Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Aug 01 2025 00:38:13 FOUS30 KWBC 010037 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 837 PM EDT Thu Jul 31 2025 Day 1 Valid 01Z Fri Aug 01 2025 - 12Z Fri Aug 01 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS=20 OF THE EAST, SOUTH, SOUTHERN PLAINS, HIGH PLAINS, & NORTHERN=20 INTERMOUNTAIN WEST... ....Mid-Atlantic into New England... The last of the organized convection with heavy rainfall is in the process of fading across portions of the Eastern Shore of the Chesapeake and portions of southern Long Island NY. Across=20 portions of southern New England and the coastal Mid-Atlantic=20 states, a non-zero chance of heavy rainfall exists for another few hours/into tonight per the 18z HREF and 12z REFS probabilities of=20 0.5"+, while areas near WV hold on to a minimal heavy rainfall risk overnight as the front drops southward. Downgraded the existing=20 risk areas and threat level to a Marginal to accommodate radar=20 reflectivity and 18z HREF/12z REFS trends. ....Gulf Coast & Southeast...=20 Existing storms are expected to fade over the next several hours until CIN sets in, so the Marginal Risk for excessive rainfall will be maintained through this evening. ....Portions of Oklahoma... An area of high moisture drifts through portions of OK overnight. The 18z HREF/12z REFS shows an increasing chance of heavy rainfall over the eastern half of the state, but without the benefit of a=20 nocturnal low-level jet. However, increasing effective bulk shear=20 is moving into the region from the northwest as a shortwave noted=20 on water vapor imagery across portions of the Central Plains moves=20 by. Hourly rain amounts to 2.5" with local amounts to 4" are=20 possible in this area. While the 12z REFS shows a broad enough=20 signal to potentially support a Slight Risk in this region, the 18z HREF implies that such a risk exists at one specific spot. Kept=20 the risk as Marginal for the time being per coordination with the=20 TSA/Tulsa OK and OUN/Norman OK forecast offices. Localized Slight=20 Risk impacts can't be ruled out, depending upon how the convective=20 evolution occurs overnight. =20 ....Rockies into the Plains... Scattered thunderstorms expected to continue from eastern=20 Arizona/New Mexico northward to Montana. While in most area In=20 general, the storms will likely lack organization for a more=20 elevated threat for flash flooding. In most spots, the threat should continue through 06z, though in the Northern Plains, the heavy rain threat could hang on into the early morning hours as a low-level jet increases available moisture and maintains instability overnight. Roth Day 2 Valid 12Z Fri Aug 01 2025 - 12Z Sat Aug 02 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CAROLINAS AND NORTH/NORTHEAST TEXAS... ....Southeast and Mid-Atlantic... During this period a potent cold front will be dropping south through the region in response to a very strong surface high pressure (above the 99.5th climatological percentile per the NAEFS) over the Great Lakes/Midwest. This will allow for moisture to pool along the front and thunderstorms to develop across the Carolinas on Friday and Friday night. Meanwhile both terrain and coastal influence are also likely, which may lead to slower storm motions and heavier rainfall amounts. This combination will also result in good convective coverage, which will ultimately likely lead to a decent degree of cell merger activity. Much of the area will have PW values nearing 2.5 inches along with abundant instability, resulting in very heavy rainfall rates over 2"/hr in this thermodynamic environment. Given the likelihood of cell mergers and westerly mean layer flow nearly parallel to the front, this should drive an isolated to scattered flash flood risk. 12z HREF guidance depicted high neighborhood probabilities (60-90%) for at least 3" in 6-hrs along the coastal Carolinas, where FFG is also highest. Additionally, along the complex terrain of the western Carolinas 12z HREF neighborhood probabilities for exceeding 3" in 6-hrs are 40-50%. Only minor adjustment to the previous outlook was to reduce the northern portion of the SLGT and MRGL based on southern trends in the progression of the cold front. Urban areas and the complex terrain of the southern Appalachians remain most at risk to