Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Aug 21 2025 01:03:35 FOUS30 KWBC 210102 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 902 PM EDT Wed Aug 20 2025 Day 1 Valid 01Z Thu Aug 21 2025 - 12Z Thu Aug 21 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL IN EFFECT FROM THE ARKLATEX/EASTERN TEXAS TO THE CENTRAL APPALACHIANS AND ACROSS=20 PORTIONS OF NEW MEXICO AND ARIZONA... 01Z Update... Removed portions of the previous Marginal Risk area. This includes much of Texas, where precipitation has ended, and the Northeast,=20 where a stable airmass is keeping rainfall rates under 0.5 in/hr in most locations.=20 A Marginal Risk was maintained from the ArkLaTex and parts of southeastern Texas, the lower Mississippi and Tennessee valleys, northeastward into the central Appalachians. Moisture and=20 instability remain sufficient for storms to continue along a cold=20 front extending across the region, with weak steering flow=20 supporting slow-moving cells. The near-term threat for locally=20 heavy rainfall resulting in isolated runoff concerns appears=20 greatest from southern Arkansas to southeastern Louisiana, and=20=20 eastward into northern and central Mississippi and Alabama. PWs are at or above 2 inches, with enough instability to support rates of=20 1-2 in/hr in spots. The latest mesoanalysis does indicate that=20 instability is waning and storms are expected to diminish over the=20 next few hours. Similarly, a Marginal Risk was maintained across parts of West Texas into New Mexico and Arizona, where isolated to scattered storms continue, but are expected to wane with the loss of daytime heating. Pereira Day 2 Valid 12Z Thu Aug 21 2025 - 12Z Fri Aug 22 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHEAST, SOUTH-CENTRAL TEXAS, SOUTHERN APPALACHIANS, EASTERN NORTH CAROLINA, UPPER MIDWEST, AND SOUTHWEST U.S... ....21Z Outlook Update... The overall forecast is on track. Small expansions were made to expand Marginal risk into more of southern Minnesota, far western Wisconsin, and far northwestern Iowa. Height falls associated with a mid-level wave over southern Canadian provinces will overspread=20 this region late in the forecast period (toward early Friday=20 morning), assisting in isolated to scattered thunderstorm=20 development. These storms should move over areas of Minnesota and=20 Wisconsin that have been wet over the past couple weeks, supporting isolated instances of excessive runoff - especially with any=20 training convection that may materialize. Elsewhere, deep convection is still expected to prompt isolated flash flooding across the Southwest and from Texas eastward through the southern/central Appalachians. See the previous discussion below for more details. Cook ....Previous Discussion... ....Southwest... Elevated monsoonal moisture lingering under the western ridge will lead to yet another afternoon of scattered convective development across much of the desert Southwest and accompanying terrain. PWATs running slightly above normal across the region will provide just enough deep layer moisture to couple with relevant diurnal buoyancy to account for periods of heavy rainfall in some of the stronger convective cores that materialize Thursday afternoon into the evening. Totals generally will fall <1" over the Southwest when looking at areal averages, however some 1-2" totals are certainly plausible in the setup with the best threat for impacts likely in those slot canyon areas, drainage across complex terrain into adjacent valleys, and remnant burn scar locations. The previous MRGL forecast was generally maintained with only minor adjustments to the edges of the risk. ....Texas through the Southeast and Southern Appalachians... Our slowly progressing front will continue to wander further south with yet another day of convective firing near and along the front as it migrates through south-central TX and towards the Gulf coast. The front will arc back into the Southern Appalachians where scattered thunderstorm activity is likely to occur for at least one more day in the terrain with the northern extent extending up into Southern WV and Southwest VA. The signal is still scattered in terms of convective coverage and flash flood prospects leaning towards isolated when assessing the HREF probs for hourly rates and FFG exceedance probs in the 12-00z window Thu/Fri. Convection will likely dissipate slowly after sunset with the loss of diurnal heating and surface decoupling putting a cap on the threat. Totals between 3-5" are most common among some of the stronger cores in guidance, a target that characteristically covers a MRGL risk with some targeted SLGT risk opportunities if the signal becomes more favorable within a favored area for flash flooding (IE: urbanized zones and terrain). For now, the MRGL risk from previous forecast was generally maintained with minor adjustments