FOUS30 KWBC 180025 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 825 PM EDT Thu Sep 17 2026 Day 1 Valid 01Z Fri Sep 18 2026 - 12Z Fri Sep 18 2026 ...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL FOR PORTIONS OF THE NORTHERN PLAINS, OHIO VALLEY, & SOUTHWESTERN U.S... 0100Z Update: Main changes to the Day 1 ERO were to curtail the outlook areas over most areas, given the observational and mesoanalysis trends. Also adjusted the Slight Risk area over the OH Valley a bit to the south and east, which too corresponded to the observational and current CAM trends, including recent HRRR guidance and HREF and REFS rainfall exceedance probabilities. Both of these Slight Risk areas align along the northern portion of the max PW axis and the tight PW gradient just to the north, where ML CAPE trends have been on the uptick over the past 3 hours per the SPC mesoanalysis. Hurley Day 2 Valid 12Z Fri Sep 18 2026 - 12Z Sat Sep 19 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE UPPER MISSISSIPPI VALLEY AND MIDWEST... ...Upper Mississippi Valley into Midwest... 21Z update... Overall the environment and setup described below is still expected, with the greatest amounts focusing over the northern Illinois and Indiana. Minor reshaping of the Marginal and Slight Risk areas were made across Minnesota, Wisconsin and Iowa to reflect the latest guidance and WPC QPF footprint. Campbell Our disturbance over the Northern Plains will continue to traverse the northern tier of the CONUS with a trajectory eastward into south- central MN, eventually into WI by Friday late-Friday afternoon. Environment will still favor heavy rainfall prospects as deep, tropical origin moisture will accompany the disturbance along with rich theta_E air that will translate to better regional instability to enhance rate potential. Current model consensus is for a wave of heavy rainfall to protrude into the area later Friday morning and continue for several hours into the evening, ending west to east as it migrates out of the Northern Plains into the Upper Midwest/western Great Lakes. Further north over Ontario, surface high pressure will situate itself over the northern half of the Upper Midwest leading to an increased low- level convergence regime that will work in tandem with the disturbance located further south. There's a sharp delineation in where the heaviest rainfall will occur on some of the guidance, and the pattern evolution makes sense given the positioning of the high pressure, and upper pattern over Canada would likely exhibit a decent area of confluence around the latitude of the Arrowhead meaning the precip field will only be able to push so far north. By Friday evening, a stout LLJ development over the Midwest will enhance the convective footprint over the region with much of the guidance showing a blossoming area of heavy precipitation across southern MN, advancing eastward with the mean flow. There is some discrepancy in the global deterministic with the vigor of the LLJ and potential for backed flow over the course of the evening in proximity to southeastern MN and western WI. The ECMWF continues to be the most potent with the development of a weak 850mb low across southern MN that would enhance the prospects for significant rainfall and back-building precip potential in the aforementioned area. The GFS is not as aggressive and provides a setup that's a touch more progressive and could limit the upper threshold of heavy rain potential. ML outputs from both the EC-AIFS and AIGFS signal the 850mb low development, although timing of when it occurs is where they differ leading to the AIGFS showing a heavier QPF signal a bit downstream into WI and less so in MN. In any case, the ML and ensemble means lean towards the more aggressive solution in the case of the evolution with this setup, so tended to lean closer to that consensus with a stronger QPF footprint still positioned over southern MN into western WI. At this juncture, the area with highest probability for seeing heavy rain is likely south of I-94 with the forecast of 2-4", on areal average, locally higher, located from Minneapolis down through southeastern MN into WI. It will be something we'll be monitoring when it comes to the area of greatest impact potential as the setup is very much conducive for significant rains with prospects for flash flooding in those urban settings located along and south of I-94. A broad high-end SLGT risk remains forecast for those areas above with the SLGT extending eastward into the Milwaukee metro and all of southern WI. This setup could very well see a higher risk forecast as we move closer and fine tune details considering the environmental conditioning and forecast synoptic evolution. ...South Florida and Keys... A Marginal Risk was raised for this period as a continuation from the Day 1 period. High rainfall rates will be possible across the southern tip of the state and across the