Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Jun 19 2025 19:19:18 FOUS30 KWBC 191919 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 319 PM EDT Thu Jun 19 2025 Day 1 Valid 16Z Thu Jun 19 2025 - 12Z Fri Jun 20 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL APPALACHIANS AND MID-ATLANTIC STATES... ....Central Appalachians, Mid-Atlantic, and the Northeast... A potent shortwave will amplify as it tracks northeast from the Great Lakes to along the northern New England/Canadian border, potentially becoming negatively tilted by tonight. This will help to deepen a surface low moving in tandem with its parent trough, which will in turn drive the accompanying cold front rapidly to the east. By 12Z Friday, this front should be offshore New England and the Mid- Atlantic, but before that occurs scattered thunderstorms with heavy rain are likely along it. The downstream environment from this front is extremely favorable for thunderstorms containing heavy rain. PWs will surge northward on SW 850mb flow of 30 kts, reaching 1.75 to 2 inches, or higher, from the Mid-Atlantic into New England, which will be above daily records according to the SPC sounding climatology. This intense PW plume will combine with a narrow corridor of MUCAPE progged to exceed 2000 J/kg from NC through ME, to support heavy rainfall within convection. Although the CAMs are generally modest with their coverage today, expect scattered to numerous thunderstorms to develop during peak heating, with modest bulk shear of 25-30 kts supporting at least isolated clusters in an otherwise pulse-type environment. Due to the impressive PWs and tall skinny CAPE profiles, max hourly rainfall could reach 2-3" according to the REFS and HREF guidance, which would be sufficient to cause runoff and isolated flash flooding. Although the flash flood risk today is generally isolated due to fast storm motions and minimal training, there is a slightly higher risk for impacts across the Central Appalachians, generally in WV and PA, where antecedent conditions (7-day rainfall more than 400% of normal according to AHPS) have resulted in severely compromised FFG as low as 1"/3hrs. Despite progressive cell motion, HREF exceedance probabilities rise to 15-25% in this area, and it is possible that even fast-moving heavy rain producers could cause impacts, so the SLGT risk has been maintained. Additionally, after coordination with WFO LWX, a targeted SLGT risk has been added for generally the I-95 urban corridor of northern VA and MD where the high-res CAMs suggest some local focusing of thunderstorms this afternoon with heavy rain which could produce as much as 3" of rainfall atop sensitive areas which received heavy rain on Wednesday. Farther north into New England, mean 0-6km winds remain quick at 20- 30 kts, but are more aligned parallel to the Corfidi vectors and the front, suggesting an enhanced training risk. However, instability and PWs are more modest, so rain intensity will be decreased as forecast by lower 1"/hr rain rate probabilities from the HREF. Although total rainfall could reach 3" in some areas from northern VT through northern ME, the excessive rain risk remains in the MRGL category. ....Southeast... The same front that will race across the Mid-Atlantic and Northeast, will trail back into the Southeast today. Although this front is likely to decay by this evening, the residual convergence zone will linger as it becomes embedded within more zonal flow south of the primary trough. Convection ongoing early this morning from AR to AL will weaken, but the accompanying outflow may merge with the weakening front to provide additional impetus for convective development thanks to robust thermodynamics characterized by PWs of around 1.75 inches and MUCAPE of 2000-3000 J/kg. This will create scattered thunderstorms this aftn/eve with rainfall rates of more than 2"/hr. Although there is some uncertainty into how the weakening front will evolve (some CAMs suggest the outflow will surge all the way to the Gulf Coast), any storms that do form have the potential to train as Corfidi vectors collapse to around 5 kts so storms could move west to east and repeat through the evening. Both the HREF and REFS guidance indicate a 30-40% chance of 3" of rain, which could produce isolated flash flooding today. ....Florida Peninsula... Bermuda-type ridge off the Southeast Coast will produce weak easterly