Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat May 10 2025 07:54:03 FOUS30 KWBC 100753 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 353 AM EDT Sat May 10 2025 Day 1 Valid 12Z 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... ....Southeast... A stationary upper low will remain parked over the Central Gulf=20 Coast through the weekend. A potent shortwave trough will pivot=20 around the upper low today, sparking convection across the=20 Southeast. At the surface, a low pressure system slowly tracking=20 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=20 precipitation on the northern side of the system as well. Heavy=20 rain should initially focus on the Florida Panhandle this morning,=20 then spread further into the Southeast and Florida peninsula=20 throughout the day.=20 Showers and storms tracking north along the frontal boundary that=20 extends into the Gulf will result in areas of repeat convection in=20 the Florida Panhandle and southeastern Alabama that will likely=20 cause high rainfall totals over the span of a few hours. Storms=20 will be capable of producing 2-3 inch per hour rain rates with=20 ample moisture (PWAT>1.5 inches) and CAPE (>1500 J/kg) to tap into. FFGs in the Florida Panhandle and far southeastern Alabama have=20 decreased to 3-5 inches in 3 hours, which will be in danger of=20 being exceeded with areas of repeat convection today. Additionally, heavy rain fell last night/early this morning in the Florida=20 Panhandle, which has helped to prime the ground and saturate soils=20 ahead of more heavy rain today. To account for this heightened=20 flood potential, a targeted Slight Risk area has been introduced=20 for portions of the Florida Panhandle and far southeastern Alabama. Elsewhere in the Southeast, persistent areas of moderate rainfall=20 are expected on the northern periphery, north of the west-east=20 oriented stationary boundary. This should result in some decent=20 rainfall accumulations today from central and northern Mississippi=20 to South Carolina. The higher flood potential will be along and=20 south of the frontal boundary where thunderstorm development is=20 favored. Hi-res CAMs have been showing the potential for some more=20 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=20 that develop will be capable of producing high rain rates of 2+=20 inches per hour and could approach the 2.5-3 inch per hour FFGs in=20 these areas. By this afternoon, showers and storms should also pick up along the Florida peninsula as diurnal heating increases. To=20 cover these threats, a broad Marginal Risk area is in place from=20 central Mississippi to South Carolina and to portions of North and=20 Central Florida.=20 ....New England... A coastal low will track along the New England Coast today with=20 stratiform rain on the northern side and likely some embedded=20 convection along the coast in the vicinity of the low pressure=20 center. The low will be fairly progressive, which will help limit=20 the flood risk, but a swath of 1-1.5 inch accumulations are=20 expected, with locally higher amounts possible, across southern=20 Maine during the day today. The highest rain rates will likely be=20 along the immediate coast where precipitation amounts could=20 approach the 1.5-2 inch per 3 hour FFGs and result in localized=20 flooding concerns. Another concern will be New Hampshire where=20 0.35-0.75 inches of rain are forecast to fall over fully saturated=20 soils, which could contribute to ongoing flooding of rivers,=20 creeks, and streams in the region. A Marginal Risk area is in place across portions of Maine and New Hampshire.=20 Dolan 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... The upper level low over the Central Gulf Coast will elongate=20 north-south as another upper level wave of energy moves over the=20 Southeast on Sunday, and the direction of the mean layer flow will=20 shift to become more directly onshore/perpendicular to the Gulf=20 Coast. Meanwhile, a strengthening southern stream jet will create=20 favorable divergence aloft in the left exit region positioned right over North Florida and southern Georgia in the afternoon/evening.=20 At the surface, an occluded low pressure system will be very slowly moving north across the Southeast with a stationary front=20 extending west to east across Georgia and South Carolina. This=20 setup will support another wave of widespread showers and=20 thunderstorms capable of producing heavy rain. Much of the region=20 will have received heavy rain the day before (Saturday), resulting=20 in more saturated soils that are potentially easier to flood.=20 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... The upper low that will impact the Southeast over the weekend will finally gain some momentum and move north towards the Tennessee=20 Valley on Monday. The accompanying surface low pressure system will slowly drift north as well, and moisture ahead of the system will=20 expand from the Southeast into the southern Mid-Atlantic. Very high PWAT values of 1.5-2 inches will spread up the Southeast Atlantic=20 Coast, and widespread heavy rain is forecast from Florida to=20 Virginia.=20 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.=20 Dolan Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8AYgEaJYvKvFpz9g2qIr2jwTZ1lr0aKqmP4N-rTzoAwr= XoxJGqTlFfGB-LG5x_MbkevZuU3z9WDObgmGhLW0Dm1it7c$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8AYgEaJYvKvFpz9g2qIr2jwTZ1lr0aKqmP4N-rTzoAwr= XoxJGqTlFfGB-LG5x_MbkevZuU3z9WDObgmGhLW0PXybSZo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8AYgEaJYvKvFpz9g2qIr2jwTZ1lr0aKqmP4N-rTzoAwr= XoxJGqTlFfGB-LG5x_MbkevZuU3z9WDObgmGhLW0G4QrXqM$=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. 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