FOUS30 KWBC 270818 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 418 AM EDT Thu Aug 27 2026 Day 1 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... ...Northeast/Mid-Atlantic... The Slight Risk area inherited from yesterday's Day 2 ERO was expanded a bit southwest to include the DC metro region. Deep-layer ascent ahead of the broad upper trough/surface front will get a boost from a developing 70-90kt upper jet streak on the lee side of this trough, adding to the frontogenetic support during peak heating (max diurnal destabilization) later this afternoon into early evening. Mixed layer CAPEs of 1000-2000+ J/Kg with PWATs approaching 1.75" will support locally intense rainfall rates (at least 2" within an hour) under the most intense cells. Simulated reflectivity guidance from multiple high-res CAM runs show the potential for multiple rounds of convection (at least 2, including the main pre-frontal QLCS), especially within the Slight Risk area. Probabilities of exceeding 3"/24hr ending 12Z Fri per the 00Z HREF peak between 40-70% within the Slight Risk area, with a similar range advertised by the REFS (though with the highest probs over northern NY into Upstate NY). The 00Z REFS in fact has a small area of 40-50% probabilities of exceeding 5" across portions of Update NY. ...Southeast and Gulf Coast... The tail-end of the longwave upper trough, with distinct shortwave energy traversing the Gulf Coast as per the 00Z guidance, along with the attendant (eventually quasi-stationary) surface front, will provide the dynamical focus for more organized (widespread) areas of convection across this region. 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. Hurley/Blanco-Alcala Day 2 Valid 12Z Fri Aug 28 2026 - 12Z Sat Aug 29 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE COASTAL CAROLINAS INTO SOUTHEAST VIRGINIA... ...Southeast/Mid-Atlantic... Some changes were made to the Slight Risk area inherited from yesterday's Day 3 ERO, most notably to extend the northern portion to include SE VA based on the latest guidance and (in particular) the bump in REFS 24hr QPF exceedance probabilities. The upper- level trough mentioned in the Day 1 discussion continues to linger in place while continuing to dig. A sharpening upper-level jet streak on the lee side of the upper trough will boost deep-layer ascent as well as low-level FGEN, increasing the convective coverage and intensity during the peak heating hours Friday afternoon into the evening hours. PWATs meanwhile area expected to range between 2.00-2.25" per the ECMWF and GFS. High PWATs, decent dynamical support, and a stalling surface front will likely lead to multiple rounds of heavy rainfall within the Slight Risk area, with the strongest cells capable of hourly rain rates of up to 3". 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 and associated surface warm front will quickly punch through/somewhat flatten the upper ridge and 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 and Portions of the Great Basin... 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. A Marginal Risk was retained over portions of the Sierra Nevada up into the Great Basin for Day 2 (Friday-Friday night). 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. Hurley/Kebede/Blanco-Alcala Day 3 Valid 12Z Sat Aug 29 2026 - 12Z Sun Aug 30 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHWEST AND GREAT BASIN INTO THE ROCKIES AND FRONT RANGE, NORTHERN PLAINS INTO THE UPPER MIDWEST, AND THE SOUTHEAST TO THE CAROLINAS... ...Southwest and Great Basin into the Rockies and Front Range... The upper high weakens/flattens 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 Sat-Sat night as above average moisture for the monsoon fuels diurnally driven widely scattered to scattered convection. The pattern would support 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. ...Northern Plains into the Upper Midwest... Pattern remains unsettled across this area during Day 3 (Sat-Sat night), owing to shortwave energy (convectively-aided) riding along the periphery of the upper ridge ("ring of fire"), along with the persistent moisture anomaly in place (PWATs ~2 standard deviations above normal). Still quite a bit of spread among the guidance QPFs; however, given the degree of instability and moisture in play, some of the stronger storms will be capable of intense short-term rainfall rates and localized flash flooding. Hurley/Roth Day 4 and Day 5 Valid 12Z Sun Aug 30 2026 - 12Z Tue Sep 01 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 the 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 the latter part of the weekend into early part of the upcoming workweek as above average moisture 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 remain 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. The combination of the surface front weakening and an increasing amount of ridging building westward from the western subtropical North Atlantic should lead to a decreasing heavy rain threat across the Carolinas and a portion of the Southeast US. 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 latter part of the weekend and into the early portion of the upcoming workweek across parts of the Upper Midwest/Northern Plains/Great Lakes. A mid- and upper-level low entering the Pacific Northwest on Sunday will result in backing mid- and upper level flow and increasing divergence aloft over the Northern Plains to the lee of the Rockies. At the same time...locally heavy rainfall will develop over parts of the Upper Midwest into the Great Lakes region as moisture flux increases between the subtropical high to the east and lowering surface pressure to the west. The threat of heavy rainfall moves north of the International border by Monday while the risk across the Great Lakes persists where isolated maximum rainfall rates of 1 to 2 inches per hour will be possible. Bann 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 $$