flow over the FL Peninsula today. As afternoon heating commences and instability climbs above 2000 J/kg, scattered afternoon thunderstorms will develop. Initially, these will occur on the sea breeze, but as the Atlantic coast sea breeze pushes westward and a weak jet streak develops downstream of a longwave trough dropping across the Southeast, additional development is likely along this boundary and due to storm mergers/boundary interactions. This will likely be most prevalent near the SW coast of FL where the Gulf Coast sea breeze will be pinned, fueling multiple rounds of slow moving thunderstorms. With PWs progged around 1.75", rainfall rates of 2-3"/hr are expected, producing hourly rainfall as much as 3" according to both the RRFS and HRRR models, and total rainfall over 5" possible (20-40%) in localized areas. Should this rain occur atop more urban areas, especially along the Sun Coast, instances of flash flooding could result. ....Upper Midwest... A cold front dropping out of Canada today will slow and stall across Minnesota this evening in response to flattening 500mb flow aloft as a ridge build atop the Southern Plains. Return flow circling this expanding ridge will amplify tonight, with 850mb winds potentially reaching above 50 kts on S/SW flow emerging from the Central Plains and surging into the front. This LLJ will resupply favorable thermodynamics into the region as reflected by an overlap of PWs above 1.25 inches and MUCAPE exceeding 2000 J/kg to support thunderstorm development. Storms that develop along this boundary may train from WNW to ESE as mean winds parallel the front, and with rain rate probabilities reaching 40-60% for 1"/hr, 2-3" of rain is possible as noted by the HREF PMM and 3"/24hr probabilities reaching 20-30. Despite relatively high FFG, training of these rain rates where they produce the locally higher stripes of rain could cause instances of flash flooding tonight. Weiss Day 2 Valid 12Z Fri Jun 20 2025 - 12Z Sat Jun 21 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PARTS OF THE=20 UPPER MIDWEST... ....Upper Midwest... An amplifying mid-level trough deepening into an impressive closed low (500mb heights below the 1st percentile according to NAEFS) will dig across the Pacific Northwest Friday into Saturday. Downstream of this feature, resultant strong deep layer lift will spread through divergence and height falls into the Northern Rockies and Northern Plains, with vorticity lobes shedding from the primary gyre to enhance downstream ascent. At the surface, a wavering front will remain draped from Montana through Minnesota, and the interaction of this baroclinic zone with the mid-level ascent will cause surface cyclogenesis in the High Plains, and this low will race northeastward, reaching the western Great Lakes by the end of D2. Downstream of the primary trough axis, low-level 850mb flow will accelerate to as much as 50 kts in response to a tightening=20 gradient, and push a plume of impressive PWs as high as 1.75 inches (above the NAEFS 97th percentile) into the Upper Midwest. This=20 plume will then be pulled northward and westward by backing 850mb=20 winds, accompanied by an instability surge (MUCAPE) of more than 3000 J/kg. This setup will support increasing convection Saturday evening/night along the elevated baroclinic zone where moisture confluence and isentropic lift overall, leading to widespread=20 shower and thunderstorm development. Storms that blossom along this boundary will likely train west to east along the front, and organize into clusters or an MCS in response to intense 0-6km bulk shear of 50+ kts. In the extreme thermodynamics, rainfall rates have a high chance (>70%) of exceeding 1"/hr, with locally more than 2"/hr rates possible (20% chance). Where training occurs, this may result in corridors of 3" of rainfall or more, with both the HREF and REFS focusing the greatest risk from eastern ND through the western U.P of MI. Despite elevated FFG in this area, these intense rates could overwhelm soils to cause instances of flash flooding, but the anticipated filtration capacity of the soils due to recent dryness limit the risk category to just a MRGL at this time. However, there appears to be a locally higher threat around Lake=20 Superior and into the Arrowhead of Minnesota. Here, recent rainfall earlier this week was significant, which has resulted in 7-day AHPS rainfall that is