FOUS30 KWBC 272054 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 454 PM EDT Thu Aug 27 2026 Day 1 Valid 16Z 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... ...16Z Update... The main adjustment to the overnight forecast includes a northward extension to the Slight Risk area in the Northeast based on the uptick in QPF depicted by 6-12Z CAM guidance. This upward trend in QPF is corroborated by 12Z RAOBs from ALY and OKX, which already depict "tall-skinny" CAPE and steadily increasing PW values tied to a low-level moisture plume advecting northward ahead of the front. Over the Gulf Coast, the Slight Risk was trimmed toward the coast based on observational trends as the MCS moves into the Gulf. Some uncertainty remains with the forecast as a well-defined outflow boundary surging out of the MCS may focus most of the thunderstorm activity south of the coastline. However, with a lingering front, MCV, and moist environment still in place any redevelopment later today will be capable of producing intense rainfall rates. In light of recent heavy rainfall, this still supports a threat of scattered flash flooding today in the Slight Risk area. Asherman ...Previous Discussion... ...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... Minor expansion made to the previous Slight Risk in the Mid- Atlantic based on the latest HREF and REFS guidance. The upper- level trough continues to linger in place while continuing to dig. A sharpening upper-level jet streak on the lee side of the upper trough will enhance deep-layer ascent as well and low-level FGEN, increasing the convective coverage and intensity during the peak heating hours Friday afternoon and evening. This forcing combined with PWs exceeding 2", 1000-2000 J/kg MUCAPE, and a stalled 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". 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. 12Z guidance has a signal for a weak circulation developing off of the Florida Panhandle over the next 24 hours, which focuses convection near the coastline. While FFGs in the are are high, a categorical upgrade could be needed if the heavy rainfall advertised from this feature trends further inland. ...Northern Plains... 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 some potential for heavy rainfall. Healthy moisture and solid instability will be present with PWs exceeding 1.5" and MUCAPE exceeding 2000 J/kg, respectively. 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. Over the Great Basin, anomalous mid-level moisture, on the order of 4-5 standard deviations above average, will stream northward from Post-T.C. Iselle and get swept into a deep mid-level trough propagating through the Pacific NW. MUCAPE values near 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. The Marginal Risk area was adjusted southward to reflect the highest HREF probability of MUCAPE exceeding 500 J/Kg -- where rainfall rates would be most intense. Asherman/Hurley 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 aforementioned disturbance in 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. Asherman/Hurley ...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 4 and Day 5 ...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PARTS OF THE WEST & NORTHERN TIER OF THE COUNTRY... ...Southwest/Great Basin/Intermountain West... A warm core high shifts from the Southern Plains into the Southern Appalachians 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 Hurricane Karina. A Marginal Risk of excessive rainfall remains for Sunday through Monday as above average moisture fuels diurnally driven widely scattered to scattered convection across the Southwest, portions of the Rockies, and parts of the Great Basin. 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 dying front near the Gulf coast region and behind a retrograding upper level disturbance moving through the central Gulf. The combination of what becomes an easterly wave and an increasing amount of ridging building westward from the western subtropical North Atlantic should lead to the heavy rain threat being constrained to portions of the northeast and north-central Gulf coast. With precipitable water values of 2"+ expected, hourly rain amounts to 3" are possible where cells merge or train. Depending upon how much rain falls during the short range period, a Slight Risk area may become necessary as we get closer in time. ...Upper Midwest/Northern Plains/Great Lakes... There is a risk of excessive rainfall persisting Sunday and Monday from the Northern Plains across the Great Lakes into the Northeast over the top of an eastward shifting warm core ridge where the flow becomes increasingly deep layer out of the west. Gulf moisture sneaks in at the low levels, which across the Plains and Midwest are more out of the southwest. Precipitable water values are expected to be in the 1.5-2" range. Where cells train or merge, hourly rain amounts to 2.5" are possible, which would be most problematic in urban areas. Changes to continuity were made to adjust to where the 12z guidance was most agreeable on a heavy rain signal which removed portions of the Midwest from the risk area. Based on the ingredients available, a Slight Risk will likely be needed somewhere between the Great Lakes and Northeast as we get closer in time. 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