FOUS30 KWBC 232016 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 416 PM EDT Sun Aug 23 2026 Day 1 Valid 16Z Sun Aug 23 2026 - 12Z Mon Aug 24 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE WESTERN AND CENTRAL U.S. AND IN THE LEE OF LAKE ERIE... ...Northern Plains to the Ozarks... 16Z update... The latest hourly guidance is showing cells producing 1-2 inches/hour tracking across central North Dakota which portions of the projected track lies outside the current Marginal Risk area. the eastern boundary of the Marginal Risk was expanded to account for the potential heavy rainfall occurring further to the east. Campbell Models continue to present a strong signal for convective development, spurred by the interaction of mid-level energy spilling off of the top of the western U.S. ridge along with a ribbon of deepening moisture (1.50-1.75+ inches). However, they continue to differ greatly on the details, especially with regards to how far south storms will develop and propagate -- limiting forecast confidence. The spread amongst the HREF members was more notable, with some, including the HRW-ARW2, bringing heavy- rainfall-producing storms farther south through the lower Missouri Valley into the Ozarks than some others, such the HRW-ARW. The REFS suite suggested greater confidence, with storm development and heavy rainfall placement remaining more focused over the Dakotas and showing high probabilities for accumulations over 2 inches along an northwest-southeast axis extending from western North Dakota to eastern South Dakota. The HREF showed a similar axis, but with lower probabilities. Given the uncertainty, the Marginal Risk drawn incorporates elements from both hi-res ensemble suites including the southern extent drawn into the Ozarks. ...Lake Erie... Shortwave energy digging south of a deep low closing off over southern Ontario will support periods of lake-enhanced shower activity later today, with the hi-res guidance continuing to present a signal for localized heavy amounts. Maintained a small, targeted Marginal Risk centered over northwestern Pennsylvania and far western New York, where both the HREF and REFS show high probabilities for amounts over 2 inches. ...Southwest and Great Basin to the Central High Plains... The overall pattern is expected to remain relatively unchanged, with the upper ridge remaining anchored over the Southwest and southern Plains. Moisture and energy moving along the western and northern periphery of the ridge will contribute to daily shower and thunderstorm activity and the potential for isolated flash flooding extending from the Southwest northward into parts of the Great Basin and east across the central Rockies into the High Plains. Pereira Day 2 Valid 12Z Mon Aug 24 2026 - 12Z Tue Aug 25 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE WESTERN U.S., THE LOWER MISSISSIPPI VALLEY, AND ALONG PORTIONS OF THE GULF AND SOUTHEAST COASTS... ...Lower Mississippi Valley to the Gulf and Southeast Coasts... 21Z update... The latest guidance and WPC QPF had a modest uptick in QPF coverage/amounts along the coast that necessitate a minor expansion of the Marginal Risk area to include more of the Wilmington metro across the northern boundary and more of the Jacksonville metro across the southern boundary. Elsewhere, the current Marginal reflects the appropriate threat level and required no alterations. Campbell Models that show convection developing and propagating south into the Ozark region late in the Day 1 period indicate the potential for storms to continue, with heavy rainfall persisting as they track south into parts of the lower Mississippi Valley during the mid to late morning on Monday. Therefore, the Marginal Risk extending from the Ozarks down into the lower Mississippi Valley was maintained for now. Farther south and east, the overnight guidance presented a growing signal for heavy amounts and isolated concerns along a ribbon of deepening moisture south of a slow-moving front extending from the Louisiana to the Carolina coasts. PWs of 1.75-2 inches interacting with weak energy moving along the base of an upper trough, including the previously noted energy dropping though the lower Mississippi Valley, may contribute to enhanced diurnal activity. A Marginal Risk was drawn along the coastal areas where the REFS shows high probabilities for accumulations exceeding 2 inches. ...Southwest to the Central High Plains... As drier air spreads into the region from the west, a decrease in activity is expected across the Great Basin. Otherwise, with the pattern remaining much of the same, another day of diurnal storms, producing isolated flooding concerns, is expected across much of the same area as Day 1. Pereira Day 3 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 Rockies and Central Plains... 21Z update... There was a modest uptick in model and WPC QPF across the Texas and into western Oklahoma therefore the Marginal Risk was expanded a bit further south to cover this trend. Otherwise the rest of the region within the Marginal Risk area remains in good order. Campbell As the upper high centered over the Southwest shifts slowly west, expect decreasing diurnal storm activity across parts of the region, especially over southern Arizona and New Mexico. A better chance for showers and storms with isolated flash flooding concerns will continue farther north from the Four Corners into parts of the northern and central Rockies. Activity is expected develop farther east out into the central Plains and perhaps as far east as the lower Missouri Valley as moisture pools ahead of a cold front pushing south through the central part of the country. While there are still plenty of differences in the details, there is a growing camp of solutions showing organized activity, with potentially heavy amounts, centering in the vicinity of western Kansas as energy moving off of the top of western ridge interacts with this deepening moisture and increasing instability. ...Central Gulf to the Southeast Coast... With ample moisture remaining in place (PWs ~2 inches), another day of slow-moving, efficient storms can be expected -- raising the threat for at least isolated flash flooding concerns, especially within urbanized areas along the coast from Louisiana to South Carolina. Pereira Day 4 and Day 5 Valid 12Z Wed Aug 26 2026 - 12Z Fri Aug 28 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM PORTIONS OF THE SOUTHWEST AND INTERMOUNTAIN WEST AND TO PORTIONS OF THE SOUTHEAST AND lOWER MISSISSIPPI VALLEYS EACH DAY EXPANDING INTO MID ATLANTIC ON DAY 5... ...Southwest and Intermountain West to Central and Southern Plains... Maintained a Marginal Risk both days as monsoonal moisture continues to fuel diurnally driven widely scattered convection covering portions of Arizona and New Mexico northward through the Four Corners into parts of the central and northern Rockies. Given the magnitude of the moisture anomalies...locally heavy rainfall rates may lead to isolated instances of flash flooding. ...Southeast to Mid-Atlantic Region... Maintained a Marginal Risk covering portions of the Lower Tennessee Valley and the Southeast US on Day 4 and Day 5 as mid- level Valid 12Z Wed Aug 26 2026 - 12Z Fri Aug 28 2026 energy continues to slide southeast into the region. As it does so,...the energy interacts with a pool of deepening moisture pooling along a weak boundary draped across the region each day. The footprint for heavy rainfall may expand and the potential for flash flooding may increase across the region and extend back into parts of the lower Mississippi Valley. By Day 5...The coverage of convection over the Mid Atlantic region should increase as a cold cold front approaches from the west/north and upper flow becomes more diffluent and the low level flow taps deeper moisture. This could lead to some locally heavy rainfall rates developing over parts of the Mid-Atlantic...perhaps as far north as Pennsylvania...on Day 5. Bann/Pereira 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 $$