Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Sep 26 2025 20:26:11 FOUS30 KWBC 262026 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 426 PM EDT Fri Sep 26 2025 Day 1 Valid 16Z Fri Sep 26 2025 - 12Z Sat Sep 27 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHWEST & IN THE VICINITY OF THE CAROLINAS... 16z Update: Not many changes needed to the inherited outlook from earlier this morning. Central to southern AZ remains the focus for potentially locally significant flash flooding. Pretty impressive southerly inflow/moisture transport combined with upwards of 2000 J/KG of CAPE will support a backbuilding convective threat today. Main question will be how much of this moves off the terrain and towards the Phoenix to Tuscon urban corridor. The initial tendency will be for convection to focus in the terrain areas, likely growing in coverage and intensity. If a strong enough outflow can push south out of this activity then we could see convection move off the terrain and into the lower elevations/urban areas. If this happens the strong southerly flow will support backbuilding/training and an increased flash flood risk. HREF and REFS probabilities support 2-4" rainfall totals where this training is able to persist. However still unclear whether these totals are able to move southward out of the higher terrain...but there is certainly some potential for this scenario. Chenard ....Previous Discussion... ....Southwest... Numerous showers and thunderstorms are expected across the Southwest, especially for much of southeastern Arizona and parts of southwestern New Mexico, which should occur primarily during the afternoon and evening hours. An upper low should slow to a crawl across southeast California. Precipitable water values should remain as high as 1.25-1.5" in spots. ML CAPE values of 2000-3000 J/kg are possible as available moisture interacts with the cold pool aloft. A Slight Risk was maintained across parts of Arizona and New Mexico, but it could be higher end Slight Risk. Hourly rain amounts to 2" with local totals to 4" are possible where cells merge and/or train. ....Southeast... A boundary will remain in place as a cold low forms near the base of an upper level trough, a feature that lingers for a while near the Southern Appalachians. Farther inland across the Piedmont, some degree of upslope flow should bring some of the higher moisture back in from the southeast. Moisture and instability lifting ahead of the closing system will support periods of showers and storms. Precipitable water values of up to 2" with MU CAPE of 1000+ J/kg across the Piedmont and 2000-3000 J/kg in coastal areas are expected, which could yield hourly rain amounts to 2.5" and local totals to 4". As the 00z HREF and 18z REFS had their strongest signal for 3"+ and 5"+ across far southwest VA and the western Carolinas, a Slight Risk was added in this update, which was coordinated with the local offices. For the time being, a Slight Risk was left out across the coastal plain of the Carolinas as recent dryness could make a higher incidence of flash flooding harder to come by, which was coordinated with the coastal Carolina offices. Roth Day 2 Valid 12Z Sat Sep 27 2025 - 12Z Sun Sep 28 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHWEST AND MID ATLANTIC TO SOUTHEAST... 20z Update: The Slight risk was maintained over the Mid Atlantic with this update. The higher confidence for potential flash=20 flooding appears to be over portions of central to southeast VA.=20 This is in the vicinity of the stationary front where lower level=20 convergence should be maximized. Instability will drop off with=20 northern extent, however we should have enough instability along=20 and just north of the front to support an axis of low topped=20 convection. This activity will likely be warm rain dominant, and=20 thus efficient at producing heavy rainfall rates. The exact axis=20 remains a bit uncertain, with model solutions varying from near=20 the NC/VA border on the southern side, to DC on the northern=20 extent. At the moment the best clustering is around Richmond, but=20 there is some opportunity for that to shift a bit north or south.=20 Where this axis does set up, localized rainfall over 4" appears=20 likely. The Slight risk was expanded a bit northward over central and eastern VA to account for this risk. Confidence is lower on the Slight risk farther south over NC and=20 SC. Certainly plenty of moisture in place, but the coverage and=20 organization of convection is less clear. Also a possibility that=20 the cells that do form tend to move along at a fast enough clip to=20 limit the flash flood coverage. The best chance of some training=20 could be along the coastal corridor where convergence and=20 instability may be a bit stronger. Overall the flash flood risk=20 over these areas may end up staying more isolated in nature...but=20 the potential for greater coverage is still there, and so we will=20 maintain the recently introduced Slight risk area. Over the Southwest a Slight risk area was added to portions of=20 southeast CA into southwest AZ. The upper level low will remain=20 over these areas on Saturday providing broad ascent for convective=20 development. Instability over these areas is forecast to be=20 greater Saturday than Friday, and PWs will remain well above late=20 September averages. Thus would expect to see greater convective=20 coverage than today, with heavy rainfall rates likely. The presence of the upper low should support slow cell motions and the=20 potential for periodic