FOUS30 KWBC 262023 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 423 PM EDT Wed Aug 26 2026 Day 1 Valid 16Z Wed Aug 26 2026 - 12Z Thu Aug 27 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PARTS OF THE LOWER MISSISSIPPI VALLEY... ...16z update... The synoptic setup mentioned in the previous discussion remains true. Minor adjustments were made to the marginal and slight risk areas based on 12z guidance. Deep layer moisture is present over the Lower Mississippi Valley, based on the latest advected layer precipitable water imagery, though low-level/surface PWATs aren't very anomalous (1-2 standard deviations). A stationary front draped across the Southeast as well as a surface wave over the Lower Mississippi Valley should act as a forcing mechanism for convective initiation along with the upper-level energy noted previously. Elsewhere, storms initiating over eastern Colorado could still produce localized flash flooding this evening over sensitive soils/surfaces. ...Previous Discussion... ...Lower Mississippi Valley through the Upper Great Lakes... An upper-level trough over the Upper Great Lakes will continue digging throughout the period, pushing a cold front into Arkansas in the evening, and remaining in place in the overnight hours. Meanwhile, a shortwave in the mid levels will dive down from the northwest, riding the eastern periphery of a ridge in the Southwest. As this shortwave approaches, localized enhancement of upper-level divergence at the 250mb level is expected, particularly over Arkansas. As such, storms are expected to fire over Missouri and Arkansas, and move into Mississippi and Louisiana by the evening and overnight. Efficient rainfall is expected, with PWATs approaching 2.25". There stands medium-to-high chances of rain rates exceeding 1"/hr per the HREF. Storms will also have the potential to backbuild given weak (5kts) and near- orthogonal Corfidi vectors relative to the mean 850-300mb winds. All that said, the latest run of CAMs guidance raised more uncertainty as to the placement to the expected evening-to-overnight rainfall than desired. There is a surprising amount of discrepancy between the 00Z runs of the HREF & RRFS particularly, with the RRFS favoring heavier rain over northern Louisiana compared to the HREF's Mississippi and Arkansas solution. The insistent heavy rainfall suggested by the RRFS will struggle to exceed FFGs in Louisiana anyway, and as such the MRGL remains in place there. Otherwise, minor expansions were made for the inherited SLGT risk area into northeast Arkansas and also into Alabama. Elsewhere, the aforementioned cold front will bring potentially heavy rainfall across portions of Illinois, Indiana, and Ohio. Rainfall rates associated could reach 0.5-1.0"/hr, spanning from Michigan through Indiana. Storms will be quite progressive and short lasting, and as such, the inherited Marginal Risk was largely kept intact besides cosmetic adjustments to account for the last run of models. ...Southwest and Rockies... Monsoonal moisture will again bring diurnal thunderstorms over the southwest, although a southwest upper-level ridge will continue to move moisture away from the Arizona and New Mexico area. As such, the inherited MRGL Risk area was merely adjusted to match the latest suite of model guidance, but remains largely unchanged. Kebede/Blanco-Alcala Day 2 Valid 12Z Thu Aug 27 2026 - 12Z Fri Aug 28 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL GULF COAST AND THE NORTHEAST/MID-ATLANTIC... ...2030z Update... The slight risk in the Northeast/Mid-Atlantic was expanded south and eastward into the Baltimore to NYC I-95 corridor due, in most part, to antecedent conditions (soil moisture/FFGs). Urban areas like Baltimore, Philadelphia and New York City will be susceptible to flash flooding. The maximum axis of QPF is likely to develop ahead of the approaching cold front and along an elevated PW and MUCAPE axis. 1-2 inches of rain are expected within the slight risk area with 2-4 inch maxima possible in the strongest storms. Rainfall rates of 1-2" are also possible in some areas. The Southeast and Gulf Coast slight risk area was shaved off and confined to the immediate Central Gulf Coast due to a notable decrease in QPF over portions of Georgia and north/central Alabama. Kebede ...Previous Discussion... ...Northeast/Mid-Atlantic... A Slight Risk upgrade was introduced over the Northeast with this update. An upper-level trough will continue to slowly inch its way eastward, enhancing upper divergence aloft, especially upwind over southern NJ and southeast Pennsylvania. A healthy influx of moisture is forecast to flow onshore from the Gulf stream to the south, with around ~20kts 850mb southerly flow expected. PWATs around 1.75-2" will support heavy rainfall which will be continuously fueled by the aforementioned low-level flow. Latest global model runs are in agreement as to the general area of the QPF core in the northeast. The 00Z run of the RRFS appears to also converge toward a northern Mid-Atlantic to Northeast solution, albeit much more bullishly compared to the global solutions. Given this, the Slight area was introduced, and the availability of more CAMs guidance should result in refinement/fine tuning of the risk area. ...Southeast and Gulf Coast... An upper-level shortwave over the eastern portion of the country will continue pushing southward, with a surface cold front moving southeastward. Upon arrival to the Gulf Coast, it will stall over Louisiana and southern Mississippi for the next few days. While approaching and eventually residing near the coast, the boundary will contend with ample moisture moving northward from the Gulf, yielding multiple rounds of storms with the potential for heavy rainfall. Flood prone urban areas such as New Orleans, Baton Rouge, Mobile, and some of the more interior areas of Mississippi dealing with the latest of multiple