444 FOUS30 KWBC 240817 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 417 AM EDT Mon Aug 24 2026 Day 1 Valid 12Z Mon Aug 24 2026 - 12Z Tue Aug 25 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ALONG THE GULF COAST FROM SOUTHEASTERN LOUISIANA TO SOUTHERN ALABAMA... ...Ozarks-Lower Mississippi Valley and Central Gulf Coast to the Coastal Carolinas... Elevated storms are expected to develop later this morning and propagate southeast from northern Arkansas into the lower Mississippi Valley mid to late morning. Following an expected decrease in activity, storms are forecast to re-intensify in the afternoon, with the consensus of the CAMs showing surface based storms organizing over portions of southeastern Louisiana, southern Mississippi, and perhaps as far east as southern Alabama. Shortwave energy dropping into the region will likely interact with a pool of deepening moisture, with PWs increasing to over 2.25 inches according to recent RAP runs. This will support very heavy rates with localized heavy accumulations likely. A Slight Risk was introduced where both the HREF and REFS indicate localized totals over 3 inches are likely. While the antecedent conditions are dry, both the HREF and REFS show probabilities for amounts exceeding 3-hr FFGs nearing 30 percent across portions of the highlighted region during the late afternoon-early evening hours -- with urbanized and poor drainage areas likely to be the most susceptible to flash flooding. Further east through northern Florida and along the Southeast Coast to the Carolinas, a ribbon of deep moisture (PWs 1.75-2+ inches) centered along and south of a slow-moving boundary will support enhanced diurnal activity, with developing storms likely to produce localized heavy rainfall rates and amounts. A Marginal Risk was maintained where both the HREF and REFS show high probabilities for amounts over 2 inches. ...Southwest to the Central High Plains... With not much change to the overall pattern, and the development of monsoon-moisture-fueled storms expected to continue, maintained a Marginal Risk covering much of the same area as previous day -- extending from portions of the Southwest northward into the Great Basin and northern Rockies, and east into the central Rockies and High Plains. Pereira 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... 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 coastalSouth 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... 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... 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. 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 $$