Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Sep 25 2025 16:00:10 FOUS30 KWBC 251559 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1159 AM EDT Thu Sep 25 2025 Day 1 Valid 16Z Thu Sep 25 2025 - 12Z Fri Sep 26 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS SOUTHEAST ARIZONA AND FAR SOUTHWEST NEW MEXICO, ALONG WITH PARTS OF EASTERN CALIFORNIA INTO WESTERN NEVADA... ....Southwest to Sierra Nevada... ....1600 UTC Update... Per collaboration with affected WFOs, have hoisted a Slight Risk area across the central Sierra (south of Lake Tahoe) into parts of western NV. This was based on some of the guidance trends, including the 06Z HREF and RRFS ensembles, and continuing with the 12Z CAMs. The 12Z HREF Z HREF probs of 0.50+ in/hr rates are aoa 70% between 19-03Z this evening, while probs of 3-hourly QPF exceeding 3-hourly FFG top out between 40-50%, which is impressive. ....Previous discussion... A cold low near the central California coast is forecast to drop southeast into southern California by early Friday. With cold air aloft overlying sufficient moisture near and east of the upper center, locally heavy amounts are possible from the Sierra Nevada east into southern Nevada and southwestern Utah. Ahead of the system, increasing southerly flow will draw deeper moisture into the Southwest from the Gulf of CA, fueling widespread shower and thunderstorm coverage, especially across southeastern and central Arizona into southwestern New Mexico. ML CAPE should rise into the 1000-2000 J/kg range. A Slight Risk was maintained across this area where cold air aloft interacts with monsoon-like moisture, with precipitable water values as high as 1.25-1.5". Hourly rain amounts to 2" are possible where cells merge and/or train. ....Northeast/Upper Ohio Valley... Unsettled weather at the present time is forecast to continue into Thursday as showers and storms stream east across the Upper Ohio Valley, WV, and southwest PA ahead of a shortwave aloft and surface frontal wave. While there may be a gap in heavy rainfall between this region and points farther east, chose to connect the areas and significantly broaden the inherited Marginal Risk. Strengthening southwesterly flow along the Mid Atlantic coast into the Northeast will support deepening moisture (PWs increasing to ~2") and ML CAPE surpasses 1000 J/kg. Hourly amounts to 2.5" are possible in this region with local totals to 4", but coverage of such amounts should be sparse enough for a Marginal Risk to still apply. ....Southern Appalachians into parts of the Gulf Coast... Amplifying mid-upper trough digging into the northwest and north- central Gulf will slow the south and east progression of the surface cold front and pre-frontal buoyant airmass. ML CAPE values should rise to 1000 J/kg or so across the southernmost Appalachians while areas of the Gulf Coast see 2000-3000 J/kg of ML CAPE. With precipitable water values at or above 2", hourly rain amounts to 2.5" with local totals to 4" are possible wherever short periods of training or cell mergers are able to occur. At the=20 present time, any flash flood issues are expected to be isolated to widely scattered. Roth Day 2 Valid 12Z Fri Sep 26 2025 - 12Z Sat Sep 27 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS SOUTHEAST ARIZONA AND SOUTHWEST NEW MEXICO... ....Southwest... Numerous showers and thunderstorms are expected across the Southwest, especially for much of southeastern Arizona and parts of southwestern New Mexico. 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, but the guidance shows more potential on the ML CAPE front when compared to Thursday, with values of 2000-3000 J/kg possible as available moisture interacts with the cold pool aloft. Widespread shower and thunderstorm coverage is expected, particularly during the afternoon and evening hours. A Slight Risk, with a footprint similar to the Day 2 area, was maintained for Day 3 across parts of Arizona and New Mexico, but it could be higher end Slight Risk. Hourly rain amounts to 2" 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. Moisture and instability lifting ahead of the closing system will support periods of showers and storms. Precipitable water values of up to 2" with ML CAPE to 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". Roth Day 3 Valid 12Z Sat Sep 27 2025 - 12Z Sun Sep 28 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF FAR WEST TEXAS & NEW MEXICO... 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 up to 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. 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, which was coordinated with the SLC/Salt Lake City forecast office. Carolinas/Southern Mid-Atlantic States... Divergence aloft around the eastern periphery of a cold low near the Southern Appalachians maintains precipitable water values of 2"+ near the coast, which should also see the greatest instability, up to 1000 J/kg. The guidance shows a bit of dispersion here in placement/northeast movement of the heavy rain area with the GFS and NAM in their usual camps (fast and slow, respectively). Hourly rain amounts to 2.5" and local totals to 4" are expected, similar to previous days. Forecast uncertainty, as much as anything else, maintained the risk level as a Marginal. Should the guidance converge, areas of risk overlap between Friday and Saturday across portions of the coastal plain of the Carolinas could see an upgrade to a Slight Risk at some point as we get closer to Saturday. Roth Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5rFIq1evtVlwy1YC8dPM6VZyWSFgQI-8Dro3_N3WA49_= snStVkXqJPbLL755PAet-tPqsJqaq4oTD5PU6_EEE5LXC0o$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5rFIq1evtVlwy1YC8dPM6VZyWSFgQI-8Dro3_N3WA49_= snStVkXqJPbLL755PAet-tPqsJqaq4oTD5PU6_EEQ4VMtUQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5rFIq1evtVlwy1YC8dPM6VZyWSFgQI-8Dro3_N3WA49_= snStVkXqJPbLL755PAet-tPqsJqaq4oTD5PU6_EEXceNI-k$=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) .