Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Jul 22 2025 20:15:36 FOUS30 KWBC 222015 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 415 PM EDT Tue Jul 22 2025 Day 1 Valid 16Z Tue Jul 22 2025 - 12Z Wed Jul 23 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE UPPER MISSISSIPPI VALLEY, SOUTHWEST, AND SOUTHEAST... ...Northern Plains/Upper Midwest... 16Z UPDATE: Very few changes were made to the outlook for this update as the overall forecast reasoning below still holds. The main change was to extend the Slight Risk area further to the west into the central Dakotas given relatively similar signal on the 12Z hi-res model guidance to areas further downstream. There is also a conditional threat over portions of Iowa, the southern half of Minnesota, and Wisconsin, where a broad Marginal Risk was maintained (and slightly expanded). There is a fairly strong mid- upper level wave evident in satellite imagery over southeast Nebraska, which should continue to lift to the northeast today and tonight. Hi-res models do attempt to fire some convection along the southern periphery later today or this evening, but overall it struggles. However, there is some uncertainty with the strength of the cap, as the wave should be tracking very close to the 11-12C isotherms at 700mb. A weaker cap than expected could lead to a more prominent training band of convection on the southern periphery, and could warrant a Slight Risk upgrade later today. PREVIOUS DISCUSSION: Regional radar mosaic across the Northern Plains depicts a migrating cluster of thunderstorms over the Dakotas with the proxy of the convective complex along the ND/SD border. Mean storm motions continue to point to the complex of thunderstorms migrating east-northeast with heavy rainfall likely to be impacting portions of eastern ND and far northeast SD into MN by the turn of the new D1. This complex will likely draw some attention for flash flooding by itself, but the environment will be somewhat favorable for the first half of the D1, so the threat will likely be more isolated with the best chance occurring in any urbanized settings. Majority of this heavy precip will likely fall north of I-94 in MN with the Minneapolis metro unlikely to see much of any impacts. The period becomes more favorable for heavy rain prospects as we move into the second half of the forecast as a stationary front currently analyzed over IA/IL into SD will lift northward through the afternoon with an appreciable advection of deep layer moisture priming the environment for the following evening. Surface low over the High Plains will slowly advance through SD during the forecast period with a cold front dropping steadily out of Canada with eyes on the Upper Midwest by the end of the D1. The proxy of the warm front and advancing cold front will lead to a tight window of enhanced sfc-850mb convergence within a formidable pool of moist, unstable air positioned across the eastern Dakotas through much of MN/WI. PWATs between +2 and +3 deviations will be present over the aforementioned areas with values >2" likely as we move beyond 00z Wed. LLJ initiation across the Upper Mississippi Valley will help trigger an area of heavy convection across the Dakotas which will eventually migrate eastward through the Red River basin into the northern half of MN. As of this juncture, the greatest threat for heavy rainfall >2" is likely over southeast ND, far northeast SD, and adjacent areas of northwestern MN with an eastward expansion over the central portion of MN (St. Cloud area). 00z HREF and REFS neighborhood probabilities for >2" are between 80-90+% for these locations with >3" (60-80%) and >5" (20-35%) well within a reasonable depiction for a widespread flash flood threat in that area of the northern CONUS. HREF EAS probabilities for >1" are also robust with 70-90% for >1" and 40-60% for >2" situated over those same areas, a testament to the growing consensus within the CAMs window as we move closer to the event. This aligns very well with the EC AIFS Ensemble mean QPF output of 1.25-1.5" in that corridor, a very strong output when you take into account the traditional low bias from the AIFS in general. The FFG's located across central MN are small deterrent for any appreciable risk upgrades as the threat targets places where 1/3/6hr indices remain a little high compared to areas a bit downstream and further south. Despite those indices, multiple flash flood warnings are increasingly likely over southeast ND, northeast SD, and central MN as we step through the evening, especially with a growing threat of training and back-building as proposed by the weakening Corfidi Upshear vectors in the CAMs as we move into the evening. This makes sense as the mean flow becomes parallel to the boundary with the approach of the front to the north. There is some chance this setup nudges north as the mid-level ridge to the south will truly flex and the warm front progression should migrate pretty rapidly to the north through the afternoon and evening, especially with the added nose of LLJ structure positioned to the south. Where this ends up is the ultimate delineation point of where training could occur and have the highest prospects for flash flooding. For now, a high end SLGT is forecast over that corridor positioned over ND/SD/northwest MN with an eastward extension through central MN prior to the Arrowhead. Pending forward propagation speed of the nocturnal convection, the risk could evolve further downstream prior to the end of the D1 period, so it will be something we will monitor closely. In any case, the SLGT risk still runs through portions of the Arrowhead into northwestern WI back into those favorable areas to the west. ....Southeast... 