FOUS30 KWBC 252348 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 748 PM EDT Tue Aug 25 2026 Day 1 Valid 01Z Wed Aug 26 2026 - 12Z Wed Aug 26 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE PLAINS AND SOUTHEAST... ...Southeast... Waning evening convection along with coastal convection in the early morning hours due to onshore flow from warm, unstable waters led to maintenance of the previous Marginal Risk area, with some reshaping to account for the more recent 18z HREF/12z REFS guidance and radar reflectivity trends. ...Southwest to Central Plains... Showers and thunderstorms were significantly inhibited across the West by 15-21C 700 hPa temperatures/a mid-level cap. In the Central and Southern High Plains, a shortwave trough sliding along the northeast periphery of a warm core ridge has supported thunderstorm development as of late which should move east to southeast with time. While hourly rain amounts could reach 2" at times, general forward progression is expected, so the Marginal Risk level remains intact though the area has been much reduced based upon 18z HREF/12z REFS guidance and radar reflectivity trends. Roth Day 2 Valid 12Z Wed Aug 26 2026 - 12Z Thu Aug 27 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE LOWER MISSISSIPPI VALLEY... ...2030Z Update... A Slight Risk upgrade was introduced with this update for portions of the lower Mississippi Valley, specifically along the Arkansas/Mississippi border with this update. A digging upper level trough over the Great Lakes will help push a cold front southeastward across Missouri and into northwestern Arkansas by Thursday morning. As this front approaches, it will uplift a very moist air mass in place across the lower Mississippi Valley. PWATs ahead of the front will be broadly above 2 inches. Thus, any and all storms that form as a result of the front's running into the moisture will be capable of producing locally heavy rainfall. Meanwhile in the upper levels, a "ridge rider" shortwave starts the day over Kansas and Oklahoma and moves southeastward into AR/MS/LA. This feature may be the primary driver for the storms, since the aforementioned cold front really enters the picture in the predawn hours Thursday morning. Regardless, the storms will be forming over a portion of Arkansas and Mississippi which was recently hit with heavy rainfall overnight last night. Thus, while today offers some time for the soils to drain and dry out, the combination of terrain effects in Arkansas, and still around normal amounts of soil moisture in both states could result in widely scattered instances of flash flooding. The storms that form over Arkansas look to generally wait to impact northern Mississippi until after sunset. Some uncertainty as to how far north the storms could occur led to the Slight including the Memphis metro. Elsewhere, Marginal Risks were expanded to include the leading edge of the aforementioned cold front moving across the Great Lakes/Midwest. Portions of Illinois, Indiana, and Ohio are still drying out from last week's heavy rainfall, and a shortwave could increase rainfall intensities into portions of northern Michigan. Out west, the Marginal Risk area was largely left intact, as the ongoing monsoonal moisture doesn't move much, allowing for yet another round of afternoon convection into repeatedly hit areas of the 4 Corners Region. While there is a minimum of precip in most of the guidance around the Lake Powell area, the proclivity for flash flooding in that area should a storm affect it led to the Marginal Risk being added there. Wegman ...Previous Discussion... ...Lower Mississippi Valley to the Tennessee Valley... A shortwave trough is expected to ride the eastern periphery of a ridge located in the southwestern U.S. and generate showers and thunderstorms of the Central Plains and Lower Mississippi Valley. Global models have begun to hone in the Ozark/Lower Mississippi Valley for the heaviest QPF core. PWAT values in the Lower Mississippi Valley are expected to exceed 2", particularly in Alabama and parts of eastern Louisiana. At the surface, a stationary boundary will remain in place as an influx of deepening moisture converges along the boundary. Further south, a continued influx of moisture from the Gulf will move north, further amplifying convergence of moisture. While global models are beginning to come into a bit more agreement as to the placement of the heaviest precipitation, spread is still high enough (spanning from northeast Arkansas to western Alabama) to limit confidence. As CAMs begin to become available, an upgrade to a SLGT may become necessary. ...Southwest and Rockies to the Central and Southern High Plains... Diurnal thunderstorms over the southwest will continue to be the theme, although a southwest upper-level ridge will continue to move moisture away from the Arizona and New Mexico area. As such, the inherited MRGL Risk area was merely adjust to match the latest suite of model guidance. Further east, a westward shift to the aforementioned ridge will bring a return of deeper moisture over Southern Rockies and High Plains. Convective activity will recommence in this area during this period and could bring isolated instance of