Keys. A conveyor of deep tropical moisture will keep pumping into the region to help enhance rainfall. With the ridge in place, weak low level winds will lead to slow storm motion thus increasing the threat for local flash flooding concerns. Campbell ...Western U.S... 21Z update... The guidance continues to show convection tracking further west across portions of Wyoming, Montana and Idaho therefore a minor westward nudge of the Marginal Risk area was made for this update. Campbell Upper low is progged to be over eastern OR while slowly migrating through the interior Northwest, maneuvering into the area of the Northern Rockies as we head into early Saturday. Ascent under the influence of the upper low will likely aid in scattered convective potential with locally heavy rainfall occurring under the low center, and within the broad diffluent axis that will situate itself over the northern Great Basin into WY/MT. Precip will be relatively progressive considering the nature of the flow ahead of the disturbance, so the prospects for more appreciable flash flooding remains low. Still, some of these cells could produce heavy enough rain to have some isolated flash flood concerns during Friday afternoon and evening through the overnight hours as the setup evolves. Best chance for convection with slower overall motions will be under the center of the upper low which is generally textbook for these types of patterns. A MRGL risk is in effect across the above areas with the best chance located over MT down into western WY. This area is consistent with a forecast of 0.5-1.5" with some local amounts of 2-3" plausible in the hardest hit areas. Rainfall rates will be between 0.5-1"/hr in the strongest cells which would preclude the necessity for a higher risk, however if the signal comes up even a little bit further, a targeted upgrade is not out of the question. A MRGL risk also exists for NM, eastern UT into CO as lingering moisture and a diurnal instability max will lead to widely scattered thunderstorms for another afternoon leading to isolated flash flood concerns for yet another day, mainly in the terrain. Kleebauer Day 3 Valid 12Z Sat Sep 19 2026 - 12Z Sun Sep 20 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MIDWEST, GREAT LAKES, AND OHIO VALLEY... 21Z update... Campbell Previous Discussion... ...Midwest and Ohio Valley... 21Z update... A minor reshaping of the western bounds of the Marginal and Slight Risk areas over Iowa was made to reflect the latest guidance trends and overlap areas of heavy rainfall on Day 2. Campbell The remnants of the convective conglomeration over the Upper Midwest will translate eastward through the southern half of the Great Lakes with heavy rainfall likely in-of western Lake Michigan areas, including Milwaukee to Chicago, extending east into lower Michigan through the first half of Saturday. Environment will remain conducive for heavy rainfall with PWAT firmly above 1.8" across the entire region, even breaching 2" for those areas west of Lake Michigan. This will provide efficient rainfall types, even in convective elements that could put down 2-4" of rainfall over the entire area leading to at least scattered flash flood potential over the impacted area(s). The highest potential for flash flooding will reside in those urban corridors littered across southern WI, northern IL, and southern MI where urban sprawl is a common occurrence in these locales. Latest ECENS probabilities for >1" are between 50-90% for the entire stretch from Madison, WI through Milwaukee into the area of Grand Rapids, extending into the Detroit metro. Considering the conservative probs typically shown on the ECENS, this signal is fairly robust and follows the general theme of the heaviest QPF axis bisecting this area. By Saturday afternoon. a strong surface ridge over northern Saskatchewan will make a quick advancement to the southeast, strengthening in the process as it moves into southern Manitoba by Saturday evening. This will press a cold front through the region and finally disperse the warm, moist environment further south into the northern and central Ohio Valley. Increasing convergence will occur within proximity to the front leading to a narrow corridor of convection to materialize Saturday evening over southern MI into the northern reaches of IN/OH which could enhance the flash flood threat further south with lingering elements over the southern peninsula of MI. Pending the latitudinal push of the front, it's plausible that flash flood concerns will continue for southern MI which would likely factor into a higher-end SLGT potential, or perhaps an upgrade in the risk if the threat is appreciable, and/or overlaps with areas hit hard earlier Saturday from the first wave of heavy rainfall. It will be something to monitor in the coming days as guidance gets a strong handle on the setup. ...South Florida and Keys... A Marginal Risk was raised for this