FOUS30 KWBC 290049 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 849 PM EDT Fri Aug 28 2026 Day 1 Valid 01Z Sat Aug 29 2026 - 12Z Sat Aug 29 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE COASTAL CAROLINAS INTO SOUTHEAST VIRGINIA AND SOUTHERN GEORGIA... ...01Z Update... All of the inherited Marginals were adjusted to account for the latest trends in Doppler Radar, as well as the latest 18Z HREF guidance. In the West, an impressive IVT for late August (>99th climatological percentile values per NAEFS from Southern CA to central WY) is likely to continue scattered showers throughout the West. Some embedded storms are possible, but upslope enhancement within an air-mass containing >90th percentile PWs could still support some localized flash flooding. In the Southwest, storms there are more thunderstorms available, but the storms are more diurnally driven and will dissipate across much of the region after 06Z. Farther east, the Slight Risk remains in place over coastal areas of the southeast and a little farther inland towards central SC. 18Z HREF guidance shows pockets of low-to-moderate chances (20-40%) for 12-hour rainfall totals >3". With some storms already contributing to flash flooding in parts of southern GA and northern FL, and storms likely to continue into tonight along the coast, the Slight still seems reasonable, but did trim off the northern most portion that was over southeast VA. Lastly, in the Midwest, a fast moving 500mb vort max will be responsible for developing scattered thunderstorms overnight. 18Z HREF probabilities do show some low- to-moderate chance probabilities (30-50%) for localized rainfall totals >2". Mullinax Day 2 Valid 12Z Sat Aug 29 2026 - 12Z Sun Aug 30 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL AND SOUTHERN ARIZONA... ...2030z Update... ...Southwest, Great Basin, and the Rockies... For Saturday (day 2), a Slight Risk Excessive Rainfall Outlook was added for portions of central and southern Arizona, including parts of the Flagstaff, Phoenix, and Tucson CWAs after collaboration. Moisture should increase due to an increase in integrated vapor transport from the Pacific ahead of an upper- trough and subtropical jet. Heavy rainfall and instances of scattered flash flooding will be supported with PWATS in the 1.5-1.75 inch range. Additionally,forecast CAPE around 1000-1500 J/Kg will support afternoon thunderstorms. The Slight Risk generally covers moderate HREF probabilities of locally heavy rainfall totals greater than 2 inches, as well as 20-30% 3 hour flash flood guidance exceedance in central Arizona and 40-60% 3 hour flash flood guidance exceedance across Tucson's CWA. A Marginal Risk of excessive rainfall remains for surrounding areas of the Great Basin and Rockies as above average moisture for the monsoon fuels diurnally driven widely scattered to scattered convection across Nevada, New Mexico, and the Colorado Front Range. 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. Additionally, areas farther north in the Great Basin and Rockies from Utah to Wyoming and Montana may see some locally heavy rainfall as anomalous moisture streams in from the Pacific ahead of a potent upper-level trough. Model guidance is indicating heavier QPF totals of 1-1.5 inches across northern Utah, eastern Idaho, and western Wyoming. Sensitive terrain would be favored for flash flooding. ...Southeast and Gulf Coast... The forecast remains on track from the previous guidance. Across the Carolinas and into the eastern Appalachians, a stalled front and diurnal heating will support afternoon thunderstorms development capable of producing localized rainfall totals of 2-3 inches with a 20% chance for exceeding FFG exceedance according to the afternoon HREF. Closer to the Gulf Coast, a combination of a moist, tropical environment and a mid-level low /surface trough may bring locally heavy rainfall and some flash flood concern as well. ...Previous Discussion... 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. ...Northern Plains into the Upper Midwest... Model guidance continues to indicate rounds of storms that form overnight in the Plains to head eastward towards Minnesota and Wisconsin. Generally, anomalous moisture and sufficient instability with lift from a shortwave should bring a general 1-2 inches of rain with embedded locally heavy rain rates that may exceed flash flood guidance. Renewed convection is possible ahead of an upper-level trough Saturday afternoon and evening across eastern Montana and western North Dakota that may also bring a localized flash flood threat. ...Previous Discussion... 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. Wilder/Campbell Day 3 Valid 12Z Sun Aug 30 2026 - 12Z Mon Aug 31 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF ARIZONA AND SOUTHERN UTAH... ...Southwest/Great Basin/Intermountain West... A Slight Risk has been added to portions of south-central to north-central Arizona, as well as south-central Utah. Similar to day 2, a persistent Monsoon environment characterized by PWATS in the 1.5-1.75 inch range and CAPE around 1000-1500 J/Kg, as well as increasing synoptic support as an upper-trough enters the Southwest, will support increasing thunderstorm coverage and periods of heavy rainfall from Arizona to southern Utah. Increasing coverage of storms is expected into the Deserts compared to day 2, particularly in central Arizona around the Phoenix vicinity. Elsewhere, a Marginal Risk of excessive rainfall remains for above average moisture fueling diurnally driven widely scattered convection across 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. Wilder/Campbell ...Previous Discussion... ..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 an 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" are possible. At this time, Marginal Risk areas remain in effect. However, there may be a need for a Slight Risk somewhere between the Great Lakes and Northeast. Campbell/Roth Day 4 and Day 5 Valid 12Z Mon Aug 31 2026 - 12Z Wed Sep 02 2026 ...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS SOUTHERN AND CENTRAL ARIZONA... ...Southwest/Four Corners/Eastern Great Basin/Rockies... On Day 4 (Aug 31), an upper trough approaching the West Coast will work together with a heat dome over the East to produce a diffluent mid-upper level wind field over the Southwest. The evening of Aug 31, NAEFS shows a strengthening IVT originating out of the tropical East Pacific that will direct >90th climatological IVT values over the Desert Southwest. The region will also see soils continue to saturate with multiple days worth of rainfall leading up to Aug 31. Given the unusually strong syoptic-scale forcing, increasing IVT and lingering 90th climatological percentile PWs, and increasingly sensitive soils, a Slight Risk was introduced for portions of central and southeast AZ this forecast update. On Day 5 (Sept 1), a longwave trough with a closed 500 mb low (H5 height anomalies around 2 standard deviations below normal) will be positioned off the Pacific NW coast. The upper trough will help tap into deep-layer moisture and direct the moisture farther north across the Western U.S.. With many of these regions having dealt with daily rounds of heavy showers and thunderstorms, and a farther northward push of anomalous moisture via the upper trough, the inherited northward expansion of the Marginal Risk area still looked reasonable as of this update. 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 (Aug 31), 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 remains fairly high at the moment. However, PWs above the 90th climatological percentile will revolve around the northern flank of the ridge and reach the Northeast by evening. With favorable synoptic-scale support and increasing moisture aloft, there is support for at least localized areas of excessive rainfall across much of NY, northern PA, and as far east as the I-95 corridor from northern NJ to southern New England. By Day 5 (Sept 1), 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 scattered thunderstorms will be likely 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. It should be noted that NBM QPF is approaching 1" near the Tug Hill, which is a signal that could support a potential Slight Risk upgrade. Especially if soils grow more saturated from rounds of storms the previous day. ...Eastern Gulf Coast... The inherited Marginal Risk on Day 4 (Aug 31) remains in place with only minor adjustments westward to include more of eastern LA. Shortwave energy moving westward underneath the closed 500 mb High will be the focus 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. Mullinax/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 $$