Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Jun 19 2025 15:54:33 FOUS30 KWBC 191554 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1154 AM 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...=20 A potent shortwave will amplify as it tracks northeast from the=20 Great Lakes to along the northern New England/Canadian border,=20 potentially becoming negatively tilted by tonight. This will help=20 to deepen a surface low moving in tandem with its parent trough,=20 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=20 with heavy rain are likely along it. The downstream environment from this front is extremely favorable=20 for thunderstorms containing heavy rain. PWs will surge northward on=20 SW 850mb flow of 30 kts, reaching 1.75 to 2 inches, or higher, from=20 the Mid-Atlantic into New England, which will be above daily records=20 according to the SPC sounding climatology. This intense PW plume=20 will combine with a narrow corridor of MUCAPE progged to exceed 2000=20 J/kg from NC through ME, to support heavy rainfall within=20 convection. Although the CAMs are generally modest with their=20 coverage today, expect scattered to numerous thunderstorms to=20 develop during peak heating, with modest bulk shear of 25-30 kts=20 supporting at least isolated clusters in an otherwise pulse-type=20 environment. Due to the impressive PWs and tall skinny CAPE=20 profiles, max hourly rainfall could reach 2-3" according to the REFS=20 and HREF guidance, which would be sufficient to cause runoff and=20 isolated flash flooding.=20 Although the flash flood risk today is generally isolated due to=20 fast storm motions and minimal training, there is a slightly higher=20 risk for impacts across the Central Appalachians, generally in WV=20 and PA, where antecedent conditions (7-day rainfall more than 400%=20 of normal according to AHPS) have resulted in severely compromised=20 FFG as low as 1"/3hrs. Despite progressive cell motion, HREF=20 exceedance probabilities rise to 15-25% in this area, and it is=20 possible that even fast-moving heavy rain producers could cause=20 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=20 front, suggesting an enhanced training risk. However, instability=20 and PWs are more modest, so rain intensity will be decreased as=20 forecast by lower 1"/hr rain rate probabilities from the HREF.=20 Although total rainfall could reach 3" in some areas from northern=20 VT through northern ME, the excessive rain risk remains in the MRGL=20 category. ....Southeast...=20 The same front that will race across the Mid-Atlantic and=20 Northeast, will trail back into the Southeast today. Although this=20 front is likely to decay by this evening, the residual convergence=20 zone will linger as it becomes embedded within more zonal flow=20 south of the primary trough. Convection ongoing early this morning=20 from AR to AL will weaken, but the accompanying outflow may merge=20 with the weakening front to provide additional impetus for=20 convective development thanks to robust thermodynamics=20 characterized by PWs of around 1.75 inches and MUCAPE of 2000-3000=20 J/kg. This will create scattered thunderstorms this aftn/eve with=20 rainfall rates of more than 2"/hr. Although there is some=20 uncertainty into how the weakening front will evolve (some CAMs=20 suggest the outflow will surge all the way to the Gulf Coast), any=20 storms that do form have the potential to train as Corfidi vectors=20 collapse to around 5 kts so storms could move west to east and=20 repeat through the evening. Both the HREF and REFS guidance=20 indicate a 30-40% chance of 3" of rain, which could produce=20 isolated flash flooding today. ....Florida Peninsula...=20 Bermuda-type ridge off the Southeast Coast will produce weak=20 easterly flow over the FL Peninsula today. As afternoon heating=20 commences and instability climbs above 2000 J/kg, scattered=20 afternoon thunderstorms will develop. Initially, these will occur=20 on the sea breeze, but as the Atlantic coast sea breeze pushes=20 westward and a weak jet streak develops downstream of a longwave=20 trough dropping across the Southeast, additional development is=20 likely along this boundary and due to storm mergers/boundary=20 interactions. This will likely be most prevalent near the SW coast=20 of FL where the Gulf Coast sea breeze will be pinned, fueling=20 multiple rounds of slow moving thunderstorms. With PWs progged=20 around 1.75", rainfall rates of 2-3"/hr are expected, producing=20 hourly rainfall as much as 3" according to both the RRFS and HRRR=20 models, and total rainfall over 5" possible (20-40%) in localized=20 areas. Should this rain occur atop more urban areas, especially=20 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 MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE UPPER MIDWEST AND PORTIONS OF THE NORTHERN ROCKIES... ....Upper Midwest... Amplified mid and upper pattern across the Western CONUS into the Northern Plains will lead to a stout area of low pressure developing across the Western High Plains, moving east through the Upper Midwest as we move through Friday into Saturday morning. South of the area will see a strong ridge pattern unfold creating an enhanced thermal gradient pattern with a budding warm sector and ample surface based instability with most guidance inferring 3000-5000 J/kg of SBCAPE across the area between the eastern Dakotas over into the Upper Midwest by Friday evening. Split jet max ejecting northeast out of the Plains will lead to increasing ascent within the diffluent area of the jet and along the eastern flank of the intensifying surface low. Thunderstorms will blossom across the Eastern Dakotas and quickly advance eastward with upscale growth thanks to the speed shear presence and strong thermal gradient