Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Sep 26 2025 00:38:23 FOUS30 KWBC 260038 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 838 PM EDT Thu Sep 25 2025 Day 1 Valid 01Z Fri Sep 26 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... ....01Z Update... ....Southwest... Moisture streaming north around a digging positively tilted trough has resulting in a corridor of showers and thunderstorms extending from the AZ/NM/Mexico tripoint north and west to west-central Nevada. The strongest storms and by far the most prolific flash flooding are across eastern Arizona into southwestern New Mexico, where atmospheric moisture levels are their highest. Heavy rain associated with scattered thunderstorms will likely continue for a few more hours with daytime heating, then will rapidly diminish in coverage and intensity after sunset. A similar scenario is also occurring up north, where a Slight Risk area covers a portion of the NV/CA border south of Lake Tahoe. In similar fashion as=20 further south, the steady rain will likely persist for another few=20 hours, followed by rapid weakening and dissipation of all=20 precipitation producing showers and storms soon after sunset. The=20 Slight Risk in this area was expanded north to cover some of the=20 flash flooding occurring southeast of Lake Tahoe, but overall a=20 very minor adjustment. ....Northeast... The inherited Marginal Risk across the Northeast was split with this update to the area around the OH/WV/PA tripoint and the northern West Virginia Panhandle, with a second one for much of southern New England. In the upper Ohio Valley, scattered showers and storms over southeastern Ohio are moving into an area hard hit with heavy rain in recent days. Thus, even small amounts of rain in this terrain may result in renewed flash flooding. Into New England, a line of moderate to heavy rain is moving into western New England, and may cause isolated flash flooding in areas that have seen more rain from the area of rain now moving across Maine, otherwise the flash flooding threat has largely ended, especially in most of PA and NY. ....Southeast... No major changes were made as ongoing clusters of showers and thunderstorms may still pose an isolated flash flooding threat into the overnight, though overall coverage should continue gradually decreasing with the loss of solar heating. Wegman 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... ....2000 UTC Update... At most, only minor changes made to the Day 2 ERO. Most notably, we expanded the Slight Risk area slightly westward, which now includes the Phoenix metro region and is closer to to Prescott and Flagstaff. This is based on the latest (more elevated_ HREF and RRFS rainfall exceedance probabilities for Friday afternoon and evening, especially the probs for 0.50-1.00+ in/hr rainfall, along with the probs of 3-hourly rainfall exceeding current 3-hour FFG (>40% within the Slight Risk area between 20-03Z). ....Previous Discussion... 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... ....2000 UTC Update... Expanded the Slight Risk area across NM, particularly northwest NM, based on the latest (12Z) deterministic and probabilistic guidance trends with the deepening low eventually cutting over southeast CA/southwest AZ by Saturday. Model QPFs seem to be convergence, which is boosting confidence to expand the Slight farther north toward the NM-CO border. The area also aligns with where the ECMWF and GFS show highest MUCAPEs (at least 500 J/Kg). ....Previous Discussion... 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... ....2000 UTC Update... Minimal changes made to the previous Day 3 ERO in this area. Still cannot rule an upgrade to a Slight Risk somewhere over the Coastal Plain from SC-VA (see previous discussion); however, the guidance spread with respect to the QPF remains quite a bit high at this time to include a targeted Slight Risk area. ....Previous Discussion... 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. Hurley/Roth Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6YmSsLc3Hz2al0zQa0LUEiA65zKbo3NmcArBMC2w4u0j= 6sIEdJQsLJb9ozBTBFysqxBt2hYZ-jgkY4SDI6ZyvOoooXk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6YmSsLc3Hz2al0zQa0LUEiA65zKbo3NmcArBMC2w4u0j= 6sIEdJQsLJb9ozBTBFysqxBt2hYZ-jgkY4SDI6ZyZmehpr0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6YmSsLc3Hz2al0zQa0LUEiA65zKbo3NmcArBMC2w4u0j= 6sIEdJQsLJb9ozBTBFysqxBt2hYZ-jgkY4SDI6ZyOY64Im8$=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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