279 FOUS30 KWBC 181559 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1159 AM EDT Fri Sep 18 2026 Day 1 Valid 16Z 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 IN SOUTHWEST MONTANA... ...Upper Mississippi Valley into Midwest... 16Z update... Latest guidance depicted higher QPF reaching southeast Wisconsin and northeast Illinois, areas that have had recent heavy rainfall and are sensitive to additional rainfall. The eastern boundary of the Slight Risk and the high-end Slight Risk were expanded about 1-2 tier of counties to the east. The highest amounts are still expected to focus over southeast Minnesota and west-central Wisconsin during this period. Campbell A disturbance over the Northern Plains will continue to track eastward from the Dakotas across southern Minnesota and into Wisconsin by late tonight. A moisture-laden atmosphere is already in place across much of the Upper Midwest will be supportive of local downpours as the disturbance approaches. The development of a low level jet will enhance the convective footprint across the Upper Midwest with much of the guidance showing a blossoming area of heavy precipitation across southern Minnesota, advancing eastward with the mean flow. Model consensus QPF remained in the 2 to 4 inch range...and the previously issued high-end Slight Risk area tended to cover the spread of higher QPF so only minor adjustments were needed. ...South Florida and Keys... 16Z update... A small northward nudge of the Marginal Risk area was made to reflect the latest model trends for South Florida. Campbell A Marginal Risk was maintained due to high rainfall rates being possible across the southern tip of the peninsula southward 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. ...Western U.S... 16Z update... Current radar and observations have organized convection lifting through portions of Utah, Nevada and Idaho that were either partially covered the Marginal Risk area or well outside of it. Some of these cells are producing locally heavy rainfall therefore the western bounds of the Marginal Risk was expanded westward to now include all of central/southern Idaho, eastern Oregon, northeast Nevada and northwest/west Utah. Additionally, with the Slight was adjusted a bit further east across south-central Montana. Campbell An upper low over Idaho will make its way into the interior Northwest and 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 situates itself over the northern Great Basin into the northern Rockies. Based on the cell behavior on Thursday and the deterministic QPF and spaghetti plots of 2 inch QPF from the GEFS and EC Ensemble member...introduced a Slight risk area in southwest Montana. The previously expressed concern about progressive cell motion working against the flash flooding potential still exists but the rates could still prove problematic. This area is consistent with a forecast of 0.5-1.5 inches. A Marginal Risk area also remained in place for a corridor from New Mexico and parts of Arizona northward into the Northern Rockies as lingering moisture and a diurnal instability maximum will lead to widely scattered thunderstorms for another afternoon leading to isolated flash flood concerns for yet another day, mainly in the terrain. Bann Day 2 Valid 12Z Sat Sep 19 2026 - 12Z Sun Sep 20 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE UPPER MIDWEST INTO PARTS OF THE SOUTHERN GREAT LAKES AND IN SOUTHERN MONTANA... ...Midwest and Ohio Valley... The environment will remain conducive for heavy rainfall with precipitable water values firmly at or above 1.8" across the entire region, locally exceeding 2 inches 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. In particular, there is potential for overlap between the rainfall on Saturday with a west-northwest to east- southeast stripe of low Flash Flood Guidance in northern Illinois which resulted from heavy rainfall 36 to 48 hours earlier. The RRFS was most supportive of this scenario but it has a bias of high QPF and higher probability of exceedance. Even so...finer scale models also show some overlap with locally heavy amounts so the Slight Risk was expanded in areal coverage and deemed a higher-end Slight Risk...especially if there are multiple rounds of convection. ...Western and Central U.S... Latest deterministic model and WPC trended upward across the Northern Rockies on Saturday as an upper low over the Northern Rockies slowly pivots eastward providing continued/prolonged diffluent flow. The flow aloft ahead of the upper low creating a threat for pockets of heavier convective potential over the Front Range of Colorado/Wyoming while also still delivering heavier precip to Montana as ascent remains strong under the closed low. As a result...there was enough support to upgrade a portion of southern Montana to a Slight Risk given the expect amounts on Saturday that would be in addition