FOUS30 KWBC 181932 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 332 PM 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 MODERATE RISK OF EXCESSIVE RAINFALL IN PORTIONS OF THE MIDWEST...PARTICULARLY EASTERN IOWA, NORTHERN ILLINOIS, AND NORTHERN INDIANA... ...Midwest and Ohio Valley... A Moderate Risk upgrade was coordinated with local WFOs primarily across portions of E IA, N IL, far S WI, far SW MI, and N IN. There is growing model signal for a concentrated corridor of heavy rainfall from multiple rounds of training convection. One round may be ongoing at the start of the period, a continuation from the current Day 1 period. Although hi-res guidance varies in the vigor and placement of the early morning convection, EAS probabilities from the HREF and REFS suggest it would be most likely around 12Z Saturday from southern Wisconsin into southwest Michigan. This may have implications for future rounds of convection if denser and more persistent cloud cover exists in those areas. In other words, what happens Saturday morning will have a significant role in the placement of the heavy rainfall maxima for the rest of the forecast period. However, there is reasonably high confidence in additional rounds of convection that should develop near the effective front as low-level flow veers and strengthens to around 30 knots. This configuration, and low-level inflow being close to parallel with the deep layer mean flow, should favor backbuilding and training. The overall risk area will be embedded within an anomalous deep moisture plume (PWs around or in excess of 2 inches, above the 99th percentile for mid- September). Combined with strong instability in areas that experience more sunshine and heating, this would be supportive of high hourly rain rates, potentially exceeding 2 inches at times. The Moderate Risk was drawn on the southern periphery of global model consensus to account for a typical bias (subsequent rounds of convection are often further into the inflow -- in this case to the WSW -- than guidance would indicate), and potential for morning cloud cover in the northern portions of the risk area. This places it closer to what the RRFS, REFS, AI-GFS and ECMWF-AIFS are depicting. Where the heavy rainfall footprints from the multiple rounds of convection overlap, there will be the potential for locally significant rainfall totals. These areas, or where intense rain rates intersect urban areas or saturated soils, significant flash flood impacts will also be possible. The Slight Risk was also expanded east to OH, far western WV, and far southwest PA, primarily for later in the forecast period as low-level westerly flow increases with PW values also on the increase. There is some potential for backbuilding and training convective bands (shown by some hi-res model reflectivity forecasts) in the vicinity of the hill country and western upslope regions of the Central Appalachian region by Saturday evening/night. ...Southern Montana... Anomalously high moisture (PWs near the mid-late September maximum) and a slow-moving mid-upper level low will provide the focus for a concentrated area of heavy rainfall in southern Montana. The primary uncertainty will be the degree of instability. Greater instability in combination with the available moisture could yield small bands of training convection with heavy rain rates on the north side of the upper low. The HRRR and RRFS depict some of the high end scenarios that could be realized with more instability, with localized 24-hour rainfall maxima as high as 7-8 inches. This would be well in excess of the 100-yr ARI thresholds, and both hi- res ensemble systems have very high neighborhood probabilities of reaching the 24-hour 100-yr ARI in southeast Montana (60-80 percent). If the instability is not realized, this could be high, but the trend has been toward the higher end of the Slight Risk probability range today, and a targeted Moderate Risk could need to be considered in future updates if guidance continues to increase. ...Elsewhere... Existing Marginal Risk areas were largely maintained with minimal edits. Scattered thunderstorms within a broader plume of anomalously high moisture could produce locally heavy rainfall and flash flooding from the Rockies to the Central Plains and as far east as the Central Appalachians. However, confidence in placement of localized heavy rainfall within the Marginal Risk area is low. Additionally, the Marginal Risk areas were maintained in West Texas and the Texas Big Bend, as well as South Florida. Lamers 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 MIDWEST AND OHIO VALLEY, AND ACROSS A SMALL PORTION OF THE NORTHEAST US... ...Ohio Valley/Southern Great Lakes... Very few changes were made to the existing Day 3 ERO. The primary adjustment was to expand the Slight Risk a bit further into the Upper Ohio Valley and Central Appalachians (SE OH, SW PA, WV). As described in the Day 2 ERO, there is some potential for backbuilding and training convective bands affecting those areas from late Saturday Night into Sunday as low-level westerly flow increases and PWs increase close to 2 inches in some spots, which is above the 99th percentile for mid-late September. Further west, the Slight Risk was mostly kept the same, and it still extends from E IA into N IL and N IN. Some of the threat may be due to a continuation of convection from late in the Day 2 period (late Saturday Night) into Sunday morning. As certainty grows in where the heaviest rain will fall Saturday and Saturday evening, it's possible the risk level could need to be increased if it seems likely repeated rounds of heavy rain will affect saturated soils into Sunday. However, confidence is low on placement or convective evolution beyond the first 36 hours of the forecast, so this is being noted as a possibility for now. Anyone within the Slight Risk area should be monitoring the potential for flash flooding, however, given just how anomalously high the moisture levels are. Farther to the east, the Slight Risk was maintained, although the ensemble support continues to wane a bit for very heavy rain rates. There is good potential for rain in portions of the Northeast, and there should be good convergence along a well defined front, but available instability may be limited. Therefore, although the chances of rain are high, this may be distributed more evenly through the forecast period rather than in intense bursts. Guidance will continue to be evaluated as to whether to maintain the Slight Risk, trim the area, or convert it to a Marginal Risk. 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. ...Elsewhere... A large Marginal Risk area was maintained from the northern Rockies through much of the Plains and into western Texas. Scattered thunderstorms embedded within a large plume of anomalously high moisture will pose the threat for localized heavy rainfall and high instantaneous rain rates. However, confidence is low on placement, so a Marginal Risk seems appropriate. The one area that may be most likely to see a risk upgrade in the future would be in the vicinity of the upper low progressing from Montana and Wyoming into the western Dakotas. A Marginal Risk was also maintained in south Florida where high rainfall rates are also expected. Lamers 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... 20Z Update... For Day 4, the biggest adjustment was the removal of the Slight Risk from southern Virginia and north-central North Carolina. The overall signal for heavy amounts has diminished, while model spread has increased. This increased uncertainty, along with relatively dry antecedent conditions, supported the removal of the Slight Risk for now. For Day 5, with a little more model support and relatively greater forecast confidence, maintained the Slight Risk in the Mid Atlantic with some relatively minor adjustments. A bigger change was the addition of a Slight Risk centered over the upper Ohio Valley from southwestern Pennsylvania to southern West Virginia. There is a decent signal for heavier QPF supported by deepening moisture along an inverted trough in this area. In addition to the growing signal for heavy rain, relatively low FFGs indicate this area is more susceptible to flash flooding. Elsewhere on Day 5, broadened the Slight Risk in New Mexico to extend into southwestern New Mexico -- where there is fairly good signal for organized amounts, supported in part my mid-level energy lifting north into that part of the state. Pereira Previous Discussion... 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 $$