flash flooding. Blanco-Alcala Day 3 Valid 12Z Thu Aug 27 2026 - 12Z Fri Aug 28 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL GULF COAST AND SOUTHEAST... ...2030Z Update... A Slight Risk upgrade was introduced with this update for portions of the central Gulf Coast and Southeast. An upper level shortwave trough over most of the eastern half of the country will continue to dig southward, allowing a surface cold front to continue to make southeastward progress across portions of the Southeast. Meanwhile, the front will run into increasing moisture along the central Gulf Coast. The combination of all of these factors will allow for multiple rounds of storms to impact the central Gulf Coast region, first with the shortwave moving through during the day, then the cold front approaching overnight. Flood prone urban areas such as New Orleans, Baton Rouge, Mobile, and some of the more interior areas of Mississippi dealing with the latest of multiple days of heavy rains will all support widely scattered instances of flash flooding. To the north and east across northern/interior Alabama and Georgia, the base of the upper level trough runs into the same tropically moist air mass in place ahead of the advancing cold front. The storm pattern expected Thursday afternoon will be much more "classic", in that storms form over Alabama in the early afternoon and progress eastward, likely in the form of multiple lines, across northern Georgia and into South Carolina. The storms moving northeastward could allow for some training as the cold front's approach makes training increasingly likely as well. Urban effect such as in Birmingham and Atlanta will locally increase the chances of widely scattered instances of flash flooding. Elsewhere, into the Northeast, the more progressive nature of the same cold front should keep the storms moving in such a way as to minimize the flash flooding potential. Very gradual eastward progression of the ridge across the Southwest should allow for a shifting of the heaviest precipitation to the east out of Utah and more into the central Plains by Thursday. The ridge should largely preclude much organization from the storms, so the Marginal really focuses on those numerous areas where either terrain or prior rainfall locally increases the flash flooding risk. Wegman ...Previous Discussion... Showers and thunderstorms capable of producing heavy rainfall will focus along a 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. Latest run of global models indicate a surprising amount of agreement regarding a QPF concentration located over the Lower Mississippi Valley, specifically Louisiana, Mississippi, Alabama, and the Florida panhandle. High FFGs, and an area characterized by wetlands ultimately resulted in an upgrade to a SLGT not being introduced. Future updates may warrant the need for a SLGT upgrade, especially if trends continue to hone in on the aforementioned region. Further west, across portions of the Southwestern US, monsoonal moisture will again fuel 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. As such, the MRGL Risk area over the Southwest was merely adjusted to match latest run of global model and ensemble guidance. Blanco-Alcala/Otto Day 4 and Day 5 Valid 12Z Fri Aug 28 2026 - 12Z Sun Aug 30 2026 ...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PARTS OF THE WEST, SOUTHEAST, & UPPER MIDWEST/NORTHERN PLAINS... ...Southwest/Great Basin/Intermountain West... A Marginal Risk of excessive rainfall exists both days across as monsoonal moisture fuels diurnally driven widely scattered convection covering parts of the region ahead of increasing upper level troughing across the Northwest. A plume of precipitable water will continue to get drawn northward on the western side of a broad mid- and upper level high. 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 on Friday and Saturday...with favored terrain, urbanized areas and burn scars being the most prone to flooding. ..Southeast... Showers and thunderstorms capable of producing heavy to excessive rainfall each day should be focused in the vicinity of a quasi- stationary front extending over the Southeast US/Gulf coast region into parts of VA. Weak and nearly impossible to time shortwave energy moving through the base of a receding large scale trough to the north and northeast will focus the convection in vicinity of the front. With precipitable water values of 2"+ expected, hourly rain amounts to 3" are possible where cells merge or train. ...Upper Midwest/Northern Plains... There is a risk of excessive rainfall developing across parts of the Upper Midwest/Northern Plains on Friday. Mid-level ridging weakens as Gulf moisture is drawn into the region on the heels of a 30 to 45 kt low level jet. The invading moisture rides over a warm front over the region on Friday. This area shifts eastward on Saturday as the mid- and upper-level flow backs to more southwesterly as a strong shortwave moves from the Pacific Northwest into Montana by early Sunday morning. Given the 1.5-1.75" precipitable water values forecast, hourly rain amounts to 2" are possible wherever cells can merge or train. Roth 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 $$