to what falls today. Over the Central Plains, scattered thunderstorms will be possible along and just ahead of an associated cold 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. ...South Florida and Keys... A Marginal Risk was continued this period as a continuation from the Day 1 period. High rainfall rates will be possible across the southern tip of the peninsula southward across the Keys with flooding a concern due to weak steering flow. Bann Day 3 Valid 12Z Sun Sep 20 2026 - 12Z Mon Sep 21 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE OHIO VALLEY AND ACROSS A SMALL PORTION OF THE NORTHEAST US... ...Ohio Valley/Southern Great Lakes... Surface low pressure moving eastward across the Mid-Mississippi Valley will help draw moisture northward into the Ohio Valley leading to a continued chance of showers and thunderstorms from Saturday into early Sunday. The rainfall amounts do not look to be blockbuster in magnitude but Flash Flood Guidance in the area is suppressed from several round of heavy to excessive rainfall. As a result...expanded the previously issued Slight Risk focused along the lower 1- and 3-hourly flash flood guidance values. Farther to the east...trimmed the areal coverage of a previously issued Slight Risk along parts of the Interstate 95 corridor from northern New Jersey into Connecticut based on shrinking deterministic QPF footprint and waning ensemble support. However...there should be good convergence along a well defined front separating strong/cool high pressure over Canada from unstable airmass to the south. Latest runs suggest that the threat of excessive rainfall should occur in the early part of the period in this area then drop as an airmass with lower precipitable water sweeps in. Rockies to the Central Plains... There continues to be a Marginal Risk of excessive rainfall from the Southern High Plains northward to the Western High Plains adjacent to the Northern Rockies as deep moisture remains in place with weak shortwave energy embedded helping to fuel scattered convection. ...South Florida and Keys... A Marginal Risk was maintained given a continuation from the Day 2 period. High rainfall rates will be possible across the southern tip of the peninsula southward across the Keys. Day 4 and Day 5 Valid 12Z Mon Sep 21 2026 - 12Z Wed Sep 23 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF THE SOUTHERN MID-ATLANTIC STATES AND NEW MEXICO... Southern Mid-Atlantic States... An upper level low/trough moves across the Upper Midwest into the Great Lakes while an upper low in the eastern Gulf shears northward ahead of the closed low and a surface high pressure system on the strong side of September climatology wedges in east of the Appalachians. Precipitable water values are forecast to be 1.75-2.25" across the Southern Mid- Atlantic States and Carolinas in the vicinity of where a confluence zone at 850 hPa will exist. Enough instability and effective bulk shear should exist for thunderstorms with heavy rainfall. Hourly rain amounts to 2.5" with local totals to 4" would be possible, given the available parameters wherever thunderstorms merge or manage to train. The best agreed upon area for Monday into Tuesday night for heavy rainfall/more numerous convection is the area in and around south to southeast VA, so a Slight Risk exists both days there. There's a bit of uncertainty concerning a heavy rain signal near WV and eastern OH and on either side of southeast VA, so left the a decent radius of a Marginal Risk around the Slight Risk. Southern High Plains... Increasing low-level Gulf moisture with time coupled with upper level Pacific moisture which may have a connection with a future northeast Pacific tropical cyclone should lead to an increasing threat for excessive rainfall with time in and near the Southern High Plains. Tuesday/Tuesday night looks better than Monday/Monday night, particularly across eastern NM where there's fairly good agreement on 2-4" areal average rain, which led to the introduction a Slight Risk area (day 5). On Tuesday and Tuesday night, enough effective bulk shear and instability should be available when combined with high moisture (1.25-1.5" precipitable water values) to allow for organized convection. Hourly rain amounts to 2.5" with local totals to 4" are easily supported in the forecast environment. Midwest/Southern Appalachians... The Midwest isn't showing a unified/cohesive signal for heavy rainfall Monday/Monday night but enough effective bulk shear, moisture, and instability should exist near an incoming upper low/trough for organized thunderstorms. The area around IN/OH/WV needs to be watched for a possibly upgrade to the Slight Risk should the guidance show a more cohesive/agreeable heavy rainfall signal considering recent ground saturation and the usual sensitivity in the Appalachians. 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 $$