period as a continuation from the Day 2 period. The environment previously described will still be present with convection likely ongoing over recent wet areas. The threat for localized flash flooding will persist across the tip of Florida and the Keys. Campbell ...Western and Central U.S... 21Z update... Convection over southwest Montana and into Idaho as depicted in the current suite of guidance was not covered by the inherited Marginal Risk. A westward expansion was made to encompass sensitive areas to potentially heavy rainfall and/or rain rates greater than 0.5 inch/hour. Campbell Our upper low over the Northern Rockies will slowly pivot east providing continued diffluent flow and sustained convective footprint across the interior west and High Plains of MT/WY. Several smaller mid-level perturbations around the base of the mean trough will eject east-northeast within the flow ahead of the upper low creating a threat for pockets of heavier convective potential over the Front Range of CO/WY while also still delivering heavier precip to MT as ascent remains strong under the closed low, as well as just to the north. Models are indicating 1-2", locally higher possible in both these regions with the area over the High Plains the most likely to see a stronger convective footprint with supercells and multi-cell clustering the probable convective modes given the increasing shear and forcing within modest instability. A broad MRGL is in place currently for both areas, encompassing a large area due to the influence from our slowly migrating upper low. Over the Central Plains, surface low will develop over the Corn Belt thanks to the pattern downstream, advancing east while dragging a cold front through the region. Scattered thunderstorms will be possible along and just ahead of the front as it advances southeastward leading to perhaps an isolated flash flood threat for any stronger storms forecast. The MRGL risk extends throughout much of the Central Plains and Missouri Valley as a result. Kleebauer Day 4 and Day 5 Valid 12Z Sun Sep 20 2026 - 12Z Tue Sep 22 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF THE MIDWEST AND NORTHEAST SUNDAY/SUNDAY NIGHT, AND ACROSS PARTS OF THE LOWER MID ATLANTIC MONDAY/MONDAY NIGHT... 2030Z Update: On day 4 (Sun-Sun night), based on the latest guidance trends (including GEFS/ECENS/CMC QPF exceedance probabilities), have nudged the Slight Risk a little farther north across the Midwest, to encompass much of eastern IA, northern IL, and northwest IN. Have also added a Slight across eastern PA-much of NJ into southern New England. On Day 5, while not an overwhelming consensus from the guidance, there are growing signals for at least a low-end Slight across parts of the Lower Mid Atlantic (central to SE VA and parts of eastern NC), ahead of the front and within the 2-2.25" PW plume. This also follows the uptick in QPF from the NBM (both deterministic and probabilistic), along with some of the AI models (AIGFS especially). Hurley Previous discussion... By Sunday/Sunday night, the upper trough makes its way east through the Northern Plains towards the Upper Midwest. A frontal boundary will be the focus for thunderstorms with heavy rainfall from West TX into the Midwest the OH Valley, then into the Mid- Atlantic States/Southern New England as a cold front approaches. Both NAEFS and ECMWF SATs show at least 90th percentile PWs in place from the MS Valley to the Mid- Atlantic. Since there is reasonable agreement, and it has rained heavily as of late, added a Slight Risk in and around Northern MO, figuring the ground won't fully dry out by then. Given the parameters present, hourly rain amounts to 2.5" with local totals to 4" are possible in the environment expected anywhere within the risk areas wherever thunderstorms merge or manage to train. For Monday/Monday night, an upper level low/trough moves towards Chicago. High enough moisture, instability, and effective bulk shear for organized thunderstorms with heavy rainfall is expected to stretch from portions of the OH Valley across the Mid-Atlantic States and southern New England within an area of decent 850 hPa confluence near and ahead of a front. Portions of the Southeast should have enough moisture/instability for heavy rain concerns around the northeast periphery of an upper level low, but it's unclear if enough effective bulk shear will be present to organize convection. A broad Marginal Risk has been introduced; there's not enough model agreement on a heavy rain axis to depict a Slight Risk at this time in and near OH/WV, but an upgrade can't be ruled out as we move closer in time to the possible impact, which bears watching. Hourly rain amounts to 2.5" with local totals to 4" would be possible, given the available parameters, anywhere within the Marginal Risk area wherever thunderstorms merge or manage to train. Roth Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt $$