612 FOUS30 KWBC 251519 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1119 AM EDT Tue Aug 25 2026 Day 1 Valid 16Z Tue Aug 25 2026 - 12Z Wed Aug 26 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN GEORGIA AND THE FIRST COAST OF FLORIDA... ...16Z Update... Only minor adjustments were needed to the risk areas across the country today. The Slight Risk in the Southeast was adjusted south and west with this update, following somewhat more moist soils, and areas more prone to flooding. Thus, portions of the northern coast of Georgia were removed with this update. Southern Georgia experienced showers and thunderstorms yesterday, which moistened the soils into the southern interior. Thus, the Slight Risk was expanded west to account for those more favorable soil conditions. Rainfall forecast amounts are little changed, so this entire Slight Risk area remains a lower-end threat. The Marginal Risk out west and into the central Plains was adjusted to carve out the Sand Hills and include a few more areas where rainfall amounts have come up from the previous forecast. Wegman ...Previous Discussion... In coordination with WFO JAX, a SLGT Risk was introduced along the I-95 corridor in Northeast Florida through the coast along Georgia, with heavy rainfall in the form of seabreeze thunderstorms expected along the coast. Low-level flow suggests an axis of convergence situating along the coast, with an onshore flow component also aiding as a lifting mechanism. The latest run of global models are in agreement in terms of PWATs forecast to exceed 2", with the American globals (NAM & GFS) suggesting PWAT value could approach 2.25", fueling efficient thunderstorms. HREF and RRFS suggest medium-to-high chances (65 and 55% respectively) of hourly rainfall rates exceeding 2". Also noted was an uptick in neighborhood FFG exceedance probs, particularly in the 3hr & 6hr FFG guidances, with now low chances (~25-30% in both the HREF & RRFS) of FFG exceedance. ...Southwest to Central Plains... Showers and thunderstorms will once again redevelop across the central and southern Rockies, albeit less expansive than previous days. An upper-level ridge will continue to remain situated over much the Southwest, expanding westward, which will inhibit moisture from moving into the southern portions of the Southwest. In the Central Plains, a shortwave trough sliding along the northeast periphery of the aforementioned ridge will support a complex of thunderstorms in the late afternoon and evening. Latest CAMs guidance suggest a quasi-linear structure which is expected to be fairly progressive. Thus, while rain rates could exceed 1"/hr at times, storms will be very short-lived, and as such, the MRGL was maintained. Elsewhere, heavy rainfall is expected to occur over the southern portions of Louisiana in association with an offshore wave. An introduction of a MRGL area was considered in this area, but ultimately was not added in coordination with WFO LIX due to high FFGs, and the region largely consisting of difficult-to-flood wetlands. Blanco-Alcala Day 2 Valid 12Z Wed Aug 26 2026 - 12Z Thu Aug 27 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHWEST AND ROCKIES TO THE CENTRAL PLAINS AND THE LOWER MISSISSIPPI VALLEY... ...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 MARGINAL RISK OF EXCESSIVE RAINFALL SOUTHEAST, MID- ATLANTIC, AND PORTIONS OF THE SOUTHWEST AND ROCKIES... 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 ...THERE ARE MULTIPLE MARGINAL RISK AREAS ACROSS PARTS OF THE WESTERN STATES...SOUTHEAST/MID ATLANTIC AND UPPER MIDWEST/NORTHERN PLAINS EACH DAY... ...Southwest and Intermountain West... Maintained a Marginal Risk both days across portions of the Southwestern US as monsoonal moisture fuels diurnally driven widely scattered convection covering parts of the desert Southwest and the Intermountain region. A plume of precipitable water will continue to get drawn northward on the western side of a broad mid- and upper level high. 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 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 region. Height falls associated with weak and nearly impossible to time shortwave energy embedded within a large scale trough to the north will focus the convection in vicinity of the front. ...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 on Friday. This area shifts eastward on Saturday as the mid and upper level flow backs to more southwesterly as the upper ridge axis gets flattened by shortwave energy entering the Pacific Northwest. 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 $$