Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue May 13 2025 15:37:01 FOUS30 KWBC 131536 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1136 AM EDT Tue May 13 2025 Day 1 Valid 16Z Tue May 13 2025 - 12Z Wed May 14 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE MID-ATLANTIC... 16Z Update: The overall synoptic pattern remains unchanged with the progression at both the surface and upper levels maintaining run to run continuity within the hi-res suite. Steady plume of moisture between 925-500mb will aid in continued moderate to heavy rain signature across the Mid Atlantic with a stronger convective focus situated across the Southern Mid Atlantic and Blue Ridge due to some marginal clearing as dry air advects aloft under the base of the upper low. Current IVT orientation remains sufficient for primed easterly upslope across the Central and Northern Blue Ridge with waves of heavier QPF migrating west-northwest from the Potomac to points west. Latitudinal push of heavier precip will occur this evening as the surface low develops across Eastern NC and moves=20 just offshore towards the VA Capes by the morning. This will align=20 the moisture advection pattern a bit further north putting areas=20 from Central and Eastern PA down into the Central Mid Atlantic=20 urban centers and Delmarva into play for some of the heavier echoes anticipated with the setup. The jury is still out on whether the=20 rates will be sufficient for the Balt/DC metro area during the=20 forecast, but the urbanization factors are a lean towards=20 maintaining continuity as any significant rainfall cores and=20 training echoes could cause some flash flood concerns east of US15. 12z HREF neighborhood probs for >3" are very high (>70%) within the Central Blue Ridge up into the Lower Susquehanna Valley and eastern slopes of the Laurels. Pockets of elevated probs (>50%) are present for >5" totals as well within the same corridor with the highest centered over the Blue Ridge thanks to the textbook easterly upslope signature within a core of higher IVT in conjunction with a reasonable theta_E maxima that aligns with the terrain. This has allowed for a general maintenance of the SLGT risk inherited with only minor modifications based on QPF output via latest HREF blended mean and prob fields.=20 Secondary and tertiary QPF maxima are positioned within the Ohio=20 Valley and portions of Eastern NC due to the proxy of the ULL and=20 modest buoyancy under the low center for the OV, and persistent=20 onshore component over Eastern NC during the first half of the=20 forecast period. Widespread MRGL risk encompasses the SLGT across=20 much of the Ohio Valley down into parts of the Southeast as=20 isolated convection pivots around the ULL with some repeated cell=20 impacts plausible given the slower mean storm motions under the=20 primary ULL.=20 Kleebauer ...Previous Discussion.. A closed upper low will gradually lift north over the Tennessee and Ohio Valleys, accompanied by a very slow-moving surface low pressure system. A stream of deep Gulf/Atlantic moisture will move into the Mid-Atlantic ahead of the system, increasing PWAT values to 1.5-2 inches. There will be enough instability (MUCAPE > 1500 J/kg) to support thunderstorm development, some of which may be strong/deep. The best chances for deep convection with high rain rates (1-2 inches) will likely be over portions of Virginia and eastern North Carolina where higher instability will reside, but persistent upslope enhancement will also contribute to elevated rain rates over portions of the Appalachians from central and southern Pennsylvania to Virginia. These areas will likely see 1-3 inches of rainfall today, with locally higher amounts in terrain and heavier convection. Heavy rain will have the potential to create isolated to scattered flooding concerns across these areas, warranting a Slight Risk of excessive rainfall. Elsewhere, a Marginal Risk of excessive rainfall is in effect for the Ohio Valley and portions of the Tennessee Valley and interior Southeast where scattered diurnal convection is expected underneath the upper low. Slow storm motion will contribute to elevated rain rates (1+ inch per hour) that could reach or exceed 1 and 3 hour FFGs (1-2 inches). Given that soils in these areas are already saturated from recent heavy rain, these rain rates could