Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Jul 19 2025 16:04:29 FOUS30 KWBC 191603 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1203 PM EDT Sat Jul 19 2025 Day 1 Valid 16Z Sat Jul 19 2025 - 12Z Sun Jul 20 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MIDWEST, THE MID-SOUTH, THE SOUTHERN MID-ATLANTIC STATES, & THE SOUTHERN APPALACHIANS... 16Z Excessive Rainfall Discussion Update... No major changes needed in the big picture. Was able to trim a bit of territory from the Slight Risk over parts of the Appalachians/Mid Atlantic where some drier air has filtered in from the north. A tight gradient in deep moisture remains and the 12Z run of the HREF continues to paint a corridor with neighborhood probabilities of 3 inch rainfall and low-end probability of 5=20 inches of rainfall to the south from convection later today and through the evening. As a result...did not tighten up the gradient quite as much as entirely possible given the probabilistic nature of the outlook. Also made a minor extension in Nebraska to account for somewhat higher exceedance probabilities just to the southwest of our previous outlook. Beyond that...few if any changes were made. Bann ....0830Z Excessive Rainfall Outlook... Corn Belt/Ohio Valley/Mid-Atlantic States/Mid-South... Ample moisture exists regionally. Some of this is due to the synoptic pattern, with the Westerlies stronger than and south of July norms which is allowing for greater effective bulk shear and thunderstorm organization. Some of the moisture influx is also due to the flow around the retrograding 3 sigma warm core ridge for mid- July moving into the eastern Gulf. Additional moisture is being imported from the direction of the former tropical disturbance. Regardless of the source, precipitable water values are forecast to crest at or above 2.25". ML CAPE could exceed 4000 J/kg due to daytime heating. Portions of IA, IL, and KY have the potential for backbuilding convection both Saturday morning and Saturday night/Sunday morning. Across IA, there is a question about how much overlap convection dropping in from the north this morning would have with convection Saturday night/Sunday morning. Within such an environment, hourly rain amounts to 4" are possible, with local amounts to 8". At the moment, these amounts are not explicitly forecast by much of the guidance, with the 00z HREF/18z RRFS showing minimal potential for 5"+ amounts, centered across IA. However, the high resolution NAM and Canadian Regional show local totals in the 6" ballpark. Large swaths of the region have had well above average rainfall during the past week, saturating soils. Since there's a bit of uncertainty, coordination with DMX/Des Moines IA and DVN/Davenport IA have left the risk area as a high- end Slight. However, should cells train for 2+ hours downstream of existing instability pools, localized Moderate or High Risk impacts cannot be ruled out. Southwest/Southern Rockies... Enough moisture is available for scattered monsoon convection again on Saturday. Hourly amounts to 2" are possible due to diurnal heating, which would be most problematic in urban areas, dry washes/arroyos, or burn scars. Roth Day 2 Valid 12Z Sun Jul 20 2025 - 12Z Mon Jul 21 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MIDWEST, SOUTHERN APPALACHIANS, & NORTHERN PLAINS... ....Midwest, Ohio Valley, & Southern Appalachians... High moisture for Saturday persists into Sunday -- precipitable water values eclipsing 2.25" -- with the core of the instability pool more significant across portions of the Ohio Valley on Sunday as warming at 700 hPa occurs across the Central Plains, slowly strengthening a mid-level capping inversion. Effective bulk shear should remain sufficient for convective organization. Inflow off the instability pool could lead to backbuilding, so the potential for 4" amounts in an hour and local 8" totals remains (the 00z NAM advertised 7"). There's not enough confidence on where the higher amounts will materialize to upgrade to a Moderate Risk, but a higher end Slight Risk appears to exist from in and near southern IL into KY, TN, southern WV, and westernmost VA. Localized Moderate or High Risk impacts cannot be ruled out. ....Northern Plains... Much of North Dakota will be at the nose of a potent low level jet across the area. A subtle shortwave in the upper level jet may be all it takes to initiate a line of storms across the region, though there may be two separate rounds: one in the morning and a second Sunday night. There is considerable uncertainty both with storm intensity and receptivity of the ground to the heavy rainfall. Used the recent guidance to trim some of the Slight Risk area away. Like continuity, believe this is a lower end Slight Risk. Southwest/Southern Rockies... Enough moisture is available for scattered monsoon convection on Sunday. Hourly amounts to 2" are possible due to diurnal heating, which would be most problematic in urban areas, dry washes/arroyos, or burn scars. Roth/Wegman Day 3 Valid 12Z Mon Jul 21 2025 - 12Z Tue Jul 22 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN ROCKIES, PLAINS, OHIO VALLEY, & SOUTHEAST... Surface boundaries/low pressure areas/upper level disturbances are on this move this period, which along with warming temperatures at 700 hPa/increased mid- level capping across portions of the Central Plains and IA should broadly limit QPF and flash flood potential. Wherever cells can manage to train/merge, hourly rain amounts up to 3" with local totals to 6" would be possible across the Plains, Ohio Valley, and Southeast, while hourly amounts up to 2" and local totals to 4" would be possible across the Southern Rockies and Southwest. At the moment, it appears any flash flooding would be on an isolated to widely scattered basis. In the Southwest and Southern Rockies, burn scars and dry washes/arroyos would be most at risk. Anywhere within these regions, urban areas would be of most concern. Roth Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7CK2hdu1ilDlcTSmgFee8Sc1Kc-h2maC9ytwI5a8wWbs= nBnHJ1iFMZ5jYXxd94Kbp24WdbcMku0YD7Ys6Ol6v_xuO4A$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7CK2hdu1ilDlcTSmgFee8Sc1Kc-h2maC9ytwI5a8wWbs= nBnHJ1iFMZ5jYXxd94Kbp24WdbcMku0YD7Ys6Ol63kaw5wA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7CK2hdu1ilDlcTSmgFee8Sc1Kc-h2maC9ytwI5a8wWbs= nBnHJ1iFMZ5jYXxd94Kbp24WdbcMku0YD7Ys6Ol6b3GHf3k$=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 3.28-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .