Subj : HVYRAIN: Excess Rain D1 To : All From : Mike Powell Date : Thu Aug 21 2025 08:27:29 FOUS30 KWBC 210719 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 319 AM EDT Thu Aug 21 2025 Day 1 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... ....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. 00z HREF neighborhood probs for >1" are running between 30-45% across the central and eastern Mogollon Rim, and across the southern extent of the Sangre de Cristos in NM. These are the areas with the best flash flood potential given the anticipated convective pattern and topographic challenges that accompany these locales. The previous MRGL forecast was maintained with little to no adjustments. ....Texas through the Southeast and Southern Appalachians... Our slowly progressing front is currently analyzed across the Lower Mississippi Valley back into the southern Ohio Valley with a general south to southeast progression over the past 24 hrs. Cold 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 with an elongated surface trough running parallel to the immediate Gulf coast from TX to FL/GA. 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 to perhaps scattered when assessing the HREF probs for hourly rates and FFG exceedance probs in the 12-00z window Thu/Fri. The strongest signature for heavy rain concerns lies within the ridges and eastern slopes of the Southern Appalachians across western NC down into the far western SC escarpment. 3hr and 6hr FFG exceedance probabilities between 35-50% exist over the above area with the time frame of interest falling in that afternoon and early evening window where diurnal instability maximum and convective temp breach overlap. 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, aligning well with the modest >3" probs and low-end >5" probs within the latest HREF iteration. Despite the totals in question, the flash flood threat will likely be more comprised of 3 and 6hr FFG exceedance compared to lone hourly rates, however some instantaneous rates upwards of 2-3"/hr in the terrain could spur some flash flood hazards, especially as storm motions remain on the weaker side and generally parallel to the topography. The previous MRGL risk was maintained given the signatures across the South and Appalachian front. Higher FFG's across the above regions were the main deterrent for a higher risk, but will be monitoring the threat closely to see if a targeted upgrade is necessary. ....Northern Plains and Upper Midwest... An energetic upper-level shortwave is forecast to move east across the northern Plains and Upper Midwest later this 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. Trends of an areal maximum across eastern SD have gained traction within the latest CAMs and matches the global ensemble means and EC AIFS Ensemble probs being highest in-of that region. This correlates with the stronger mid-level vorticity advection entering the area to couple with the best low-level instability axis along the southern edge of the front. The previous MRGL risk was generally maintained with some minor trimming on the eastern edge to account for lower potential downstream given timing of convective advancement among the hi-res. Kleebauer $$ --- SBBSecho 3.28-Linux * Origin: ILink: CCO - capitolcityonline.net (454:3/105) .