147 FOUS30 KWBC 280806 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 406 AM EDT Fri Aug 28 2026 Day 1 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 AND COASTAL GEORGIA... ...Southeast/Mid-Atlantic... The HREF and REFS guidance continue to depict much of the Carolina Coastline and surrounding areas to reach/exceed 2 inches during this period. Local enhancement can be expected during the afternoon and evening hours. 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 with the strongest cells capable of hourly rain rates of up to 3". The latest guidance concentrates the highest rainfall QPF along the coastline from eastern Georgia to southeast Virginia. The inherited Slight Risk was expanded southward to cover all of South Carolina coastline and into eastern Georgia. Of note, a signal persists for a weak circulation to develop offshore 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 progressive thunderstorm complex is expected to move through the northern states as a shortwave trough and associated surface warm front rapidly advances through/somewhat flattens the upper ridge. Although PWs exceeding 1.5" and MUCAPE exceeding 2000 J/kg will be favorable for for producing heavy rainfall, the threat for localized flash flooding will be limited given the progressive nature of individual storms. A Marginal Risk was maintained. ...Southwest and Portions of the Great Basin... Monsoonal moisture will continue to 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 remains in effect across the region. Campbell/Asherman Day 2 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... A quasi-stationary frontal boundary will be draped across the Gulf Coast region during this period and will continue to provide a focus for convection capable of producing heavy to excessive rainfall along with aforementioned disturbance in the central Gulf. 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... Shortwave energy will be traversing the edge of the upper ridge along with the persistent moisture anomaly in place (PWATs ~2 standard deviations above normal. 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. A Marginal Risk spans from the Dakotas to Wisconsin and portions of Upper Michigan. ...Southeast and Gulf Coast... The tail-end of the longwave upper trough, with distinct shortwave energy traversing the Gulf Coast, along with the attendant (eventually quasi-stationary) surface front, will provide the dynamical focus for more organized (widespread) areas of convection across this region. Multiple rounds of potentially heavy rainfall for this period will 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. A Marginal Risk area remains in effect for this period. ...Southwest... The upper ridge will continue its gradual eastward shift across the Southwest which in turn will also shift the footprint of the heaviest precipitation. For this period, it is expected to set up mainly east of Utah into the Central High Plains. Given the ridge, storms will generally lack organization therefore the Marginal Risk continues to focus on the areas where either terrain or prior rainfall could locally increases the risk of flash flooding. Campbell Day 3 Valid 12Z Sun Aug 30 2026 - 12Z Mon Aug 31 2026 ...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PARTS OF THE WEST,NORTHERN TIER OF THE COUNTRY AND THE GULF COAST... ...Southwest/Great Basin/Intermountain West... A Marginal Risk of excessive rainfall remains for 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... Convection near the stalled/dying frontal boundary and retrograding upper level disturbance moving through the central Gulf will persist through this period to continue to the threat for isolated instances of flash flooding. 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. ...Upper Midwest/Northern Plains/Great Lakes... The upper ridge will continue to shift eastward during this period while the lower levels see and uptick in Gulf moisture spreading across the Plains and Midwest. Precipitable water values are expected to be in the 1.5-2" range. Where cells train or merge, hourly rain amounts to 2.5". At this time Marginal Risk areas remain in effect, however with time, there may be a need for a Slight Risk somewhere between the Great Lakes and Northeast as we get closer in time. Campbell/Roth Day 4 and Day 5 Valid 12Z Mon Aug 31 2026 - 12Z Wed Sep 02 2026 ...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PARTS OF THE WEST, MIDWEST TO NORTHEAST, AND EASTERN GULF COAST... ...Southwest/Four Corners/Easter Great Basin/Rockies... For both Days 4 and 5, we are maintaining a Marginal Risk in areas along the outer periphery of the flattened, broad upper ridge. The guidance continues to show a fairly active northern stream, with several jet streaks and attendant shortwave energy riding along the perimeter of the upper ridge. On Tuesday-Tuesday night, this includes the Four Corners and parts of the Southwest where diurnal instability and continued favorable deep-layer moisture (PWAT anomalies 1 to 2 standard deviations above normal per the 00Z GEFS) will again favor scattered-numerous monsoonal activity with an isolated risk for flash flooding. On Day 5 (Wed-Wed night), a fairly vigorous longwave trough with a closed 500 mb low (H5 height anomalies around 2 standard deviations below normal) will drop into the Pacific NW Coast. This dynamic system will help draw the deep-layer moisture anomaly farther north across the Western U.S., thereby allowing for an expansion of the Marginal Risk area well northward (to the Canadian border). While the global QPFs currently do not show much in the way of organized (focused) areas of heavy rain, the combination of the deep moisture, increasing upper difluence over a broad area, and diurnal instability will maintain at least a localized flash flood risk. ...Upper Midwest/Lower Great Lakes/Northeast... On Day 4, an upper level jet streak traversing far northern New England will provide a focus for more organized convection within the favorable right-entrance region of this jet streak, along the outer perimeter of the upper ridge. The spread in the model QPFs at this point is quite high, however the signal is there to support at least localized areas of excessive rainfall across much of NY and the Northeast outside of northern New England. By Day 5, the Marginal Risk area broadens a bit (from the Upper Midwest east through a bit more of the Northeast Region), again with more upper jet streak energy and associated vort lobes riding along the flatted outer rim of the upper ridge. While the activity appears to be active across this corridor, it should also be fairly transient. This along with the high spread in the guidance QPF would support capping the ERO risk at Marginal for now. ...Eastern Gulf Coast... Carried over the Marginal Risk area from the current Day 3 ERO to the new Day 4 outlook (12Z Mon-12Z Tue), though given the guidance trends, trimming much of eastern LA that is in the Day 3 outlook. Shortwave energy moving westward underneath the closed 500 mb High will be the foci for potentially more numerous (organized) convective clusters during peak diurnal heating, especially when interacting with the Atlantic and Gulf sea breezes. By Day 5 (12Z Tue-12Z Wed), the mid/upper ridge presses closer to the Gulf Coast, thus lowering the 0-6km bulk shear values and reducing the convective coverage and intensity. The global QPFs are notably lower on Day 5 compared to Day 4, and as such we opted to not include a Marginal on Day 5. Hurley 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 $$