scattered instances of flash flooding, with the remainder of the Carolinas flat and sandy soils capable of soaking in most of the intense rainfall. The frontal boundary will also linger across the remainder of the Southeast and into the Lower MS Valley, where additional instances of flash flooding are possible given weak steering flow and 2-2.25" PWs. However, most storms should become outflow dominant and be short-lived after a brief period of intense rainfall and rates up to 3"/hr. ....Southern Plains... The cold/stationary front continues to stretch westward towards the southern Plains along with PW values near 2" Friday into Friday night. While most activity should remain pulse-like, a surface wave and meandering MCV could trigger locally heavy rain in northern/northeast TX Friday night. The location remains somewhat uncertain, but this area is highlighted by the 12z HREF as having 6-hr probabilities for >5" at 30-40%. Additionally, the RGEM and ECMWF included elevated QPF values above 2-3" with the 12z runs in a very similar location. This potential collocated with the Dallas- Fort Worth metro led to enough confidence for a targeted Slight Risk. Expect refinements in future outlooks as more CAM runs become available and/or the MCV at play becomes more trackable. ....Rockies and Plains... Scattered convection is expected to span from Montana to New Mexico into Friday. The most favorable environment for heavy rainfall will likely setup from Montana into the Dakotas, which is where mid level troughing will be paired with the highest PW anomalies in the CONUS...above the 90th percentile climatological early August peak values. Combine this with CAPE forecast around 2000 J/kg and the environment appears conducive to areas of localized flash flooding. Hourly rainfall rates up to 1.25-1.5 inches/hour possible over the Northern Rockies/Montana and up to 2 inches/hour across the Dakotas. There is also the potential for an MCV across the Dakotas to provide a focus for locally intense rainfall, but the location of this heavy rain remains too uncertain for an upgrade to a Slight Risk. This potential will be evaluated in future outlooks. Snell Day 3 Valid 12Z Sat Aug 02 2025 - 12Z Sun Aug 03 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHEAST... ....South and Southeast... The strong front mentioned will continue pressing southward, draped from the Southeast westward across the Deep South, Lower Mississippi Valley and over Texas. High moisture content and instability along and ahead of the front interacting with the right entrance region of the upper jet will promote heavy to excessive rainfall in thunderstorms that could cause localized flash flooding. Very heavy rainfall rates are expected and should drive an isolated to scattered flash flood risk. A Slight Risk remains in effect for portions of the Lowcountry South Carolina, southern Georgia and far northern/northeast Florida. The front stretching west across the Gulf Coast states in a moist and unstable environment could promote scattered diurnal thunderstorms into Saturday thus maintaining the Marginal Risk. ....Rockies and Plains... Shortwaves rounding the eastern edge of the upper ridge in the western U.S. is forecast to interact with the same front that bends back across the Great Plains, where rounds of rain and thunderstorms are likely with possibly multiple MCSs. High rain rates should be supported at times with the above normal moisture and instability with some areas possibly seeing repeated rounds of storms as the front begins to lift as a warm front. Location in heaviest rainfall remains too uncertain for an upgrade to a Slight Risk at the moment and MCS activity may bode too progressive for widespread flash flooding, thus the region is covered by a Marginal Risk. Snell/Campbell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9a0kGCs-FzmVGnSU2J7PrE3lMzq0g8XiHFSAxS5PUPUs= mZNs8HeTa8iLn5sZC2qVcRC2nVBZEEBoJVd9dlvQokmBf3E$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9a0kGCs-FzmVGnSU2J7PrE3lMzq0g8XiHFSAxS5PUPUs= mZNs8HeTa8iLn5sZC2qVcRC2nVBZEEBoJVd9dlvQX7nGG6A$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9a0kGCs-FzmVGnSU2J7PrE3lMzq0g8XiHFSAxS5PUPUs= mZNs8HeTa8iLn5sZC2qVcRC2nVBZEEBoJVd9dlvQ2n_-r4Q$=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. 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