FOUS30 KWBC 200108 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 908 PM EDT Sat Sep 19 2026 Day 1 Valid 01Z Sun Sep 20 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... ...01Z Update... ...Central Plains to the Central Appalachians... A Moderate Risk remains in place across portions of the Midwest, extending from eastern Iowa to southwestern Michigan and northwestern Indiana. It is expected that scattered showers and storms will continue along this axis through the remainder of the evening, followed by more organized activity that is already developing over the central Plains and lower Missouri Valley and is expected to propagate northeast through the remainder of the evening. This latest round of developing activity is supported by a well-defined shortwave and a steady transport of moisture from the south. The north-south gradient of the Moderate Risk and surrounding Slight and Marginal Risks was tightened some. However, the Slight Risk was elongated a little farther east and west. There has been some model signal for shower and storms to propagate east and produce locally heavy amounts within the typically more runoff sensitive and lower FFG parts of West Virginia. There has also been some signal for locally heavy amounts developing back along the axis of deeper moisture and stronger southwesterly inflow extending back across southern Iowa into northwestern Missouri and eastern Kansas, where activity is now firing. Within the Moderate Risk area itself, both the 18Z HREF/REFS support localized additional amounts of 2-3 inches overnight -- which given the very moist soils from previous heavy rains is likely to produce additional runoff concerns. ...Northern to Central High Plains... Anomalous moisture interacting with a closed low will continue to produce widespread precipitation across the region, with additional localized heavy amounts possible through the remainder of the evening and into the overnight. Instability has been limited across much of Montana, keeping rates in check. With instability expected to remain a limiting factor, the Moderate Risk was removed with this update. A Slight Risk was maintained for parts of south- central to southeastern Montana where low level easterly inflow overlapped by mid-to-upper level forcing may help produce some locally heavier amounts overnight, which may produce localized runoff concerns given the low FFGs produced in part by earlier rains. Some of the heaviest amounts overnight may occur farther south, supported by the greater instability extending north from the central High Plains into parts of western South Dakota and eastern Wyoming. A Slight Risk was maintained where the hi-res ensemble suites show some potential for localized amounts over 2 inches in this region. ...Southwest into the Southern Plains... Made some minor adjustments based in large part on mesoanalysis and radar trends, including extending the Marginal Risk northeastward across the Texas Panhandle and western Oklahoma -- reflecting ongoing activity that is being supported by an axis of deeper moisture and low level convergence. Pereira Day 2 Valid 12Z Sun Sep 20 2026 - 12Z Mon Sep 21 2026 ...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL OVER PORTIONS OF THE MIDWEST AND OHIO VALLEY AS WELL AS THE NORTHERN HIGH PLAINS... ...2030Z Update... ...Midwest and Ohio Valley... A modest adjustment was made to the inherited Slight Risk based on expected progression of the frontal boundary over the Midwest. Most guidance indicates the western portion of the front will push further southward from the Great Lakes towards the Middle Mississippi Valley, likely aided by growing outflow due to repeated rounds of storms to the north Saturday and through Saturday night. This would set up the axis of highest instability/forcing and potential for training along the front from northeastern Missouri east through central Illinois and Indiana. The RRFS maintained an axis further to the northwest but this seems more unrealistic based on the ongoing convection. Given the influence of ongoing convection, additional changes are possible. There is higher confidence in both the placement and potential for heavier rainfall/flash flooding further east across the Upper Ohio Valley. Storms may be ongoing at the start of the period before a renewed round in the afternoon, and rainfall overall from Saturday will lead to more sensitive antecedent conditions compared to further west where storm coverage will be further south than the day before. A bit higher/anomalous moisture pooling in the valley will also bring a greater chance of heftier totals in the 2-5" range. ...Northern High Plains... Minimal areal adjustments were made to the inherited Slight Risk over portions of the northern High Plains based on the now available CAM guidance. A slow moving upper-low over the Rockies will begin to progress eastward as an open wave later Saturday and into the day Sunday encouraging a renewed round of storm development. Anomalously moist upslope flow (PWs remaining 2 to 2.5 standard deviations above the mean) will continue to promote efficient rainfall with rain rates of 1-1.5"+ per hour. The CAM guidance shows the potential for localized totals in the 2-4" range particularly within the Black Hills and vicinity. The region will also be more sensitive to additional rainfall following a prior bout of storms and heavy rainfall on Saturday. Elsewhere, the broader Marginal Risk across the Plains was maintained as is. There likely is a lower potential for even isolated flash flooding across the central Plains northeast into the Middle Missouri Valley and Upper Midwest given more limited instability north of the front and away from terrain induced upslope flow. However, there was still enough of a signal for some multi-inch totals in the guidance particularly into the Missouri Valley so left the Marginal Risk as is for now. Similar to Saturday, afternoon thunderstorms fueled by a plume of anomalously high moisture will lead to some very heavy downpours and isolated flooding/ponding concerns for South Florida. Putnam ...Previous Discussion... ...Ohio Valley/Southern Great Lakes... Very few changes were made to the placement of the Day 3 Excessive Rainfall Outlook as it propagated into the Day 2 period. There was a tightening of the east-west length and a subtle southward shift to match the latest guidance but maintained the focus for heavy rainfall in the Ohio Valley, There remained some potential for backbuilding and training convective bands affecting those areas from late tonight into Sunday. 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 removed as model and ensemble QPF dwindled. Cool, stable air being fed out of Canada by a sprawling area of high pressure will overspread the area and limit rainfall rates over an increasing amount of New England and the Northeast states. A boundary separating this airmass with moister and more unstable conditions (still fairly modest magnitude of instability) to the south may still prove to be the focus for convection capable of producing moderate to heavy rainfall rates but the signals for heavy rainfall have become weaker and more isolated in nature. 