Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu May 15 2025 15:52:42 FOUS30 KWBC 151552 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1152 AM EDT Thu May 15 2025 Day 1 Valid 16Z Thu May 15 2025 - 12Z Fri May 16 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF=20 THE NORTHERN PLAINS, CENTRAL MIDWEST, OHIO VALLEY AND MID-=20 ATLANTIC... ....Northern Plains... 16Z Update: The previous forecast remains on track with the lack of sufficient instability likely to keep much of the Dakotas within a broad moderate stratiform precip shield leading to lower probability of flash flooding despite forecast totals. A tongue of higher theta_E within the eastern flank of the low will allow for a strong cell or two to pivot up from Eastern SD into Southeast ND with a small sector potentially having the "best" threat for flash flood prospects. This will be the area of interest the next 6-12 hrs before conditions settle to majority stratiform, likely leading to an end of any risk for the back half of the forecast. Kleebauer ...Previous Discussion.. A well-developed low will track north into the eastern Dakotas into this afternoon, with a slow and chaotic track through tonight. This track is supported by a nearly stationary upper level low, with a potent shortwave trough rounding its southern periphery, thus increasing the divergence aloft. While the low itself will be able to tap into some of the Gulf moisture across much of the eastern half of the country, this is not expected to result in significant flash flooding. For North Dakota, any associated instability is likely to stay east of the low center. Meanwhile, the cold conveyor belt feeding the comma-head region of the surface low will be unable to also advect instability into the region. Thus, while the 24 hour rain forecast remains consistent with continuity with a forecast for 2-3 inches of rain across Western North Dakota=20 through 12Z Friday, the lack of instability will keep that rainfall as stratiform, and therefore unlikely to cause flash flooding. ....Mid-Atlantic... 16Z Update: Scattered convective development through the afternoon and evening will generate some low-end flash flood concerns within the region. The areas of interest are likely the more sensitive App/Blue Ridge areas where FFG's are incredible low (<0.5"/hr) where any convection could cause problems within a remnant, moist airmass, as well as the corridor between Philadelphia to Newark=20 where a weak surface reflection along the Jersey coast will=20 provide a narrow axis of elevated theta_E on the western flank of=20 the circulation. Coverage of convection will be widely scattered in nature and not nearly as prevalent to the prior days of impact, so not anticipated widespread concerns this go-around. Across Western PA, much of the threat will be coincident with the advancement of=20 the warm front into the region promoting a larger convective threat compared to areas further east. 12z HREF neighborhood probs for=20 >1" and >2" are pretty robust in and around the Pittsburgh metro=20 out to the western slopes of the Laurel Highlands. The progressive=20 nature of the activity should curb a higher threat, but rates=20 between 1-2"/hr at peak intensity will be enough to cause some=20 localized flash flood concerns. The MRGL was relatively unchanged=20 within any areas in the Mid Atlantic with a small expansion north=20 towards far Southwest NY state given QPF trends.=20 Kleebauer ...Previous Discussion.. Multiple rounds of rainfall are likely to impact portions of the Mid-Atlantic through this Day 1 period. This will be on the heels of several days of more widespread heavy rain that has occurred over the past few days. While today should be a wind down of the coverage of heavy rain, it's still possible that any storms that form today into tonight will merge together, train, or remain so slow moving as to cause localized flash flooding. As an occluded front associated with the low over the Northern Plains approaches, it will turn the predominant storm track towards the east and northeast. This will be a significant change in the storm motions from previous days, and one that should ultimately keep any flash flooding producing storms as few and far between. Since the soils are very moist and prone to flash flooding, the lessening coverage of storms over the area would still favor the continuation of the Marginal Risk for today. Wegman ....Midwest and Ohio Valley... Warm front progression across the eastern flank of the large cyclone over the Northern Plains will advance quickly to the northeast with a trailing cold front rapidly sweeping east in its wake. Warm sector development across the Central Midwest and Northern Ohio Valley will denote a large expanse of elevated=20 instability with 12z CAMs signaling MUCAPE between 2000-3000+ J/kg from Southern MI/WI to points south. This will allow for heavy=20 convective cores within the warm sector to materialize providing a=20 much greater threat for elevated rainfall rates between 1.5-2"/hr as the environment remains ripe with PWAT anomalies solidly +1 to +2 deviations according to NAEFS and global deterministic output. Neighborhood probs of exceeding 3hr FFG indices maintain a modest depiction between 15-30% for an area encompassing the Milwaukee/Chicago urban corridor through the Northern Ohio Valley with focus along I-90, including a bullseye ~40-45% prob situated between Cleveland and Fort Wayne. Considering the massive urban footprint within that region and relatively low FFGs, decided to extend the MRGL risk to cover for the low-end to potentially modest threat later this evening.