FOUS30 KWBC 250111 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 911 PM EDT Mon Aug 24 2026 Day 1 Valid 01Z Tue Aug 25 2026 - 12Z Tue Aug 25 2026 ...THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL FROM THE CENTRAL GULF COAST TO THE SOUTHEAST COAST AND ACROSS PORTIONS OF THE SOUTHWEST...CENTRAL ROCKIES AND CENTRAL HIGH PLAINS... ...West to Rockies... Ongoing monsoonal convection will maintain an isolated flash flood threat across portions of the West until the loss of daytime heating and increasing lower level inhibition ends the threat for localized 1+ in/hr rainfall rates. There is a non-zero threat for flash flooding later in the overnight period but most indications are for any lingering storms to have weakened beyond a flash flood concern beyond 06Z. ...Central High Plains... Low level convergence at the leading edge of convection moving eastward across portions of east-central CO into weak upslope flow over western KS within a corridor of 1000-2000 J/kg MLCAPE may allow a flare up in thunderstorm intensity overnight. Potential exists for 1 to 3 inches (locally higher) with rates of 1-2 in/hr. ...Central Gulf Coast to Southeast Coast... lingering thunderstorms near a frontal boundary over southern GA/northern FL will continue a limited flash flood concern over the next few hours. Later in the night, forecasts of a surface low forming along the front in GA and increasing instability with low level winds from the south beneath diffluent flow aloft may be enough to ignite nocturnal storms near/north of the front. Farther west into the lower MS Valley, thunderstorms from Monday afternoon/evening had stabilized inland areas north of the Gulf Coast. However, high PWs over 2 inches and short term forecasts for a return of southerly flow from the Gulf and decreasing inhibition in the vicinity of the lower MS Valley/Gulf Coast could allow for an isolated threat for nocturnal convection and isolated flash flooding through 12Z Tuesday. Otto Day 2 Valid 12Z Tue Aug 25 2026 - 12Z Wed Aug 26 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHWEST AND ROCKIES TO THE CENTRAL PLAINS AND ALONG THE GULF AND SOUTHEAST COASTS... ...Southwest to the Central Plains... 21Z update... Consensus has the convection progressing further east across the southern High Plains than some of the previous cycles. WPC followed this trend with the latest QPF and as such the threat for excessive rainfall and isolated flash flooding has increased warranting a modest expansion of the Marginal Risk across the Texas Panhandle. Elsewhere the Marginal remains appropriate to the level of risk. Campbell Diurnal shower and thunderstorms activity will develop once again across portions of the Southwest into the central and southern Rockies. However, the footprint for these storms will likely be less than earlier in the week as the upper high begins to shift west -- capping or directing moisture away from parts of the Southwest and Great Basin. Therefore, continued to leave southern Arizona and New Mexico and much of the Great Basin out of the Marginal Risk. The models continue to signal a chance for organized storms and potentially heavy amounts across portions of the central Plains. Energy emanating from the West and sliding off of the top of the western U.S. ridge is expected to interact with an area of increasing moisture that will be deepening ahead of cold front dropping through the central U.S. While models continue to differ on the details, they have presented a stronger signal for organized activity over portions of southern Nebraska into Kansas. But given the uncertainty and what are expected to be fairly progressive storms, maintained just a Marginal Risk for now. ...Gulf and Southeast Coasts... Another day of potentially heavy diurnal showers and storms are expected from portions of the Florida Panhandle to coastal South Carolina. PWs are forecast to remain at or above 1.75 inches, fueling efficient-rainfall-producing-storms. Farther west, the consensus of the guidance shows much of the heavier activity developing and remaining offshore with a weak wave. There is some potential for heavy amounts developing onshore, but at this point they look to be mostly confined to the immediate coast of southeastern Louisiana, where flash flooding is not a typical concern. Pereira Day 3 Valid 12Z Wed Aug 26 2026 - 12Z Thu Aug 27 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHWEST, ROCKIES, AND CENTRAL SOUTHERN HIGH PLAINS AND FROM PARTS OF THE LOWER MISSISSIPPI TO THE TENNESSEE VALLEY... ...Lower Mississippi Valley to the Tennessee Valley... 21Z update... Guidance is suggesting that some of the heavier rainfall will make it further south into Mississippi and Alabama for this period therefore the Marginal Risk was extended further southeast for portions of southern Mississippi, central Alabama and far western Georgia. Campbell A series of shortwaves sliding off of the western U.S. ridge and then along the base of a trough amplifying over the eastern U.S. are expected to generate showers and storms across portions of the region. While the models differ greatly on the details, there is some general indication that one of the more defined waves moving across the region will help develop organized storms and some greater potential for heavy accumulations over parts of the lower Mississippi Valley. Backing flow and deepening moisture may also support heavy rainfall developing farther east into parts of the Tennessee Valley, but model spread is significant there as well -- limiting forecast confidence. ...Southwest and Rockies to the Central and Southern High Plains... 21Z update... QPF coverage once again saw an increase across portions of the southern High Plains with the latest cycle. WPC QPF followed suite raising coverage and amounts for the Oklahoma and Texas Panhandles. The eastern boundary of Marginal Risk was expanded further east across southwest Kansas, western Oklahoma,Texas panhandle and southeast New Mexico. Campbell Overall, not much change from the previous day with diurnal activity likely to be very limited across southern Arizona and New Mexico and the Great Basin. Farther east, storms that develop along the eastern front of the Rockies are expected to propagate more south than the previous day and remain mostly limited to the High Plains. With the ridge shifting west, expect to see some return of deeper moisture and convective activity over a greater portion of the southern Rockies and High Plains this period. Pereira Day 4 and Day 5 Valid 12Z Thu Aug 27 2026 - 12Z Sat Aug 29 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM PORTIONS OF THE SOUTHWEST AND INTERMOUNTAIN WEST AS WELL AS PARTS OF THE SOUTHEAST AND MID ATLANTIC... ...Southwest and Intermountain West to Central and Southern Plains... Maintained a Marginal Risk both days across portions of the Southwestern US as monsoonal moisture fuels diurnally driven widely scattered convection covering portions of Arizona and New Mexico. Given the plume of precipitable water values nosing back into to the Arizona desert on the western side of a broad mid- and upper level high...local downpours leading to instances of flash flooding are possible. Farther north...deep moisture and weak shortwave energy riding around the ridge axis associated with the upper high will combine with southerly flow of deep moisture in low-levels to the lee of the Front Range to support more showers and thunderstorms capable of producing heavy rainfall rates and flash flooding. Locally heavy rainfall rates may lead to isolated instances of flash flooding here as well. ...Southeast to Mid-Atlantic Region/Southern New England Coast... Showers and thunderstorms capable of producing heavy to excessive rainfall each day should be focused along a quasi-stationary front extending over the Southeast US/Gulf coast region into parts of the Mid Atlantic. Height falls associated with weak shortwave energy will embedded within a broader trough will aid and focus the convection in vicinity of the front. In addition...the approach of a front in the northern stream trough will tap into some deeper moisture being drawn northward ahead of the above-mentioned front to bring an isolated threat of flooding on Thursday before the northern stream front sweeps off shore later in the Day 4 period. ...Upper Midwest/Northern Plains... There is a risk of excessive rainfall developing across parts of the Upper Midwest/Northern Plains on Friday as low level flow of Gulf moisture is drawn into the region on 30 to 45 kt low level jet and rides over a surface warm front over the region. Otto/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 $$