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. Bann Day 3 Valid 12Z Mon Sep 21 2026 - 12Z Tue Sep 22 2026 ...THERE IS A BROAD MARGINAL RISK AREA FROM PARTS OF THE SOUTHERN MID-ATLANTIC STATES AND NEW MEXICO... ...2030Z Update... Coverage of the inherited Marginal Risk across the southern Mid- Atlantic states was reduced given a cold air damming pattern setting up throughout the region early next week. The latest guidance shows that the cold front will push southward through the day Sunday over Maryland and northern Virginia and into southern Virginia and North Carolina by Monday morning. Rain chances after this further north will be much more limited with lower rates. However, forcing and moisture convergence along the front as it stalls along the central Appalachians will keep heavier rain chances and isolated flash flooding a concern here. Otherwise, the previous forecast for the Southern High Plains remains on track. A Marginal Risk remains in place for South FLorida as another day of thunderstorms fueled by a plume of anomalously high moisture will bring some very heavy downpours and the risk of some isolated flooding/ponding for urban areas. Putnam ...Previous Discussion... Southern Mid-Atlantic States... An upper level low/trough moving from the Upper Midwest into the Great Lakes while an upper low in the eastern Gulf shears northward ahead of the closed low will set the stage for showers and thunderstorms on Monday. Model spread increased while the overall signal for heavy amounts diminished during the past 24 hours. Given the increased uncertainty combined with relatively dry antecedent conditions, was not inclined to reintroduce a Slight Risk area. However...isolated instances of excessive rainfall are possible Southern High Plains... Increasing low-level Gulf moisture with time could lead to an increasing threat for excessive rainfall in and near the Southern High Plains as mid-level flow increases over the central US and starts to push a surface front eastward. Precipitable water values should generally be around 1.75 inches from Texas northward into the Mid-Mississippi Valley with enough effective bulk shear and instability to result in organized convection capable of producing heavy rainfall with the associated risk of flash flooding. 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 the expected rainfall in the Day 1 period and how much of that lingers into Day 2. Bann Day 4 and Day 5 Valid 12Z Tue Sep 22 2026 - 12Z Thu Sep 24 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF THE APPALACHIANS AND NEW MEXICO... 20Z Update... No major changes for most of the previous outlook areas. Given the lingering axis of deep moisture (PWs ~2.0 inches) -- fueling the potential for additional heavy rainfall -- continued a Marginal Risk into Day 5 across parts of South Florida. Otherwise, made minor adjustments based on the 12Z guidance. Pereira Previous Discussion... ...Ohio Valley and Mid-Atlantic... Mid level troughing with multiple embedded shortwaves will slowly track eastward into the OH Valley and Mid-Atlantic, interacting with a backdoor front. Should be plenty of moisture overrunning this front, resulting in what should be a broad area of steady rainfall. However, quite a bit of uncertainty with the details, as a coastal low should eventually develop offshore, helping advect drier air in from the north over portions of the Mid-Atlantic. This trend has resulted in a southward trend in the rainfall axis on day 4. Also of question is the degree of instability, with the backdoor front likely making a push all the way into the Carolinas, leaving a more stable airmass to its north. Given these uncertainties we opted to drop the central/southeast VA Slight risk with this update. Overall just enough uncertainty on placement, and questionable peak rainfall rates given the limited instability, to be confident in a scattered flash flood risk. We will keep a Marginal though, as plenty of moisture is in place, so even just a bit of CAPE could drive some low topped convection and locally higher rates. We will continue to monitor and possible a Slight risk is reintroduced somewhere over the region as we get closer and confidence increases. The Slight risk over PA and WV on day 4 (Tue) was maintained. An inverted trough should help focus convection here, and while still weak, up to ~500-1000 j/kg of CAPE could develop. In addition, this area has lower FFG than the coastal plain, and should see additional rain in the days leading up to Tuesday, potentially lowering FFG further. Similar setup on day 5 (Wed), just with even more uncertainty as model spread increases. The degree of model spread supports keeping the risk level at Marginal, with the understanding an embedded Slight risk is possible as we get closer. The best chance appears to be from TN/KY into OH and WV as the upper low moves overhead and decent instability is forecast. If the 00z EC and AIFS are correct, a Slight risk will be needed on future updates. However, model spread is high and run to run continuity is low. The GFS is more aggressive along the coastal plain with an offshore low bringing heavy rain inland. However, it generally seems like an outlier at this time, with both the GEFS mean and AIGFS more offshore (although still more aggressive than the non-ncep guidance). ...New Mexico... A favorable setup for heavy rain and scattered to numerous instances of flash flooding appears to be setting up for New Mexico on days 4 and 5, and probably continuing into days 6 and 7 as well. Conditions will likely get more sensitive with time as we see multiple days in a row with areas of heavy rainfall, and this may very well end up being a MDT risk event at some point during the Tue-Fri timeframe. Broad troughing in the west will place NM persistently within the right entrance region of a strong upper jet. One stronger wave will move to the north Tuesday, helping enhance lift, with a few more subtle waves on Wednesday providing ascent. We will also have easterly upslope flow in the low levels, sufficient instability, and eventually a possible PW connection to an eastern Pacific tropical system. This combination of forcing, instability and well over 90th percentile PWs for September, suggest heavy rainfall and a growing flash flood risk. We will carry Slight risks for now given we are several days out and some uncertainties continue with the details, but overall this seems to be an above average confidence forecast for this lead time, and would expect to see some flash flood issues arise. Can not rule out the potential for locally significant flash flooding in that Tue- Fri timeframe given the setup, so we will need to closely monitor trends. Chenard 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 $$