to maintain general MCS structure. Inflow around the eastern flank of the low will aid in the overall maintenance of the expected complex with smaller convective cells forming out ahead and being integrated into the complex as it maneuvers through MN. Areal QPF within the latest blend and ensemble bias corrected forecast indicates a general 0.5-1" coverage with maxima likely to locally exceed 2" over a short period of time within a zone extending through Central MN into the Arrowhead. Will monitor for backbuilding within the flanking portion of any complex which could lead to local enhancement of precip and enhance flash flood prospects in pockets where the MCS crosses. For now, the progression of the MCS would favor limited training potential and more impacts based on enhanced rates given the favorable upper level dynamics, instability, and rich moisture pull within the eastern side of the surface low. The previous MRGL risk was maintained with only some minor adjustments based on QPF distribution this forecast. ....Northern Rockies... Guidance remains pretty locked in for the anticipated evolution of the next disturbance entering the Pacific Northwest with convective impacts forecast for the Northern Rockies and adjacent areas to the east. Extremely anomalous mid- level low will dig into the Pacific Northwest Friday, pushing 500mb heights to below the 00Z minimum of the climatological database from NAEFS. Where intense height falls/PVA combine with the increasing diffluence along the LFQ of a Pacific jet streak, surface pressure falls will result, leading to a deepening low pressure moving from ID into MT. The increasingly backed low-level flow ahead of this system will cause 850mb winds to surge out of the E at 15-25 kts, lifting favorably to upslope into the terrain while additionally transporting a ribbon of PWs above 1 inch into the High Plains and banking into the eastern side of the Rockies. This will cause expanding precipitation, with embedded rainfall rates above 0.5"/hr where a narrow corridor of instability exists over northern MT. Locally heavy rainfall of 1-2", with locally higher amounts, will result, leading to the potential for isolated flash flooding atop FFG that is only 1-1.5"/3hrs. The previous MRGL risk was maintained given the expected evolution with general run to run continuity among deterministic and global ensemble means. Weiss/Kleebauer 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 MONTANA... ....Northern Great Lakes into the Northeast... Shortwave progression out of the Upper Midwest will ride along the northern periphery of our expansive ridge as we move through Saturday. Ensemble consensus pins the energy over the Northern Great Lakes with remnant thunderstorms propagating through the Michigan U.P. into adjacent Ontario with a break in convective activity in the CONUS before the disturbance re-emerges over Upstate NY and Northern New England. Some discrepancy on the northern extent of the shortwave progression as the ECMWF bias is further north with sights on Western ME and far northern VT/NH as the impact points. Other guidance is suggesting a bit further south into Upstate NY through Central New England with heavier precip threats more likely into the central and northern Hudson Valley. This is a period to monitor closely as flash flood impacts will be tied to the progression of a fairly robust complex as it exits out of Canada. QPF distribution is skewed towards heavier precip located within North Country of NY state with some secondary maxima over Northern New England down through the Adirondacks. Heavy rain over the U.P. may be progressive enough to curb significant impacts, but the eastern portion of the U.P. is the area of concern with the vacating complex as it moves quickly through the region. The MRGL risk inherited was adjusted slightly to account for the uncertainty in the exact progression of the complex. ....Northern Montana... Our potent mid-upper trough will meander over the Pacific Northwest with a strong shortwave migrating out of the base of the trough, eventually closing off right along the International Border to the northeast of Glacier National. Surface low development in-of the closed upper reflection should provide a narrow axis of convergence just outside the eastern slopes of the Rockies generating a period of heavy rainfall in the adjacent valleys to the east of the mountains. Height falls across the region will be significant creating favorable mid-level lapse rates within any convective scheme along with ample moisture advection east of the terrain. The threat is very much tied to the northern tier of MT where the convergence pattern is greatest. The threat is less pronounced further south and east comparatively, however some inference of a few mid-level perturbations ejecting downstream over Northern WY into Southern MT could provide a secondary area of focus for heavy rainfall. For now, the main threat within the QPF distribution is for locally heavy rainfall in that small zone of convergence under the surface low formation near the border. Local 1-2" is forecast in that area with some ensemble QPF output closer to 3" not too far away into Canada. Targeted MRGL risk for that area east of the terrain remains with little change from previous forecast. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5c1GRLA-w9n_TvgubvDdUZj-QSVFLKb0mGno3Yddq2Ev= DZFGYx_WFiKv7STtjSh1o2ATxM4g__hFMFMQI0-fSYgBALc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5c1GRLA-w9n_TvgubvDdUZj-QSVFLKb0mGno3Yddq2Ev= DZFGYx_WFiKv7STtjSh1o2ATxM4g__hFMFMQI0-fy78o16Y$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5c1GRLA-w9n_TvgubvDdUZj-QSVFLKb0mGno3Yddq2Ev= DZFGYx_WFiKv7STtjSh1o2ATxM4g__hFMFMQI0-fLhf8LOk$=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) .