days of heavy rains will all support widely scattered instances of flash flooding. Rainfall and thunderstorms will bring heavy rainfall to Alabama and Georgia as well, and the previously mentioned trough encounters a similarly moist airmass being pushed northeastward in the lower levels. This will support several rounds of storms in the form of multiple lines across Georgia and parts of South Carolina. The storms moving northeastward could allow for some training as the cold front moves through, and could bring widely scattered instances of flash flooding. ...Southwest... As similarly stated in previous outlooks, a very gradual eastward progression of the upper ridge across the Southwest should allow for a shifting of the heaviest precipitation to the east out of Utah and more into the central Plains. The ridge should largely preclude much organization from the storms, so the Marginal continues to focus on the areas where either terrain or prior rainfall could locally increases the risk of flash flooding. Blanco-Alcala/Wegman Day 3 Valid 12Z Fri Aug 28 2026 - 12Z Sat Aug 29 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE COASTAL CAROLINAS... ...2030z Update... A marginal risk was introduced over portions of the Sierra Nevada up into the Great Basin for this Friday. Anomalous mid-level moisture, on the order of 4-5 standard deviations above average, will stream northward from Tropical Storm Iselle and get swept into a deep mid-level trough propagating through the Northwestern U.S. on Friday. MUCAPE values between 250-500 J/Kg working their way north through the southern Great Basin/Sierra should be enough to produce heavy rainfall and at least isolated flash flooding out of the available moisture. Kebede ...Previous Discussion... ...Southeast/Mid-Atlantic... A Slight Risk was introduced in this update on the coast of the Carolinas, as a stationary boundary remains in place fueling repeated thunderstorms. The upper-level trough mentioned over the previous days' discussion continues to linger in place while continuing to dig. A mid-level shortwave will move to the base of the trough and will focus convection to the vicinity of a stationary front. This front will continue to remain in place throughout the day, while a secondary cold front moves in behind from the Great Lakes. Low level flow will begin to take the shape of an inverted trough, with southerly flow moving onshore from just off the coast in the Atlantic. PWATs at this time are expected to approach 2.25" per the ECMWF and EC-AIFS. As it moves onshore, an axis of moisture convergence is expected to develop and aid in lifting moist air for the development of thunderstorms. High PWATs will bring the potential of hourly rain rates of up to 3", particularly where cells merge or train. As such, scattered instances of flash flooding will be possible along the coast. Heavy rainfall is also expected in the surrounding areas extending from the Gulf Coast to southern Virginia, in which isolated instances of flash flooding are possible. ...Northern Plains... A plume of upper-level energy by way of a shortwave trough will quickly move through the eastern Dakotas and western Minnesota and Iowa. This will bring a progressive complex of thunderstorms into the area, with potential for heavy rainfall. Healthy moisture and solid instability will be present, with PWATs exceeding 1.5", and MUCAPE exceeding 2000 J/kg. However, given the progressive nature of thunderstorms, the Marginal risk area was maintained. ...Southwest... Monsoonal moisture continues the fuel diurnally drive convection along the southwest. A plume of moisture will drive PWATs around 1.5-1.75" northward along much of the Southwest. This will lead to renewed chances of isolated flash flooding, as has been similar for much of the monsoon season. Blanco-Alcala Day 4 and Day 5 Valid 12Z Sat Aug 29 2026 - 12Z Mon Aug 31 2026 ...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PARTS OF THE WEST, SOUTHEAST, PLAINS, UPPER MIDWEST, & GREAT LAKES... ...Southwest/Great Basin/Intermountain West... A warm core high weakens across the Southern Plains as troughing becomes increasingly established over portions of the West and Northern Plains. Moisture gets funneled in between the two features, which may have a connection to a future northeast Pacific tropical cyclone. A Marginal Risk of excessive rainfall remains for this weekend as above average moisture for the monsoon fuels diurnally driven widely scattered to scattered convection. The degree of moisture expected could lead to hourly rain amounts to 2" wherever cells sit, merge, or train. This could lead to isolated flash flooding...with favored terrain, urbanized areas, box/slot canyons, arroyos/dry washes, and burn scars being the most prone to flooding. ..Southeast... Showers and thunderstorms capable of producing heavy to excessive rainfall each day should be focused in the vicinity of a quasi- stationary front near the Gulf coast region and behind a retrograding upper level disturbance moving through the central Gulf, luring the dying front west as an easterly wave Sunday and Sunday night. Increasing ridging poking in from the western subtropical North Atlantic should lead to a decreasing heavy rain threat across the Carolinas with time. With precipitable water values of 2"+ expected, hourly rain amounts to 3" are possible where cells merge or train, particularly near the Gulf Coast. ...Upper Midwest/Northern Plains/Great Lakes... There is a risk of excessive rainfall persisting through the weekend across parts of the Upper Midwest/Northern Plains/Great Lakes. Precipitable water values of 1.5 to 1.75 inches should already be in place...having been drawn into the region prior to this weekend. That should be enough to result in hourly rain amounts to 2" are possible wherever cells can merge or 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 $$