where thunderstorm development is favored. Hi-res CAMs have been showing the potential for some more organized convection late this morning into this afternoon. Given a favorable environment with ample moisture and instability combined with 35-45 knots of effective bulk shear, strong thunderstorms that develop will be capable of producing high rain rates of 2+ inches per hour and could approach the 2.5-3 inch per hour FFGs in these areas. By this afternoon, showers and storms should also pick up along the Florida peninsula as diurnal heating increases. To cover these threats, a broad Marginal Risk area is in place from central Mississippi to South Carolina and to portions of North and Central Florida. Day 2 Valid 12Z Sun May 11 2025 - 12Z Mon May 12 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHEAST... ....20Z Update... A few changes were made to the ongoing Slight Risk area across the Southeast for Sunday. In coordination with BMX/Birmingham, AL forecast office, the Slight was expanded westward across much of central Alabama in the expectation of additional heavy rains wrapping around the northwest side of the upper level low over Louisiana. In addition to rainfall today causing localized flash flooding, the area has seen multiple days of heavy rains recently, so the soils are near saturation and therefore likely to flash flood with the expected 1-2 inches of additional rain expected Sunday and Sunday night, though localized totals from thunderstorms will be higher. The other update was to expand the Slight Risk to include all of the South Carolina coast with this update. The Carolinas will be on the "warm" side of the system so abundant Gulf moisture will be drawn north, supporting training thunderstorms at times across all of the South Carolina coast. Including the lighter rains the area is seeing right now, soils will be saturated more quickly with the additional rainfall Sunday through Sunday night. The surrounding Marginal Risk was nudged northward in North Carolina as well as into Arkansas and Tennessee around Memphis. A narrow area of training showers and storms is probable across northern Mississippi, western Tennessee and eastern Arkansas. However, given the lack of moisture overall, especially as=20 compared with areas further east, expect that rainfall rates will not be high enough to cause anything more than isolated flash flooding, despite somewhat saturated soils in that area. The forecast will be monitored for a possible Slight Risk issuance around Memphis with future updates. Wegman ....Previous Discussion... The upper level low over the Central Gulf Coast will elongate north-south as another upper level wave of energy moves over the Southeast on Sunday, and the direction of the mean layer flow will shift to become more directly onshore/perpendicular to the Gulf Coast. Meanwhile, a strengthening southern stream jet will create favorable divergence aloft in the left exit region positioned right over North Florida and southern Georgia in the afternoon/evening. At the surface, an occluded low pressure system will be very slowly moving north across the Southeast with a stationary front extending west to east across Georgia and South Carolina. This setup will support another wave of widespread showers and thunderstorms capable of producing heavy rain. Much of the region will have received heavy rain the day before (Saturday), resulting in more saturated soils that are potentially easier to flood. There is good agreement among models of widespread 2-3 inch totals through the period for North Florida and southern and central Georgia, with locally higher amounts up to 4+ inches possible. Hi- res CAMs (available through the first half of the period) suggest that storms developing Sunday afternoon will have the potential to produce rain rates of 2+ inches per hour. Given the moist, unstable air mass in place, some stronger/deep thunderstorms will be possible as well that could contribute to rain rates exceeding the 1 and 3 hr FFGs of 3-4 inches. Some global models (mainly the ECMWF and CMC) are suggesting high moisture pushing even further inland with rainfall totals of 2-4 inches potentially reaching into western North Carolina along the upslope region of the southern Appalachians. Some heavy rainfall potential will also exist across portions of Mississippi and Alabama where showers and storms may develop in the vicinity of the surface low pressure center. A broad Marginal Risk is in place from northern Mississippi through much of Alabama to South Carolina and western North Carolina and for portions of Central Florida. There is an embedded Slight Risk area for North Florida, southern and central Georgia, and portions of eastern Alabama and southern South Carolina, which covers areas with the highest rain rate/flash flood potential. Dolan Day 3 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... With increasing amounts of rainfall across the Southeast in the forecast, the Slight Risk area was expanded in all directions with this update.=20 Most notably, portions of western North Carolina and far southern Virginia are very close to needing a Moderate Risk upgrade. Increasing rainfall, likely training, and abundant influx of Gulf/Atlantic moisture into the east facing slopes of the southern Appalachians may impact areas hard hit from Hurricane Helene last September. Thus, with some sensitivity in that area to prolonged rains, including embedded heavy rains, the area may need that upgrade with coming updates. For now there remains some uncertainty as to where training lines of storms will set up relative to the most impacted areas, as it may take prolonged heavy rain, rather than long duration light to moderate rain, to cause flooding impacts. For this reason the Moderate was skipped for now. A small northward trend was noted in the guidance, leading to more rainfall into the Appalachians and Piedmont of southern Virginia. The Slight Risk was expanded north with this update to the Shenandoah Valley, as well as into the Richmond metro. Down south, in coordination with MFL/Miami, FL Forecast office, the Slight Risk was expanded south along the coast through Miami. Stationary thunderstorms are likely along the sea breeze and a stalled front in that area. Should these storms form over the urbanized I-95 corridor, flash flooding is likely as a result. The Slight was expanded west to the Atlanta Metro due to long=20 duration repeating rounds of rain expected to move over the city.=20 The Marginal Risk was expanded well to the west into Arkansas for another day of widely scattered but potentially training thunderstorms.=20 Wegman ....Previous Discussion... The upper low that will impact the Southeast over the weekend will finally gain some momentum and move north towards the Tennessee Valley on Monday. The accompanying surface low pressure system will slowly drift north as well, and moisture ahead of the system will expand from the Southeast into the southern Mid-Atlantic. Very high PWAT values of 1.5-2 inches will spread up the Southeast Atlantic Coast, and widespread heavy rain is forecast from Florida to Virginia. Inland areas in the upslope region of the Appalachians have lower FFGs in the rage of 2.5-3 inches per 6 hours while FFGs elsewhere are a bit higher at 3-4+ inches per 6 hours. The synoptic setup with prolonged onshore flow from the Atlantic will favor enhanced rainfall along the southern Appalachians, which is observed in model output from all available global models. There will be a high chance for 2-3 inches of rain, with potential for up to 4 inches, from northern Georgia to southern Virginia, which would likely cause at least scattered flash flooding concerns. High rainfall totals will also be possible in convergent areas along the coast from Florida to North Carolina, and 24 hour QPF totals are forecast to reach 1.5-2 inches with locally higher amounts expected with stronger convection. Many areas from Florida to South Carolina will have already received heavy rain over the weekend, which will increase the chance of flash flooding concerns with additional heavy rain on Monday. There is a fairly large Slight Risk area in place to cover the higher rainfall potential from Central and North Florida through eastern Georgia and the Carolinas to southern Virginia. The highest risk area within this region will be the southern Appalachians, which will need to be monitored in future forecast updates for potential ERO upgrades. Surrounding the Slight Risk area, a Marginal is in place from Florida to Virginia. Dolan Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9eTfuP9HN8W_puq3Tis7WlOw0WxeQ24w6MpKkcDU01gy= 8GY1d7aWQJo3_D1TCfy_HDQ2d6Ns-t_wEMEjCWKxt-FW2vY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9eTfuP9HN8W_puq3Tis7WlOw0WxeQ24w6MpKkcDU01gy= 8GY1d7aWQJo3_D1TCfy_HDQ2d6Ns-t_wEMEjCWKx6H2VRs4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9eTfuP9HN8W_puq3Tis7WlOw0WxeQ24w6MpKkcDU01gy= 8GY1d7aWQJo3_D1TCfy_HDQ2d6Ns-t_wEMEjCWKxeXNouqs$=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) .