Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Jul 27 2025 23:28:53 FOUS30 KWBC 272328 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 728 PM EDT Sun Jul 27 2025 Day 1 Valid 238Z Mon Jul 28 2025 - 12Z Mon Jul 28 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE UPPER MS VALLEY AND THE OH VALLEY TO THE MID ATLANTIC... 23Z Special Update... ....Upper MS Valley... Repositioned the Slight Risk over the Upper Midwest/MS Valley=20 further south across southern Minnesota into northern Iowa. A=20 cluster of storms over the area is currently producing rainfall=20 rates over 2 in/hr within some of the stronger cells. With deep=20 moisture in place and an environment favorable for backbuilding and training storms, additional heavy rainfall is likely into the=20 evening. The heavy rainfall threat is expected to diminish by the=20 overnight as a cold pool develops and storms begin to propagate=20 more quickly to the southeast.=20 For more details on the near-term threat, see WPC MPD #813. ....Ohio Valley to the Mid Atlantic... Shifted the Slight Risk further to the south, where storms=20 continue to develop along a slow-moving, west-east oriented outflow boundary. Further to the north, drier and more stable air has=20 spread south across the Northeast, so the Slight Risk that extended north across portions eastern Pennsylvania, New Jersey, and=20 Upstate New York was removed. ....Southwest... Made only minor adjustments to the Marginal Risk over New Mexico and southeastern Arizona. Pereira Day 2 Valid 12Z Mon Jul 28 2025 - 12Z Tue Jul 29 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE NORTHERN PLAINS... ....Plains of Eastern Montana to parts of the Upper Midwest... 21Z update... The latest guidance noted a north and eastward shift in the QPF axis. The previous forecast focused the higher amounts across eastern South Dakota and northern Iowa, whereas now it is more over south/central Minnesota and western Wisconsin. With this trend the Marginal Risk was reduced out of Wyoming and southwest South Dakota, northeast Nebraska and southern Iowa and then expanded over northeast Montana, North Dakota, central Minnesota and central Wisconsin. Areal averages of 1 to 3+ are still expected but hourly rainfall rates may reach or exceed 1.5 inches/hour with any of the expected storms. Campbell A surface cold front and mid-level shortwave trough will be approaching an area rich in moisture and instability over parts of South Dakota and Minnesota/Iowa on Monday afternoon. With low level flow likely to increase the amount of moisture transport into the region during the evening with a corresponding increase in moisture flux convergence along the front...the potential exists for excessive rainfall especially in the event of cell training or repeat convection. Uncertainty remained with respect to placement in the numerical guidance...but the RRFS seemed too far north. The WPC deterministic QPF was closer to a GEFS/ECMWF ensemble mean position...which still left some room for excessive rainfall to build south. Consequently...allowed for a somewhat broader apron of Slight Risk than shown by guidance. Surrounding the Slight risk area...only modest changes were made to the Marginal risk area extending northwest into the plains of eastern Montana where a secondary QPF maximum is expected. The expectation is that there will be two discrete QPF area with a risk of excessive rainfall associated with each...but maintained the one area given the range of possible solutions. ....Ohio/Tennessee Valleys to the Carolinas... Much of the region will continue to have a very deep column of moisture in place during this period. Flow is expected to be slow which will bolster the potential for storms to backbuild and given that some locations across the region have steep topography, isolated flash flooding may arise. Some of the CAMs show rainfall rates of 1-1.5+ inches/hour possible. A broad Marginal Risk was raised, spanning from southeast Missouri to the Carolinas. ....Southwest US... A continuation of flow with deeper moisture into the Southwest US will linger on Monday and Monday night...with a continued chance for scattered late day and evening convection across parts of New Mexico and a small portion of Arizona that extends northward along the southern and central Colorado front range. Bann Day 3 Valid 12Z Tue Jul 29 2025 - 12Z Wed Jul 30 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE WESTERN HIGH PLAINS... Northern Rockies eastward to Great Lakes and southward to the Southwest US... 21Z update... A previously noted for the D2 period, the latest QPF trends continue to be shifted eastward for this period as well. The axis of concentrated amounts went from northeast Wyoming to Nebraska, to now as far east as northern Iowa. The Slight Risk was elongated to now include most of northern Iowa. The Marginal Risk was nudged a bit further north across Wisconsin. Campbell Building mid-level ridge over British Columbia and Albert will result in lowering heights over parts of the northwestern US and increasing mid- and upper flow. This results in an increasingly well defined surface boundary that serves to focus showers and thunderstorm on Tuesday. Ingredients for thunderstorms to produce heavy to potentially excessive rainfall appear to be aligning from southeast Montana into northern Nebraska along a corridor of 1000 to 2000 J per kg instability and 1 to 1.25 inch precipitable water values...and even some 1.5 inch PW values in northern Nebraska. Showers and thunderstorms are also likely to spread eastward toward the Great Lakes where the instability and synoptic scale forcing looks to be weaker. In addition...another round of late day convection is possible from the Southwest US into the Rockies and some of this moisture may approach...if not come into play,,,across the western High Plains late in the period. ....Southeast US... 21Z update... The footprint across the Southeast increased, leading to the westward expansion of the Marginal across Southeast Louisiana and northward into South Carolina and western portions of North Carolina. Campbell Precipitable water values in excess of 2 inches will be in place with 1000-1500 J per kg of CAPE in place and weakening of the broad upper level ridge should lead to a somewhat better chance for showers or thunderstorms on Tuesday. The GFS and ECMWF both show hints at weak vorticity centers in the vicinity to help focus/support some activity. The expectation is that isolated but very intense rainfall rates could lead to isolated instances of excessive rainfall. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7DrScxuXkvWbQD8cmgm95XLSK4qdeHQBuiccvw6dWz8Z= Yy1rFQKrNU5fNbO8R3fG3RH1tyrMgKO-aA7wAn8ccAztUY4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7DrScxuXkvWbQD8cmgm95XLSK4qdeHQBuiccvw6dWz8Z= Yy1rFQKrNU5fNbO8R3fG3RH1tyrMgKO-aA7wAn8c4b8R5NA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7DrScxuXkvWbQD8cmgm95XLSK4qdeHQBuiccvw6dWz8Z= Yy1rFQKrNU5fNbO8R3fG3RH1tyrMgKO-aA7wAn8cHv4zems$=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) .