more than 300% of normal. Despite that, FFG has remained elevated and exceedance probabilities from the HREF are just around 15%, but the REFS is much higher (30-40%). While the REFS has been a bit overdone with recent events, the uptick in QPF noted in model trends atop sensitive areas is concerning for a slightly higher risk for flash flooding despite generally fast=20 storm motions. After coordination with WFO DLH, a targeted SLGT=20 risk was added to enhance the messaging for potential impacts. ....Northern Rockies... The same anomalous upper low closing over the Pacific Northwest will help deepen a surface low over WY through Saturday. Intense downstream southerly flow over the Plains will surge elevated PWs and instability northward, with backed flow invof the low pulling these thermodynamics NW into the Northern Rockies. This will result in the accompanying theta-e axis lifting NW into the region as a modest TROWAL, helping to enhance elevated instability which would otherwise be confined to the east across the High Plains. The easterly flow around this low pressure will favorably upslope into the terrain as well, providing additional ascent, which in the presence of a ribbon of PWs above 1 inch will result in expanding precipitation with rainfall rates likely eclipsing 0.5"/hr at times. Slow moving and repeating showers and thunderstorms, embedded within a broader stratiform rain shield, are expected, especially immediately adjacent to the higher terrain. This will produce an axis of 1-3" of rainfall which may result in runoff and isolated instances of flash flooding. Weiss Day 3 Valid 12Z Sat Jun 21 2025 - 12Z Sun Jun 22 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE NORTHERN GREAT LAKES, INTERIOR NORTHEAST, AND NORTHERN ROCKIES... ....Northern Great Lakes through Northern New England... A stationary front draped from ND through Michigan will transition to a warm front and lift northeast on Saturday. This evolution will be driven by an expanding mid-level ridge centered over the TN VLY, which will result in rising heights and developing SW flow from the Plains into the Great Lakes. This synoptic change will push the boundary northeast as a warm front, along which a shortwave ejecting from the amplifying closed low in the west will track east from Minnesota to Vermont in the pinched and progressive flow. This overlap of ascent will drive increasing convection which will likely organize into clusters due to 30-40 kts of bulk shear, and then track progressively around the amplifying ridge. Storm motions will be quick to the east, but favorable thermodynamics to support heavy rain rates of 1-2"/hr could result in isolated instances of flash flooding, primarily where any repeating convection occurs. ....Northern Rockies...=20 The closed and anomalously strong 500mb low across the interior=20 Pacific Northwest will close off near southern Alberta, at least=20 briefly, before opening and beginning to pivot the entire trough=20 eastward by Sunday morning. The surface low pressure over WY on=20 Saturday will respond by lifting northeast beneath the core of this mid-level low, resulting in an axis of accelerated low-level flow=20 to draw moisture and instability northwest into the Northern=20 Rockies. The impressive deep layer ascent through height falls=20 combined with upslope ascent on easterly flow will result in=20 widespread moderate to heavy rain, with embedded convection=20 producing locally 0.5"/hr rainfall rates, especially beneath the=20 elevated instability/TROWAL pivoting southward. With=20 repeating/training cells with moderate to heavy rain moving across=20 areas just east of the higher terrain expected, this could result=20 in axes of as much as 3" of rain as forecast by 10-20%=20 probabilities from the ECENS and GEFS. The inherited MRGL risk=20 remains in place, with only minor adjustments tailored for updated=20 guidance. Weiss Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7WGfnYT1VACouYRC4sIgniElYyS8eplyy92zYLU1rtAQ= B7myvKemjvv3MuhnQs0IJccAtVESfnLj2tmAEriAI2T7_JQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7WGfnYT1VACouYRC4sIgniElYyS8eplyy92zYLU1rtAQ= B7myvKemjvv3MuhnQs0IJccAtVESfnLj2tmAEriAp-A6xmA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7WGfnYT1VACouYRC4sIgniElYyS8eplyy92zYLU1rtAQ= B7myvKemjvv3MuhnQs0IJccAtVESfnLj2tmAEriAUJhED-8$=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) .