Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed May 14 2025 00:07:27 FOUS30 KWBC 140007 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 807 PM EDT Tue May 13 2025 Day 1 Valid 01Z Wed May 14 2025 - 12Z Wed May 14 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE MID-ATLANTIC AND OHIO VALLEY... 01Z Update: Realigned the outlook areas a bit based on trends in=20 satellite and radar imagery across portions of West Virginia and=20 the adjacent portions of the Mid-Atlantic. Maintained the Marginal=20 risk area over portions of southeast Ohio where convection with=20 heavy rainfall occurred earlier in the day had started to weaken=20 with the loss of daytime heating and upstream cloud tops were=20 warming and decreasing in areal coverage. Even so...there were=20 enough signals to suggest additional showers could redevelop. Over the Mid- Atlantic region, confluent flow has been strengthening=20 the focus for training cells aligning with terrain...and the=20 approach of an upper level trough been aiding pressure falls over=20 portions of Virginia. With surface dewpoints in the 65 to 70 degree range leading to surface based CAPE values on the order of 2000 J=20 per kg...the environment has been supporting some locally intense=20 downpours, With loss of solar insolation...the expectation is for=20 rates to taper off in the 02Z to 04Z range. In addition to the loss of daytime heating...cells will begin to encounter a more stable airmass as they head north and should result in a more stratiform rainfall later. Bann 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 just offshore towards the VA Capes by the morning. This will align the moisture advection pattern a bit further north putting areas from Central and Eastern PA down into the Central Mid Atlantic urban centers and Delmarva into play for some of the heavier echoes anticipated with the setup. The jury is still out on whether the rates will be sufficient for the Balt/DC metro area during the forecast, but the urbanization factors are a lean towards maintaining continuity as any significant rainfall cores and 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. Secondary and tertiary QPF maxima are positioned within the Ohio Valley and portions of Eastern NC due to the proxy of the ULL and modest buoyancy under the low center for the OV, and persistent onshore component over Eastern NC during the first half of the forecast period. Widespread MRGL risk encompasses the SLGT across much of the Ohio Valley down into parts of the Southeast as isolated convection pivots around the ULL with some repeated cell impacts plausible given the slower mean storm motions under the primary ULL. 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 SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE NORTHERN PLAINS... ....Northern Plains... 20Z Update: The newest update has added a regional SLGT risk across portions of North and South Dakota along with the northern fringes of Nebraska. The setup will yield a multi-round period of convection within an amplifying mid-level pattern where strong mid and upper ascent will couple with a persistent north-south oriented surface trough bisecting the Dakotas. Afternoon destabilization within the confines of a sharpening surface trough will lead to scattered convection with slow storm motions as the mesocyclones will likely be anchored to the boundary in question. Theta_E gradient is well advertised outlining the general positioning of the surface trough within all major deterministic with boundary layer flow showing a primed convergence pattern in-of the boundary. MUCAPE between 1000-2000 J/kg will provide ample instability for strengthening updraft capabilities which will ultimately tap into an improving moisture advection regime downstream of a potent shortwave trough. The trough in question will become a larger factor for the back half of the forecast as the trough axis will swing eastward by late-Wednesday afternoon with an increased layer of diffluence positioned from the northern Front Range up into the Dakotas. A powerful vorticity maxima on the base of the trough will pivot quickly to the northeast allowing for a broadened large scale forcing mechanism that will trigger a widespread swath of convection as it enters into the Northern High Plains. Guidance is mostly in tandem with the positioning of the heaviest precip located across central and southern SD with the convective pattern likely to initiate and grow upscale rapidly as the activity presses eastward. LLJ implementation overnight will only exacerbate the threat as strengthening shear profiles via local forecast soundings indicate an environment that will be mature quite nicely as it advances near and over the I-80 corridor. There will be an area where both convective schemes will align providing the areal QPF average maxima with totals between 1.5-2.5" likely with localized totals between 3-5" plausible where there's the general overlap. 12z HREF EAS probs for >1" are very high (60-80%) within a large swath of Southeast SD where the CAMs are most bullish with the overlapped QPF distribution from both convective patterns. A secondary maxima extends up into South- Central ND where the primary impacts will be stronger cells in-of the surface trough with some scattered convective elements as the potent mid-level vorticity maxima approaches early Thursday morning. There's still some spread on the northern extent, but the magnitude of the potential is worthy for the implementation of a SLGT risk up into portions of Bismarck's CWA. The SLGT is most prominent across southern and Southeast SD where there's growing consensus on a dual convective threat with an overlap between two impact periods in the forecast window. A MRGL encompasses the general SLGT with heavy precip likely to fall as far south as Southwest NE, but consensus is forward propagation speeds should limit the threat, especially when you add the Sandhill regions very high FFG indices. Kleebauer ....Mid-Atlantic... 20Z Update: Broad MRGL remains with little variation from run to run within the recent deterministic output. CAMs continue to pinpoint a round of convection developing under the primary upper circulation as it maneuvers eastward through VA. The axis of greatest impact will occur over Southeast VA up through the western shores of the Chesapeake with a secondary maxima across Northern NJ into the Lower Hudson of NY. The latter has a low-prob compared to the anticipated convective threat further south due to lack of instability and more of a steady-state stratus rainfall with some embedded heavier rain cores. A SLGT risk was mulled over across Southeastern VA, but CAMs differed on the magnitude and positioning of the greatest convective potential. A targeted SLGT is possible if future CAMs have a better handle and promote a more significant probabilistic output for heavier rainfall potential. For now, localized 1-2" amounts are most likely when assessing the modest EAS signal for >1" across Eastern VA (20-35%) and a much lower >2" signal (<5%) in the same region. The MRGL was maintained given the above data interpretation. Kleebauer ...Previous Discussion.. 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... 20Z Update: Little updated was necessary for the previous D3 MRGL inheritance across the Northern Plains and Upper Midwest. There's a solid QPF magnitude and distribution within the current risk area, but a majority of the precip will be subject to a strong axis of deformation with rates capped between 0.75-1.25"/hr during peak intensity. The heavy rain axis is also subject to the evolution of the eventual closed upper circulation that will manifest over the Northern Plains, so positioning is favored over ND, but the longitudinal placement is spread among the relevant deterministic. Ensemble mean QPF footprint and ML output are relatively close, but still slightly off set. In either case, locally enhanced rainfall from intense dynamics will likely preclude some type of localized flooding potential despite the lack of true thermodynamic instability factors. Decided continuity was the best way to go for the update and will assess in later forecasts for perhaps a targeted upgrade. Kleebauer ...Previous Discussion.. 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!7E4Ndh9k6Ou0-IJ_9dBEktltobqIWBen40aZh0WYsLg0= JEzEzsp5mFNpRDS0jE4QlLGw-5zvsIrln5oW4SttfKbPX_o$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7E4Ndh9k6Ou0-IJ_9dBEktltobqIWBen40aZh0WYsLg0= JEzEzsp5mFNpRDS0jE4QlLGw-5zvsIrln5oW4SttHrHyNUw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7E4Ndh9k6Ou0-IJ_9dBEktltobqIWBen40aZh0WYsLg0= JEzEzsp5mFNpRDS0jE4QlLGw-5zvsIrln5oW4SttlSEmocA$=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) .