cell mergers. Chenard ....Previous Discussion... ....Southwest/Southern Rockies... An upper low remains fixed across the Lower Colorado River while Gulf Moisture streams into the TX Big Bend and New Mexico, moving up the Rio Grande. Precipitable water values of 1.25-1.5" are forecast, which when combined with 1000+ J/kg of ML CAPE could yield hourly amounts to 2.5" and local totals to 4" where cells train and/or merge. Believe the risk level from continuity (Slight) conveys the risk appropriately at this time for portions of far west TX and NM. Closer to the upper level center across the Southwest, slow-moving convection is possible under the cold pool aloft, with ML CAPE of 1000-2000 J/kg of instability and precipitable water values in the Lower Desert of 1-1.25" expected. Based on the decline of the 1000-500 hPa thickness across UT towards 5670 meters, limited the northern extent of the Marginal Risk to southern UT which is about the northern extent of the 0.75" precipitable water value. ....Carolinas/Southern Mid-Atlantic States... Divergence aloft around the eastern periphery of a cold low near the Southern Appalachians and what is now a tropical disturbance moving through the Bahamas as a possible tropical cyclone on Saturday team up to maintain precipitable water values of up to 2.25" across the coastal plain, which should also see the greatest instability, 1000-2000 J/kg. The Piedmont should see times when the MU CAPE rises above 500 J/kg. This is a PRE-like set up for the region, so the heavy rainfall potential has a higher upside than usual. There has been some convergence in the model guidance QPF- wise, and both the 00z HREF and 18z REFS had a cohesive and agreeable enough heavy rain threat of 3"+ for a new Slight Risk area from roughly Richmond VA south on both sides of the fall line into eastern SC. Hourly rain amounts to 3" and local totals to 6" are possible, which is potentially problematic anywhere within the Slight Risk area. A higher risk level can't be ruled out in future updates. Roth Day 3 Valid 12Z Sun Sep 28 2025 - 12Z Mon Sep 29 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF FAR WEST TEXAS AND NEW MEXICO... 20z Update: We did opt to remove the small Slight risk along the SC/NC coast with this update. Most guidance suggests the heaviest rainfall this period will remain offshore, with greater impacts onshore beginning on day 4 (Monday into Tuesday). There is certainly some potential for locally heavy rain along coastal areas, but at the moment the risk appears localized in nature.=20 Thus a Marginal risk should be enough to cover this threat for=20 now and we will continue to monitor trends. Little to no change was needed over the Southwest with this=20 update. Portions of NM into far west TX still look on track for an=20 isolated to scattered flash flood risk as mid/upper level=20 troughing moves eastward. Chenard ....Previous Discussion... Southwest/Southern Rockies... The combination of an upper low moving to near the Four Corners, a kicker upper level trough moving into CA, and moisture feed from the Gulf into NM persist into Sunday. Precipitable water values remain above 1" locally in the Desert Southwest and 1-1.25" in the Southern High Plains. Pockets of 1000+ J/kg of ML/MU CAPE are forecast to develop and/or persist. A Slight Risk still appears reasonable across portions of far west TX and NM, with a broader Marginal Risk surrounding that. The guidance appears to be trending somewhat drier across the Great Basin, so limited the northern extent of the Marginal Risk area when compared to continuity. Florida northward into the Southern Mid-Atlantic States... A system now considered a tropical disturbance over Hispaniola should lie somewhere north of the Bahamas and east of Florida as a possible tropical or hybrid cyclone on Sunday. The upper level environment doesn't look kind for whatever emerges, and there are two scenarios -- a more convective system that remains offshore during the medium range period or a more sheared system that attempts to occlude when it approaches landfall early next week. The guidance is not ideal in differentiating which is correct during these scenarios, historically. Regardless, moisture will not be a problem regionally -- precipitable water values locally 2"+. Coastal areas will see 1000+ J/kg of ML CAPE while 250+ J/kg of MU CAPE invades portions of VA. The guidance has slowed down the approach of the tropical low since yesterday, but the other players in the flow pattern remain -- the upper low near the southern Appalachians/southern Cumberland Plateau and the front near the East Coast. A PRE- like set up appears to be in the cards, with moisture streaming into the front from the south. The best signal for heavy rainfall on Sunday into early Monday appears to be in the vicinity of Myrtle Beach, and considering previous days of rain, thought a new Slight Risk in that area would be prudent. In theory, hourly rain amounts to 3" should be possible, which would be most problematic should thunderstorms merge or train. Roth Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8D-yRoqOmKEr3QqccSN0O-qwsJwkziezGu4czyd0yK3z= sMFdgvHwMhSdW5Z4oFnVwB6XT_-nxe05Uz-V0O9rWs9EfOs$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8D-yRoqOmKEr3QqccSN0O-qwsJwkziezGu4czyd0yK3z= sMFdgvHwMhSdW5Z4oFnVwB6XT_-nxe05Uz-V0O9r3eepMfc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8D-yRoqOmKEr3QqccSN0O-qwsJwkziezGu4czyd0yK3z= sMFdgvHwMhSdW5Z4oFnVwB6XT_-nxe05Uz-V0O9rn4REM70$=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) .