16Z UPDATE: The main change was to expand the Slight Risk to the northwest into southwest North Carolina, Upstate South Carolina, and a bit more of eastern Georgia. Abundant deep moisture is present across the region, with precipitable water values generally above the 95th percentile, even for the heart of summer, and deep layer mean winds at or below 10 knots. Unsurprisingly, hi-res model guidance generates a variety of localized rainfall maxima across the region, with basically every hi-res model generating a localized max in excess of 4 inches somewhere in the new Slight Risk area (and generally more than one maximum). Of course, the placement of the maxima is more uncertain, but it does seem to be favored in the Lowcountry of South Carolina and adjacent portions of southeast Georgia, where simulated reflectivity from the hi-res models tend to indicate an abundance of convection along the sea breeze merging and colliding with developing convection further inland, leading to complex mergers and larger clusters and/or 2-3 hour periods of training. A secondary signal, somewhat weaker overall, but still standing out from a regional perspective, exists in southwest North Carolina and far Upstate South Carolina. Confidence was not high enough in placement of concentrated convection in Florida to introduce any Slight Risk areas there, but it would not be surprising to see some additional FFWs later today. A Marginal Risk was also introduced over the Mid-South, from SE MO into S IL, W KY, NW TN. Hi-res guidance is more consistently showing a brief period of scattered, slow-moving convection in a very unstable and moist environment this afternoon and evening. PREVIOUS DISCUSSION: Elevated PWATs and instability across the Southeastern CONUS will maintain a posture of widely scattered thunderstorms capable of locally enhanced rainfall rates between 2-4"/hr that could spur flash flood prospects through the course of the daytime hrs. A weak surface reflection along the SC coast with a weak quasi- stationary front aligned west to east across the Southeast will lead to enhanced regional convergence that could allow for slow- moving heavy thunderstorms to train over any area in proxy to these features for a few hrs. These are the types of setups that can provide a sneaky 4-7" maxima in any given location, especially near the low pressure center along the Carolina coastal plain. Surface trough extending off the southern flank of the low will bisect southeast GA down through northern FL with guidance consistently pinning a more defined heavy precip axis within the confines of this area. FFG's remain very high across that specific area, so the threat for widespread flash flooding is very low, especially when considering the probabilities for >5" are still running <25% for a majority of that region, a signal necessary for more appreciable impacts to occur in these zones. 00z HREF neighborhood probs for >5" are higher than previous forecasts for the coastal Carolina's with the max probs (30-45%) located from Morehead City, NC down through Charleston, SC. Multiple population centers are encompassed in that zone, the most likely locations to see any flash flood prospects due to urbanization factors. EAS probabilities for >1" are also pretty elevated over the Upper SC coast (>50%) with a modest output over the area referenced above. The synoptic and thermodynamic evolution support a strong coastal convergence pattern, especially with the afternoon sea breeze during the life cycle of the surface low, so the upgrade to SLGT from the last forecast update maintains merit. Considering the current signals, there was no reason to deviate from the previous forecast, so the SLGT risk remains due to continuity. ....Southwest... 16Z UPDATE: The new outlook is very consistent with the previous one, with the Slight Risk expanded slightly northward into the mountainous region of south-central Colorado, based on improved model QPF signal. An area of greater concern for flash flooding may be in southeast Arizona near the Tucson metro later today, with higher probabilities of exceeding flash flood guidance based on the latest 12Z HREF. Another period of monsoonal type convection will enhance localized flash flood prospects across eastern AZ and much of NM/CO. Persistent moisture advection along the western flank of a ridge positioned across the Mississippi Valley will be the driver for the threat as scattered to widespread showers and thunderstorms will litter the Southwestern CONUS up into the Central Rockies by this afternoon, carrying through the evening hrs. Any area within the confines of the above regions will have a threat for impacts due to convective cores likely producing rates >1"/hr with upwards of 2"/hr in the strongest cells. Areal QPF average is generally between 0.25-0.75", but neighborhood probabilities for >1" over eastern AZ into NM/CO are well above 70% with >2" probs running between 40-70% over the more terrain favored areas in both locations. The most prominent signal for rainfall eclipsing 1" aligns within the Sangre de Cristos and the eastern side of the Mogollon Rim near the AZ/NM line. These are the most likely locations to see heavy rainfall and heightened flash flood prospects this period with the best chances over any remnant burn scars. Mean storm motions will be generally weak, so any convective cores will run the chance of impacting the same areas for multiple hours before collapsing or moving away from their initiation points. A broad SLGT risk encompasses much of NM, southwest TX, and southeast AZ with a bit of an extension into CO along the Sangre de Cristos. Lamers/Kleebauer 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.=20 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=20 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=20 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=20 backbuilding, and the presence of strong instability in addition to the anomalously high moisture levels would support intense rain=20 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!6-25e3donKAtvBNOsAcgRltUOTEySwbRGoS8qKN426wn= rvHdEgylS4UixgSchbuPR1FvGB6xIYK5l5RgPqTPtZuZHak$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6-25e3donKAtvBNOsAcgRltUOTEySwbRGoS8qKN426wn= rvHdEgylS4UixgSchbuPR1FvGB6xIYK5l5RgPqTPJ92H5bQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6-25e3donKAtvBNOsAcgRltUOTEySwbRGoS8qKN426wn= rvHdEgylS4UixgSchbuPR1FvGB6xIYK5l5RgPqTPxPZ1FPw$=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) .