Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Jul 23 2025 01:06:12 FOUS30 KWBC 230106 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 906 PM EDT Tue Jul 22 2025 Day 1 Valid 01Z Wed Jul 23 2025 - 12Z Wed Jul 23 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE UPPER MISSISSIPPI VALLEY AND SOUTHWEST... ...Northern Plains/Upper Midwest... Few changes made this evening across the Dakotas and Upper Midwest where a Slight Risk remains in effect through tomorrow morning. Current radar representation shows scattered showers/storms moving in an east-northeast direction as mean-layer winds remain in this orientation at about 30 to 40 knots. A frontal boundary draped across the region will provide some focus for repeating cells across northern MN into southeast ND, where HREF neighborhood probabilities for exceeding 3 inches by 12z Tues are highest (40-60%). PWs are up around 1.7-2.0" in the region, which is above the 99th climatological percentile per the NAEFS ESAT and allows for the potential for localized extreme rainfall amounts. Late afternoon CAMs, while likely overdone, do depict some localized totals of 5-8". FFG remains lowest across the Dakotas (below 2" in 3-hrs for some spots) and highest across the wooded terrain of MN and WI, limiting the flash flood potential somewhat. Further to the southeast across IA, WI and southeast MN, a well- defined MCV will lift north this evening and interact with a warm front in the region to provide some potential for locally heavy rainfall. Instability and moisture are plentiful, but a mid-level warm nose may cap convective potential and remains highly uncertain in latest CAMs. The 22z HRRR tries to spin up convection close to the center of the MCV over west-central WI by 12z, but is mostly alone. Thus, maintained a Marginal Risk here. ....Southeast... Based on radar trends and decreasing surface instability the risk area was lowered and confined to parts of the Southeast where ongoing thunderstorm activity remained. Additionally, activity may refire along the surface boundary across the central/northern FL Peninsula tonight as well as near the southern Appalachians as a weak mid-level wave passes over. Maintained a Marginal Risk in these areas. ....Southwest... Following radar and satellite trends to mostly maintain the outlook area across the Southeast tonight, with a Slight Risk extending from southeast AZ to West TX, including much of southern/central NM where ongoing scattered Flash Flood Warnings are in effect. Scattered convection should linger until about 06z or so as instability wanes. See MPD #780, #781, and #782 for more information. Did expand the Marginal Risk into the TX Hill Country as convection seems to be able to extend north from northern MX (supported by the 23z HRRR). There is some westerly flow entering the area as an=20 upper trough passes overhead and 2000 J/kg will allow for updrafts=20 to maintain for a least a few more hours. Snell Day 2 Valid 12Z Wed Jul 23 2025 - 12Z Thu Jul 24 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE UPPER MIDWEST... ....Upper Midwest... Models depict high confidence in the overlap of a deep moisture plume (with precipitable water values near 2 inches and the 99th percentile for late July) and strong instability across portions of the Upper Midwest on Wednesday. Combined with deep layer mean flow oriented roughly parallel with a slowly advancing front, there are plenty of ingredients in place to support very heavy rain rates and flash flooding. Organized convective lines or clusters may be ongoing at the start of the period from the Dakotas into northern or central Minnesota, northern Wisconsin, and Upper Michigan, and could already be producing localized corridors of heavy rainfall and flash flooding. The exact mesoscale pattern and evolution through the morning hours will be critical to determining the placement of heavy rainfall through the entire Day 2 period, and therefore the Slight Risk was generally maintained and somewhat expanded for this update. Regardless of the mesoscale details, the overall atmospheric ingredients should support convection either redeveloping into the afternoon and evening hours, or persisting and reintensifying along mesoscale boundaries. At present, the most favored locations for heavy rainfall seem to be in Upper Michigan and Northern Wisconsin, where ensemble model agreement is strongest, but it would be possible to the southwest through southern and central Minnesota into portions of eastern South Dakota, eastern Nebraska, and northwest Iowa as well. Machine learning first guess ERO fields from CSU show much of the region solidly at the Slight Risk level, and this seems both reasonable, and largely consistent with the inherited outlook. The overall environment and 12Z HREF probabilities strongly suggest hourly rain rates exceeding 1 inch in organized convective lines and clusters, and they may exceed 2 inches in the most intense convection at times. As occasional periods of training should occur, localized totals reaching 4-6 inches in a few hours do seem plausible, and that would favor some isolated to scattered flash flooding across the region. ....Florida and the Coastal Southeast... A Marginal Risk was maintained with only small modifications for Florida and the coastal Southeast. A westward drifting upper level trough enveloped in