FOUS30 KWBC 310827 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 427 AM EDT Mon Aug 31 2026 Day 1 Valid 12Z Mon Aug 31 2026 - 12Z Tue Sep 01 2026 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF UTAH... ...Desert Southwest and Four Corners... A Moderate Risk for excessive rainfall and flash flooding was maintained for portions of Utah for this period as widespread convection is expected. Dual shortwaves are progged to lift northward (one from Mexico, one from California), bringing enhanced ascent to a region already experiencing pronounced synoptic lift through impressive diffluence in the right rear quadrant of a strengthening and meridionally advancing jet streak. This deep layer lift will act upon extreme thermodynamics characterized by PWs above 1" lifting as far north as the Utah Swell, with +2 sigma PWs pooled across much of Arizona, Utah and western Colorado. While there is some uncertainty as to how quickly convection will develop due to morning CINH (owing from overnight thunderstorms moving north from AZ), once MLCAPE does re-develop it should reach 500-1000 J/kg to support another day of widespread convective activity. Within the extreme thermodynamics, the robust lift will generate thunderstorms which have a 20-30% chance of producing hourly rainfall above 1 inch. This will be despite generally progressive storms on 0-6km mean winds of 15-20 kts, faster over Utah and Colorado, and it is important to distinguish that convection over southern AZ will move much more slowly. However, bulk shear of 30-40 kts will help organize storms, and aligned propagation vectors indicate a strong threat for repeating cells, enhancing the rainfall risk. Impacts will be most significant over sensitive features like dry washes, slot canyons, and burn scars, but intense rain rates repeating anywhere on Monday could cause excessive rainfall related impacts. ...Upper Midwest/Lower Great Lakes/Northeast... Showers and thunderstorms are expected fire in the proximity of two frontal boundaries that will be draped across the region during this period. Individual storms will have the potential to produce rain rates of 1-2 inches/hour. FFG exceedance is modest (less than 15%) so maintained the Marginal Risk are that spans from the Minnesota Arrowhead to southern New England. ...Gulf Coast... A mid-level closed low that has been meandering around the northern Gulf the past few days is beginning to organize as reflected by an NHC invest area (AL97). The latest guidance continues to show this feature drifting westward toward the upper Texas coast by Tuesday morning coast under the ridge centered over the Mississippi and Tennessee Valleys. The accompanying plume of tropical moisture noted via a large pool of PWs of 2-2.25" will help support slow moving thunderstorms along the Gulf Coast through Monday, although any focused heavy rain is progged to remain off the coast at this time. A Marginal Risk area was maintained from the upper Texas coast to the just east of the Alabama/Florida state line. Campbell/Weiss Day 2 Valid 12Z Tue Sep 01 2026 - 12Z Wed Sep 02 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE UPPER TEXAS COAST AND FAR SOUTHWEST LOUISIANA... ...Central Gulf Coast... Guidance varies in timing and intensity however there is consensus for Invest AL97 to move onshore the upper Texas coast on Tuesday morning bringing an increasing threat for heavy rainfall. Several inches of rain may materialize given PW values greater than 2.25 inches and an increasing low level flow to intensify ascent across this region. A Slight Risk was maintained although shifted to reflect the latest guidance and WPC QPF placement. ...Portions of the Southwest... The multiday, heavy rainfall event over the Southwest/West will drastically reduce coverage and intensity during this period in response to the mid-level trough moves onshore the California coast. This will lead to height falls well into the Southern Rockies and Central Plains. This not only will shift the favorable corridor for ascent, but also push the greatest moisture anomalies farther east and out of the Southwest. For portions of Arizona, New Mexico and souther Colorado a modest threat will continue for this period although cell motion will likely be more progressive in nature thus limiting the threat for flash flooding. A Marginal Risk remains in effect. ...Great Lakes to New England... Multiple mid-level disturbances will track through the flow from the Great Lakes to the Northeast and in tandem with a stationary frontal boundary draped over the region, will bolster ascent. The resulting lift will act upon PWs pooled south of the boundary that will approach 2" (+1.5 sigma) to support fast moving thunderstorms (0-6km mean winds 25-35 kts) but that will contain 1+"/hr rain rates. The latest suite of guidance suggests multiple rounds of convection on Tuesday due to the weak mid-level impulses, and as bulk shear climbs to 30-40 kts, repeating rounds of convective clusters could produce as much as 3" of rain. A Marginal Risk area remains in effect for this region. Should models trend higher with amounts/repeating rounds cause the heaviest rain to fall atop the more vulnerable soils around the Tri-State area of Connecticut, New York and New Jersey, an upgrade to a Slight risk may be needed. ...Gulf Coast and Florida... Persistent easterly flow across the Florida Peninsula will once again supply tropical PWs around 2.25" to the region to support scattered afternoon convection. This setup should allow for a rapid expansion of thunderstorms on the westward advancing sea breeze before pinning storms along the western/Gulf side of the Peninsula later in the aftn. While coverage is a but uncertain, a weak mid- level impulse progged to move across the area may act to enhanced diurnal activity, and with rainfall rates likely reaching 2-3"/hr at times, this could produce excessive rainfall within urban areas of the Sun coast therefore the Marginal Risk area was maintained. Campbell/Weiss Day 3 Valid 12Z Wed Sep 02 2026 - 12Z Thu Sep 03 2026 ...