Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Jul 22 2025 15:55:06 FOUS30 KWBC 221554 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1154 AM 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=20 Plains depicts a migrating cluster of thunderstorms over the=20 Dakotas with the proxy of the convective complex along the ND/SD=20 border. Mean storm motions continue to point to the complex of=20 thunderstorms migrating east-northeast with heavy rainfall likely=20 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=20 attention for flash flooding by itself, but the environment will be somewhat favorable for the first half of the D1, so the threat=20 will likely be more isolated with the best chance occurring in any=20 urbanized settings. Majority of this heavy precip will likely fall=20 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=20 prospects as we move into the second half of the forecast as a=20 stationary front currently analyzed over IA/IL into SD will lift=20 northward through the afternoon with an appreciable advection of=20 deep layer moisture priming the environment for the following=20 evening. Surface low over the High Plains will slowly advance=20 through SD during the forecast period with a cold front dropping=20 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=20 lead to a tight window of enhanced sfc-850mb convergence within a=20 formidable pool of moist, unstable air positioned across the=20 eastern Dakotas through much of MN/WI. PWATs between +2 and +3=20 deviations will be present over the aforementioned areas with=20 values >2" likely as we move beyond 00z Wed. LLJ initiation across=20 the Upper Mississippi Valley will help trigger an area of heavy=20 convection across the Dakotas which will eventually migrate=20 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=20 northwest into southwest North Carolina, Upstate South Carolina,=20 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=20 layer mean winds at or below 10 knots. Unsurprisingly, hi-res model guidance generates a variety of localized rainfall maxima across=20 the region, with basically every hi-res model generating a=20 localized max in excess of 4 inches somewhere in the new Slight=20 Risk area (and generally more than one maximum). Of course, the=20 placement of the maxima is more uncertain, but it does seem to be=20 favored in the Lowcountry of South Carolina and adjacent portions=20 of southeast Georgia, where simulated reflectivity from the hi-res=20 models tend to indicate an abundance of convection along the sea=20 breeze merging and colliding with developing convection further=20 inland, leading to complex mergers and larger clusters and/or 2-3=20 hour periods of training. A secondary signal, somewhat weaker=20 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=20 Southeastern CONUS will maintain a posture of widely scattered=20 thunderstorms capable of locally enhanced rainfall rates between=20 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=20 Southeast will lead to enhanced regional convergence that could=20 allow for slow- moving heavy thunderstorms to train over any area=20 in proxy to these features for a few hrs. These are the types of=20 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=20 will bisect southeast GA down through northern FL with guidance=20 consistently pinning a more defined heavy precip axis within the=20 confines of this area. FFG's remain very high across that specific=20 area, so the threat for widespread flash flooding is very low,=20 especially when considering the probabilities for >5" are still=20 running <25% for a majority of that region, a signal necessary for=20 more appreciable impacts to occur in these zones. 00z HREF=20 neighborhood probs for >5" are higher than previous forecasts for=20 the coastal Carolina's with the max probs (30-45%) located from=20 Morehead City, NC down through Charleston, SC. Multiple population=20 centers are encompassed in that zone, the most likely locations to=20 see any flash flood prospects due to urbanization factors. EAS=20 probabilities for >1" are also pretty elevated over the Upper SC=20 coast (>50%) with a modest output over the area referenced above.=20 The synoptic and thermodynamic evolution support a strong coastal=20 convergence pattern, especially with the afternoon sea breeze=20 during the life cycle of the surface low, so the upgrade to SLGT=20 from the last forecast update maintains merit. Considering the=20 current signals, there was no reason to deviate from the previous=20 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. 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 MINNESOTA, NORTHERN WISCONSIN, AND THE WESTERN U.P. OF MICHIGAN... ....Upper Midwest... The progression of convection from the prior D2 will bleed over into the front half of D3 with the cold front slowly migrating eastward allowing some advancement of the organized convective pattern to spill over into the Arrowhead of MN and adjacent northern WI and western U.P. There's a better consensus on this occurring given the aligning mass fields from relevant deterministic, as well as an agreement on the pattern progression as the surface low over the northern plains finally lifts northeast along the front and clears into the northern lakes by Wednesday afternoon. The final 12hrs of the HREF blended mean QPF output was pretty clear in a heavy precip presentation across northern WI through the U.P with the focus likely along and south of the southern lake shores of Superior. Deterministic depictions are upwards of 2-4" in additional QPF spilling over from the previous forecast period leading to some 24hr totals from 00z Wed to 00z Thu running upwards of 4-5" over the western U.P and northern WI. This would entice the threat for flash flooding due to the anticipated rates between 1-2"/hr with higher intra-hour rates expected given the favorable environment of PWATs >2". The cold front will still be slowly pushing eastward through the second half of the period with guidance indicating another wave of low pressure