deep, tropical moisture, will favor slow-moving scattered convection with locally heavy rain rates. Probabilities for 24-hour rainfall totals reaching 5 inches will decrease relative to the Day 1 period (Tuesday afternoon and night), as weakly negative vorticity advection aloft and broad northeasterly flow should suppress convective coverage a bit and keep the most active areas of convection pinned closer to the coast. Therefore, the main change was to slightly reduce the inland extent of the Marginal Risk over the Southeast. Nevertheless, where low-level convergence is maximized and convection may be able to maintain intensity for a longer duration, localized 2 inch per hour rain rates will be possible, and that could lead to localized impacts. ....Central and Southern Rockies and Central Plains... A broad Marginal Risk was included for a large portion of the Central Plains region and the Rocky Mountain Front. This is due to a broad zone of ample moisture, with precipitable water values near or over the 90th percentile, and strong instability, but a general lack of prominent forcing mechanisms. Hi-res models unanimously show corridors of several inches of rainfall scattered across the region, but vary considerably in placement such that some models may have upwards of 3-4 inches of rain in a specific location and others are dry. Therefore, confidence is lacking in upgrading any portion of the large Marginal Risk to a Slight Risk at this time, but in general we do expect localized heavy rainfall and flash flooding somewhere within the broader region. Lamers Day 3 Valid 12Z Thu Jul 24 2025 - 12Z Fri Jul 25 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE CENTRAL PLAINS INTO THE UPPER MIDWEST... ....Central Plains to Upper Midwest... For this outlook update we upgraded to a Slight Risk generally 60-70 miles either side of a line from Manhattan, Kansas to Milwaukee, Wisconsin, including Chicago and Kansas City. The driving factor is an impressive plume of deep moisture, with model forecasts of precipitable water values well in excess of 2 inches. All the global ensemble systems and the ECMWF AIFS ensemble indicate this is likely (around 60 percent) to nearly certain (around 90 percent) in that same corridor, depending on the ensemble system you select. Not only is there confidence in this occurring, but the context is important as well -- PWs this high are above the 99th percentile and closing in on record levels. As this draws closer in time, the risk area is likely to be further refined and adjusted, and placement of heavy rainfall will likely be influenced by convective outflows and mesoscale details from convection the day prior. For now, the Slight Risk was placed where ensemble model signals for at least an inch of QPF were strongest. General alignment of the mean flow vectors, low-level inflow, and orientation of the overall front would favor some training and backbuilding, and the presence of strong instability in addition to the anomalously high moisture levels would support intense rain rates and flash flooding. ....Central High Plains and Central and Southern Rockies... A continuation of the pattern with strong instability and above normal moisture levels will continue to favor scattered convection and localized heavy rainfall. As with the Day 2 period (Wednesday and Wednesday Night), there is a general lack of strong forcing, so confidence on placement of heavy rain is limited. Therefore, a broad Marginal Risk outlines the general threat area, primarily over the Central Plains and Rocky Mountain front in NM, CO, WY. ....Southeast... No major change to the overall thinking in the Southeast, so a Marginal Risk was maintained. The main limiting factor for a more organized heavy rain and flash flooding threat will be the upper level trough and associated surface trough or low remaining well out over the Gulf, and as a consequence some weak height rises over land areas. Nevertheless, deep moisture will be abundant across the region, and a return of southerly low-mid level flow from Florida into the southeast Atlantic Coast may cause the plume of deep moisture to expand back into the Carolinas. A Slight Risk may eventually be needed, but that will probably come down to greater confidence in mesoscale details, and guidance that allows for more confidence in specific areas of concentrated convection. ....Northern California and Adjacent Areas... Confidence is growing in thunderstorm activity over N CA, S OR, and into the northern Sierra on Thursday, with increasing model QPF. Ensemble mean precipitable water from the ECMWF ensemble is near or above the 90th percentile and global models also show CAPE values approaching 1000 j/kg. All of that would support more organized convection with heavy rain rates that may be able to sustain itself long enough to produce localized flash flooding threat. Lamers Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_oHECtYsq35RpoxFrA5uP97hZEiCfSB30xNGib_OR8ia= W33J2OtNG3APunUxzOCbtNviutYeLFcC9kuAkOWUZkPT0R8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_oHECtYsq35RpoxFrA5uP97hZEiCfSB30xNGib_OR8ia= W33J2OtNG3APunUxzOCbtNviutYeLFcC9kuAkOWUsmKQbUE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_oHECtYsq35RpoxFrA5uP97hZEiCfSB30xNGib_OR8ia= W33J2OtNG3APunUxzOCbtNviutYeLFcC9kuAkOWUjUa9www$=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) .