=20 Kleebauer ....Low to Mid-Mississippi Valley... A weak southern stream perturbation will eject northeast out of the Southern Plains and into the Mississippi Valley by later this evening. Relatively unstable airmass situated across the ArkLaTex up through Western TN will allow for scattered thunderstorms to development and press northeast with the mean flow. 12z CAMs are about 50/50 on the threat for some heavier convection initiating over Arkansas and moving towards the Memphis metro with some of the output pretty bullish on the threat (2-3" locally). The=20 environment is favorable, but the prob fields are not too enthused due to the lack of agreement among the HREF members. The threat is non-zero, but falls below the necessary threshold for a MRGL. Plus, the main areas of concern are likely the metro areas of Little Rock to Memphis, so the coverage/footprint for flash flooding is small. Decided not to add a risk area, but wanted to make a note considering the threat is still plausible and will be monitored closely for any additions.=20 Kleebauer Day 2 Valid 12Z Fri May 16 2025 - 12Z Sat May 17 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE OHIO VALLEY... A stalled and shearing apart occluded front over the Ohio Valley early Friday morning will dissipate through the day as its associated rainfall ends. However, a reinforcing strong shortwave will round the southern periphery of the vertically stacked low, which will be pushing east from the Northern Plains and into the upper Great Lakes. This shortwave and associated push of cooler air in the form of a cold front will tap into the moisture and instability largely in place ahead/southeast of the cold front over the Ohio Valley, resulting in multiple areas of storms forming Friday afternoon from southern Illinois through West Virginia. While some training is possible as the storms are forming, they're likely to develop into a single line, where any training would be from pre-line convection that will likely be moving into and being absorbed by the line. The line of storms will continue south into the Mid-South through Friday night, albeit with lessening flash flooding potential as the line becomes the only area of storms around. As with areas further east, soil moisture levels all up and down the Ohio Valley are at or above normal for this time of year, and the presence of moisture to the order of 1.5 to 1.75 inches of PWAT will support strong thunderstorms capable of 2+ inch/hour rainfall rates. Given the sensitivities of the rivers and streams in the area, the combination of heavy rain and sensitive soils supported the continuance of the Slight Risk area, with some expansions included to account for more the suite of guidance. Wegman Day 3 Valid 12Z Sat May 17 2025 - 12Z Sun May 18 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM CENTRAL TEXAS THROUGH NORTHERN ALABAMA AS WELL AS FOR PORTIONS OF THE NORTHEAST... ....Texas to Alabama... Another round of afternoon and evening thunderstorms is expected from northeast Texas into northern Alabama Saturday and Saturday night. Continued influx of Gulf moisture and instability on southerly flow will clash with the cooler, dry air mass behind a cold frontal passage over much of the Midwest on Friday. The storms will form along this frontal interface. With that said, the forcing in the upper levels should be greatly reduced over this region as compared with previous days, as the upper forcing on Saturday will be associated with a fast moving and weaker shortwave. This should limit the coverage of storms. Once again however, recent heavy rains in this area continue to keep the soils wet, and therefore more prone to flash flooding. Should the forcing increase or amount of moisture off the Gulf increase compared to current forecasts, then it's possible a targeted Slight may eventually be needed, with the DFW Metroplex area currently appearing to be at greatest risk for an upgrade. ....Northeast... Periods of heavy rain will move across the Northeast Saturday afternoon ahead of a potent upper level low moving across the Great Lakes. The storms will occur ahead of the upper low. There is some uncertainty on storm coverage and intensity with moisture amounts being limited in this area, but it is made up for by wetter than average soils in this area of New England and New York. The inherited Marginal Risk was trimmed out of most of Maine, as soils there are drier than normal, which should allow any rainfall (which should be less than an inch) to be absorbed by the dry soils. The greatest flash flooding risk is likely in the Green and White Mountains where terrain will worsen any potential flash flooding. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5PgW0Pl8fIYw8z4BudR9FsTyMwEBizyaGQ_tJMPbvPaX= iJTsxDxqDLKl2vWnNGTgt-aQYlUf8jDQFck7FQTVgvsOCtY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5PgW0Pl8fIYw8z4BudR9FsTyMwEBizyaGQ_tJMPbvPaX= iJTsxDxqDLKl2vWnNGTgt-aQYlUf8jDQFck7FQTVfjY2qp8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5PgW0Pl8fIYw8z4BudR9FsTyMwEBizyaGQ_tJMPbvPaX= iJTsxDxqDLKl2vWnNGTgt-aQYlUf8jDQFck7FQTV8XQ3y8s$=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) .