create isolated instances of flash flooding. Dolan Day 2 Valid 12Z Wed May 14 2025 - 12Z Thu May 15 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE NORTHERN PLAINS AND MID-ATLANTIC... ....Northern Plains... A deep upper level trough will push across the northern Rockies into the northern Plains late Tuesday into Wednesday, resulting in a strengthening low pressure system tracking from Wyoming through North Dakota. Stratiform rain with some embedded convection is expected on the northern and western side of the low, which should result in a swath of 0.5-1+ inches of rainfall across central and eastern Montana on Wednesday. Storm total rainfall will be higher across Montana due to additional rain falling at the end of the Day 1 period (Tuesday night). The main concern with this activity will be the duration, with rainfall rates potentially reaching 6 hour FFGs. This could support isolated flash and river flooding concerns. Over the Dakotas, convection is expected to develop along a strengthening cold front that will push east across the region. With favorable upper level and surface support, some convection may be deep enough to produce 1+ inch per hour rain rates that could reach or exceed the 1 and 3 hr FFGs (around 1.5-2.5 inches). Conditions will be supportive of localized heavy rain in stronger convection/thunderstorms, with plentiful moisture (PWATs 1-1.5 inches) and instability (CAPE > 1500 J/kg) in place. The Marginal Risk of excessive rainfall in the northern Plains includes portions of Montana and the Dakotas where isolated to potentially scattered instances of flash flooding will be possible. It's possible an upgrade may be needed for the Dakotas if precip rates and QPF trend higher, but we should have a better idea of potential impacts once the period is fully within range of the CAMs. ....Mid-Atlantic... Soils are expected to be thoroughly saturated across the Mid- Atlantic by Wednesday due to heavy rain earlier in the week, which will likely result in a lingering flash flood threat on Wednesday. The slow-moving surface low is forecast to become nearly stationary over the region while gradually weakening, resulting in scattered precipitation through Wednesday night. Enough moisture and instability are expected in the vicinity of the surface low to support convection with relatively higher rain rates that could lead to flooding. The 00Z HREF shows a high probability (60-80%) of 1 inch per hour rain rates from the Maryland and West Virginia Panhandles through Virginia to eastern North Carolina and even a low/isolated probability (~20%) of 2 inch per hour rain rates. A Marginal Risk is in place from southern Pennsylvania through North Carolina to northeast South Carolina to account for this flood potential. Dolan Day 3 Valid 12Z Thu May 15 2025 - 12Z Fri May 16 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE UPPER MIDWEST AND NORTHERN PLAINS... An occluded low pressure system will lift northeast across the Upper Midwest, with showers and storms ahead of the occluded and cold fronts and some precipitation wrapping around the backside of the system into the northern Plains as well. Slow storm motion and repeat activity look like they will be the main threat, which may result in rain rates that may reach or exceed the 3 hour FFGs. There will be plenty of moisture in place to support elevated rain rates with PWAT values reaching 1-1.5 inches. However, instability may be a limiting factor. The best instability is expected to be further south along the cold front, displaced from the higher QPF values. Based on available guidance, a Marginal Risk area should suffice to account for the low end flash flood potential. Precipitation is forecast to extend across the Great Lakes region as well, but rain rates will likely be moderate at best, not supporting a flash flood threat at this time. Dolan Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!55rrlPUHWG2rNolCcre_pdlPErLeC48tGl2cYfB26pII= xCq8H_q_PsTl0Di5TSQdbNw4_P8zHeCDOev0-6pZhjG9ZTQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!55rrlPUHWG2rNolCcre_pdlPErLeC48tGl2cYfB26pII= xCq8H_q_PsTl0Di5TSQdbNw4_P8zHeCDOev0-6pZKNu7nOc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!55rrlPUHWG2rNolCcre_pdlPErLeC48tGl2cYfB26pII= xCq8H_q_PsTl0Di5TSQdbNw4_P8zHeCDOev0-6pZui_lhxc$=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) .