based on 00z QPF footprint. ....Northern Plains and Upper Midwest... An energetic upper-level shortwave is forecast to move east across the northern Plains and Upper Midwest Thursday afternoon through the overnight. A strong cold front trailing a strengthening surface low over southern Canada will aid in surface based forcing to couple with the improving mid and upper dynamics as the trough migrates along the Canadian border. Sufficient buoyancy and moisture anomalies of 1-2 deviations above normal will exist along and ahead of the front coinciding with a strong, yet progressive line of thunderstorms that will be capable of isolated flash flood prospects thanks to hourly rates between 1-2"/hr expected. Guidance remains steadfast in its interpretation of the setup lending confidence in maintaining continuity from previous forecast with only small adjustments on the western flank of the risk area to account for timing adjustments. Kleebauer Day 3 Valid 12Z Fri Aug 22 2025 - 12Z Sat Aug 23 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHWEST, SOUTHEAST, AND CENTRAL ROCKIES ... ....21Z Outlook Update... Minor spatial adjustments were made to the outlook, particularly to add southeast Texas and southwest Louisiana to the Marginal Risk. Models depict a convective signal in those areas, but vary with respect to specific timing. Nevertheless, a weak front and favorable kinematics for slow-moving convection should encourage a few spots of heavy rainfall near the Houston and Lake Charles areas (and vicinity), prompting low-end flash flood probabilities. The Marginal Risk was also expanded across the Southwest. A favorable pattern for clusters of thunderstorms to evolve from higher terrain westward toward the Lower Colorado River Valley exists, and both convective coverage and instability/moisture profiles suggest at least isolated flash flood potential across the area. A Slight Risk upgrade was considered, and may be needed in later outlook cycles. See the previous discussion below for more details. Cook ....Previous Discussion... ....Southwest to Central Rockies... Western ridge will shift to the west by the end of the week with the monsoonal moisture pattern also migrating westward to include more of the Great Basin and areas around the Colorado river basin. PWAT anomalies ~ +1 deviation and strong surface destabilization will yield a fairly widespread convective regime across the Southwestern part of the CONUS with heavy rain signatures littered among the activity. Rates up to 1-1.5"/hr in the strongest cores will likely promote flash flood concerns in those more prone dry washes, slot canyons, and burn scars residing over the region. The threat extends into the Central Rockies as shortwaves round the eastern flank of the ridge and aid in convective initiation over the terrain of Central CO. Signal currently runs within the MRGL threshold, but will assess for prospects of targeted upgrades if any specific area stands out as we get closer to the event with CAMs input. ....Southeast... The slow-moving front that will plague the South through the week will finally hit a road block once it reaches the Gulf coast becoming a focal quasi-stationary front by Friday morning. Hurricane Erin will be exiting off into the northwest Atlantic by this point, but the prominent in-flow trailing the storm will continue across the Western Atlantic leading to pooling moisture situated along the confines of the boundary. PWATs will surge with areal average 2.25-2.5" encompassing a majority of the Gulf coast from LA over towards the Southeast coastal plain between Wilmington, NC down into northern FL. The combination of elevated deep layer moisture, instability, and enhanced surface convergence along the front will yield some fairly prolific rainfall makers in any convective scheme during the period. The best signature for heavy rain is more likely to occur over the SC/GA coast where there's a stronger sfc-850mb convergence signature near the coast leading to an area of ~2" of precip currently forecast in the area. This is even prior to hi-res model forecasts, a testament to a growing signature for heavy rain prospects that could very well necessitate an upgrade in future forecasts, especially as we add more CAMs input. For now, a broad MRGL exists across a good portion of the Southeastern U.S. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-avA1ytoYkFATol_6UDlG4bZXIFaZbO_51WeZFKjuYXi= nceny5HoD5LgfRKOAe1KeG9SrEVAqIFXl0O39XIPI8wG3eo$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-avA1ytoYkFATol_6UDlG4bZXIFaZbO_51WeZFKjuYXi= nceny5HoD5LgfRKOAe1KeG9SrEVAqIFXl0O39XIPx7FHm98$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-avA1ytoYkFATol_6UDlG4bZXIFaZbO_51WeZFKjuYXi= nceny5HoD5LgfRKOAe1KeG9SrEVAqIFXl0O39XIPOPdK3F0$=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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