Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri May 16 2025 00:51:29 FOUS30 KWBC 160049 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 849 PM EDT Thu May 15 2025 Day 1 Valid 01Z Fri May 16 2025 - 12Z Fri May 16 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE NORTHERN PLAINS AND PARTS OF THE CENTRAL MIDWEST/OHIO=20 VALLEY... Maintained the Marginal risk over portions of the Midwest/southern Great Lakes region along and ahead of a QLCS from the Great Lakes region overnight. The progressive nature of the system should=20 mitigate some of the flash flooding risk caused by locally intense=20 rainfall rates and reflects only a minor change to the on-going=20 Marginal risk area. Farther to the east...removed the Marginal risk area from eastern Pennsylvania southward into the Carolinas where=20 the airmass has less instability and most of the high=20 resolution/convective allowing guidance has little convection=20 moving into the area prior to 12Z on Friday. Across the Northern Plains...guidance continues to focus the risk of locally heavy rainfall near the upper level low meandering eastward overnight with 1 to 1.5 inch amounts possible. Given some overlap with areas where flash flood guidance has been lowered by recent moderate to heavy rainfall...saw little reason to deviate much from the previous outlook. Bann 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... 20Z Update: A minor expansion of the SLGT risk was added to encompass much of Eastern TN with emphasis on the I-75 area and adjacent Smokey Mountains. There's a solid consensus on how the nocturnal QLCS will progress with the line likely to move ESE out of KY the centroid of the complex likely to follow a well-defined theta_E gradient positioned from Western KY down through Eastern TN. 12z HREF is now depicting the full time frame for the D2 and pretty much all CAMs has the heaviest axis of precip bisecting the area over Southern KY down into the northern tier of TN with a western inflection near Paducha and eastern inflection towards the I-75 corridor between Knoxville/Chattanooga. Areal average QPF is now upwards of 2-3" with local maxima as high as 5", a range that will cause flash flood issues within a large area of above normal soil moisture anomalies as indicated via the latest NASA SPoRT output. A high-end SLGT is now forecast within the zone above with a broad SLGT risk still encompassing the rest of KY, the northern two-thirds of TN, and the Ohio River basin covering Southern IL/IN/OH into WV. A targeted upgrade is not out of the question in the coming updates, but the progressive nature of the precip maybe the biggest saving grace for the setup. Scatterd bouts of thunderstorms across PA and Ohio will offer some isolated threats of flash flooding due to compromised soils thanks to the preceding convective impacts the days prior. There's some=20 potential for an initial complex to move out of the Tennessee=20 Valley into the Central Mid Atlantic on Friday evening, however=20 there's only a few pieces of guidance offering the potential, so=20 have foregone a small MRGL expansion into the region, but will be=20 something to monitor with trends in the CAMs in future updates. Kleebauer ...Previous Discussion.. 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... 20Z Update: Little changes were necessary for either risk area for the period. Deterministic variance in QPF maxima and orientation of the overall precip footprint limited the ability for any potential upgrade across the risk area over the Southern Plains through the Mississippi Valley. There's a chance there is some expansion to the north pending the evolution of a smaller mid-level vorticity maxima ejecting eastward out of the Southern Rockies. Guidance is split on the handling of the shortwave energy, so there could be some additional MRGL risk coverage to the north near the OK/KS/MO/AR intersection. With more time for resolution, decided to maintain the nil in the location, but will monitor over subsequent updates. The MRGL across the Northeast U.S. remains within the lower end of the risk threshold with the best chances likely within the valleys tucked into the Green and White Mountains. Scattered convective coverage will allow for localized QPF maxima between 1-2" with potential for rates to reach ~1"/hr at peak intensity. The environment remains moist with PWAT anomalies between +1 to +2 deviations, enough to typically concern flash flood prospects within the complex terrain of Northern New England. The inherited MRGL was maintained with no real change. Kleebauer ...Previous Discussions.. ....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!_iX-E3Wwcj-wLWXt7P76mtIQZkBuX3w2DtalgvHSxGti= QxNtvMkyAI12A_uOEk1ILpzSpOTJ678kIYMnV1eqOSt0Q5w$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_iX-E3Wwcj-wLWXt7P76mtIQZkBuX3w2DtalgvHSxGti= QxNtvMkyAI12A_uOEk1ILpzSpOTJ678kIYMnV1eqSjJtgvw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_iX-E3Wwcj-wLWXt7P76mtIQZkBuX3w2DtalgvHSxGti= QxNtvMkyAI12A_uOEk1ILpzSpOTJ678kIYMnV1eq_1L-UHU$=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) .