FOUS30 KWBC 190829 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 429 AM EDT Sat Sep 19 2026 Day 1 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... ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF SOUTHERN MONTANA TODAY INTO TONIGHT... ...Midwest and Ohio Valley... Maintained the Moderate Risk area that was coordinated on Friday for portions of the Midwest and Ohio Valley. The model signal remained strong for a concentrated corridor of heavy rainfall from multiple rounds of training convection. One round will be on-going over parts of southwestern/southern Wisconsin at the start of the period after leaving a stripe of excessive rainfall that began over far southeast Minnesota and made its way into southwest Wisconsin. Questions remain about the ramification lingering rainfall and cloud cover have on convection evolution later in the period. With 1.5 to 3inches of rainfall that has already fallen as of 0730Z in parts of southwestern Wisconsin and more expected...the Moderate risk area was expanded northward. 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. Maintained the placement of the Moderate Risk area primarily on the southern periphery of global model consensus to account for a typical bias (subsequent rounds of convection are often further into the inflow of unstable air) than guidance would indicate. It still lines up well with the depiction shown by the RRFS, REFS, AI- GFS. and ECMWF- AIFS. Where the heavy rainfall footprints overlap, the potential for locally significant rainfall totals with considerable impacts are possible. 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 Ohio, far western West Virginia, and far southwest Pennsylvania, 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... Upgraded to a Moderate Risk in parts of southern Montana as deterministic and ensemble guidance QPF continued to increase. 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. The primary uncertainty continued to be with 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 WPC deterministic QPF increased to nearly 3.5 inches while the 00Z HRRR and RRFS continued to 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. They also depicted neighborhood probabilities in excess of 70 percent at the 100-yr ARI threshold for 24 hours in southeast Montana. If the instability is not realized, this could be high, but the trend has been toward the higher values over the past few outlook cycles. As a result...a targeted Moderate Risk was hoisted. ...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 remained low. Additionally, the Marginal Risk areas were maintained in West Texas and the Texas Big Bend, as well as South Florida. Bann Day 2 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 THE OHIO VALLEY... ...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... 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... ...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 $$