potentially enhancing the convective pattern across the Upper Midwest as we move into Wednesday evening. There's a growing consensus on heavy precip redeveloping across the central and northern plains between NE/SD and migrating east-northeast along and ahead of the slow-moving cold front. With flow remaining parallel to the boundary across a majority of the cold front positioning, heavy convection with some back-building/training evolutions are favored which would undoubtedly lead to, at least some scattered flash flood concerns. The highest probabilities sit firmly over northeast NE up through the southern half of MN where ensembles delineate the most favorable proxy of the cold front. QPF means are running between 1-2" from Minneapolis down towards Omaha with the prob fields off the latest ECENS and EC AIFS Ensemble depicting the highest threat across southwest MN towards Minneapolis with the second round of convection. This was enough to extend the previous SLGT risk further to the southwest to cover for the threat with even some potential for extend further back into NE if the consensus grows and/or magnitudes of rainfall increase further. This window (00-12z Thu) will need to be monitored closely. ....Southeast... A slowly westward migrating upper trough off the Southeast coast and attendant surface low over the northern Gulf will lead to locally heavy rainfall prospects from the central Gulf coast over into Florida. The signal for rainfall exceeding 3" remains very spotty within the varying deterministic, however the threat remains for isolated heavy convection anywhere along the South Carolina coast down through the northern half of the FL peninsula over into the central Gulf coast. Overall, the threat remains on the lower end of the MRGL risk category, but environment and proximity of favorable surface pattern maintains the threat for a few robust cells that could easily overproduce with PWAT forecasts firmly above 2.25" with some of the coastal plain likely to see PWATs exceeding 2.5" at times. ....Central and Southern Rockies... Ample moisture, instability and weak mid-level perturbations in proximity to the Continental Divide will lead to slow-moving bouts of convection that will amplify heavy rain prospects that could induce flash flooding. Terrain complexity and the slow-moving nature suited by lackluster mean flow and cell anchoring over the topography spell for a sneaky flash flood risk across the central and southern Rockies with a general maxima positioned across CO, west of I-25. A MRGL risk exists for the threat, but if signals within the CAMs continue to grow as we move closer to the threat window, a targeted upgrade is not out of the question. For now, have maintained continuity with the MRGL and will await for the 12z CAMs output as they will have the full D2 output/prob fields to assess for the threat. Kleebauer Day 3 Valid 12Z Thu Jul 24 2025 - 12Z Fri Jul 25 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL PORTIONS OF THE CENTRAL AND HIGH PLAINS, MIDWEST, AND SOUTHEASTERN U.S... ....Central Plains to Great Lakes... Cold front progression and sheared mid-level shortwave ejecting out of the Rockies will become an integral part in enhancing regional convective output from the Central Plains up through the Great Lakes. The environmental parameters such as favorable thermodynamic properties (Elevated MUCAPE), weak mid-level forcing, and sfc-850mb convergence in correlation to the front will provide enough of a mechanism to drive scattered to widespread areas of thunderstorm activity capable of producing localized flash flooding. Good news is the models have the threat in question. The bad news is the consensus on where is pretty much up in the air. The most favorable areas in the ensemble means are across the central Midwest back into the Central Plains with a secondary maxima focused across the Great Lakes. The prospects for a SLGT are relatively high within the bounds of the broad MRGL risk, but the threat for scattered flash flood concerns is very much warranted with the signal. ....High Plains... Another closed low off CA will allow for a broad diffluent mid and upper level regime with a multitude of smaller shortwaves migrating northeast out of the Four Corners. Scattered convective pulses will likely transpire over the Front Range down along the Continental Divide. Any area within these bounds could be subject to heavy precip with a potpourri of individual deterministic outputs signaling the threat for 2-3+" in any location. Not enough of an organized area to produce anything more than a broad MRGL risk encompassing the above areas, so maintained continuity from previous forecast. ....Southeast... Low pressure along the central Gulf coast will entice yet another period of heavy convective potential from LA back into FL/GA. PWATs remains very high (>2.25") for the period with ample instability situated along the Gulf coast and areas just inland. This is a prime setup for somewhere to see over 5" of rainfall in a spot, but the question of exactly where is very much up in the air. Just assessing the SLP positioning, the best threat is likely over Southern LA, mainly along and south of I-10. This is a signal within multiple global outputs, and coincides with the best convergent winds on the northern side of any surface circulation. In any case, still kept the previous MRGL inherited just due to the lack of a discernible organized maxima, but don't be surprised to see a targeted upgrade somewhere along the central Gulf coast as we inch closer to the time frame of interest. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6hQLm46WJYaeNzzigGGcLLN9FGBQtFl-kmeLWYEQxxRd= jlHN26WS9aIEk75xrZTVsszYg4qVsvt8ad0pK18zv5xAh9M$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6hQLm46WJYaeNzzigGGcLLN9FGBQtFl-kmeLWYEQxxRd= jlHN26WS9aIEk75xrZTVsszYg4qVsvt8ad0pK18zduIyQKk$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6hQLm46WJYaeNzzigGGcLLN9FGBQtFl-kmeLWYEQxxRd= jlHN26WS9aIEk75xrZTVsszYg4qVsvt8ad0pK18z2OOluUk$=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) .