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PARTS OF THE NORTHEAST AND EASTERN TEXAS... ...Northern Mid Atlantic & Southern New England... Showers and thunderstorms will focus along the frontal boundary sprawled across the region. PW values of 1.75-2" expected to be pooled over the region to help support heavy rain, with hourly rates up to 2.5 inches with total accumulations nearing 5 inches for some locations. A Slight Risk is in effect for much of central/southeast New York, northeast Pennsylvania, northern New Jersey, western Massachuesetts and western Connecticut. ...Northern Plains and Midwest... Lingering MCS activity possible early in the period across the Upper Midwest. PWAT anomalies remain elevated at times mid- week, with values of 1.5-2" forecast, however the spread in the deterministic QPF is higher than average. A broad Marginal Risk spans from Minnesota/Iowa eastward to the Northeast. ...Southeast TX... After landfall, the guidance QPF signal moves minimally, which is concerning, as random pieces of guidance drop 6"+ per day. Precipitable water values of 2-2.5" would certainly support hourly amounts to 3" with additional local totals to 6". The low-level inflow could exceed the mean wind and could also be antiparallel to the flow aloft if the system were to become a tropical cyclone. If enough effective bulk shear is present, local totals could end up substantially higher than listed above, especially if the system stalls/dawdles. A Slight Risk was raised for this period as it overlaps a majority of the D2 QPF footprint, thus reduces the FFGs over that part of the state. See NHC Tropical Weather Outlooks for more information on its chances of TD/TS formation. Should it develop into a tropical system there may be a need to further upgrade categories with time. ...In and near FL... Heavy rainfall possible due to a combination of onshore flow from a warm and unstable subtropical Atlantic and Gulf and moisture/remnant energy from what was tropical cyclone Dolly. Coverage is expected to be spotty therefore a Marginal Risk best highlights the level of threat for flash flooding. Campbell/Roth Day 4 and Day 5 Valid 12Z Thu Sep 03 2026 - 12Z Sat Sep 05 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FRIDAY-FRIDAY NIGHT ACROSS MUCH OF SOUTHERN ARIZONA... ...Southwest/Four Corners/Central-Southern Rockies... After a brief respite on Day 3 (Wed-Wed night), moisture returns Thursday and especially by Friday and into next weekend per the guidance. The 00Z GFS was not utilized during this forecast cycle, given the the deeper, more neutrally-tilted full-latitude trough along the West Coast, which draws any potential tropical moisture farther east-northeast closer to northern Baja and the Southwest. Nevertheless, the ECMWF/CMC/UKmet show deep-layer moisture returning by Friday and especially by the weekend, and the non-GFS consensus QPF (including the NBM) has come up. Per collaboration with WFO Tucson, Phoenix, and Flagstaff, have hoisted a Slight Risk on Day 5 across much of southern AZ. ...Middle-Upper TX Coast and East-Central TX to Southwest LA... Maintained the Marginal Risk on Day 4 (Thu-Thu night), which would be dependent on the track and evolution of the current low pressure system over the northern Gulf. NHC currently has a high (70%) chance that this feature would develop into a short-lived tropical depression or tropical storm before reaching the Upper TX or southwestern LA Coast by Tuesday morning. However, the big question would be how long this feature will linger as a rainfall threat inland before dissipating. The 00Z ECMWF and especially CMC are notably slower with the inland progression and dissipation, and as such have maintained the Marginal Risk on Day 4, with the emphasis of it being early in the period (i.e. Thursday morning). ...Upper Midwest/Ohio Valley/Appalachians/Parts of the Northeast... Mid/upper level trough (initially closed H5 low) pivoting through eastern Ontario and Quebec late this week will enhance the upper level confluence south of this feature towards the upper ridge. This will result in at least a couple of 90-100kt jet streaks traversing the along/near the U.S.-Canadian border, providing the necessary dynamical support (deep-layer ascent) for more organized convection (potentially growing upscale into an MCS), again tracking along the outer periphery of the flattened upper ridge, ahead of the surface frontal boundary that will be dipping southward Thursday through early Saturday across the OH Valley and central Appalachians. For now, we have maintained the Marginal Risk on Day 4 (inherited from yesterday's Day 5 ERO), while shifting the outlook area a bit farther south on the new Day 5 ERO (12Z Fri-12Z Sat). Deep-layer moisture anomalies ahead of the front remain high, with with PWATs around 2 standard deviations above normal per the 00Z GEFS and ECENS. ...Central Gulf Coast and Florida... A combination of onshore flow from a warm and unstable subtropical Atlantic and Gulf and Dolly's remnants moving into the region are a recipe for heavy rainfall. However, any heavy rain signal is spotty and not showing any real run to run model continuity at the present time. The Marginal Risk of excessive rainfall from yesterday's Day 5 ERO was maintained on the current Day 4 outlook, but if Dolly ends up more organized than currently expected, a higher risk threshold could be necessary. ...Southwest/Southern Rockies... The combination of a nearby front at the leading edge of a strong western trough for this time of the year and a monsoon surge up the Gulf of CA caused by a future tropical cyclone is expected to cause a substantial rise in the precipitable water values across Thursday and Thursday night, ending up near 2" by Thursday morning. Any shortwave moving through the base of this trough could cause a convective uptick, so the real question is whether or not any outflow cirrus from the as-yet-to-form tropical cyclone moves into the area to inhibit daytime heating. There are more than enough ingredients available for a Marginal Risk area, which was added in this update. Hourly rainfall up to 2.5" would be problematic in urban areas, burn scars, dry washes/arroyos, and box/slot canyons. Hurley/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 $$