713 FOUS30 KWBC 291556 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1156 AM EDT Sat Aug 29 2026 Day 1 Valid 16Z Sat Aug 29 2026 - 12Z Sun Aug 30 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL AND SOUTHERN ARIZONA AS WELL AS FOR PORTIONS OF THE INTERMOUNTAIN WEST... ...16Z Update... ...Intermountain West... A Slight Risk upgrade was introduced with this update for portions of the Intermountain West from central Utah through the Yellowstone region of southwest Montana. A deep positively tilted trough over the Pacific Northwest will swing across the Intermountain West today. Anomalous moisture to 1 inch PWATs, or 3 sigma above normal for this time of year over portions of the region will be drawn northeastward with the incoming upper level shortwave. This will support scattered shower and thunderstorm development across much of the region this afternoon, not including the ongoing showers and storms ongoing across northern Utah and southwest Wyoming. The storms will be fast moving to be sure given the fast upper level flow. However, this will be offset by likely backbuilding, so that light to moderate rainfall persists for multiple hours in some areas. Of course, with the many mountain ranges in the area, it's likely some of the storms will be locally enhanced by upslope flow, which will slow their movement locally somewhat as they get "hung up" on the mountains. Guidance suggests that MUCAPE values across this region will range from 500 to 1,000 J/kg. This too will support at least widely scattered convective development. ...Arizona... The Slight Risk area is largely the same across this region, though the threat into metro Phoenix is high enough to include the Valley of the Sun into the Slight Risk area, given the urban concerns there. The heaviest rains will be in the mountains surrounding Phoenix, but they are likely to still affect the valley in spots. ...Northern Plains and Great Lakes... The Marginal Risk was split with this update with the ongoing MCS expected to evolve to more of a widely scattered thunderstorm threat over the next few hours. Much of Minnesota is behind these storms and clear of any flash flooding threats. Into the Dakotas, the threat is more into tonight with scattered convection with localized areas of backbuilding posing and isolated flash flooding threat. ...Southeast... The entire Florida Peninsula was added to the Marginal Risk area as numerous showers and storms are tracking west towards the East Coast. That convection will reform over the Peninsula later this afternoon. An urban and small stream flooding threat remains. See MPD 1141 for more details on the near-term flooding threat. Further north into the Carolinas. A stalled out front is supporting nearly stationary thunderstorms mostly into eastern North Carolina. See MPD 1140 for more details. Wegman ...Previous Discussion... ...Southwest, Great Basin, and the Rockies... A Slight Risk for excessive rainfall was maintained for southern and central Arizona where HREF probabilities of locally heavy rainfall totals greater than 2 inches, as well as 20-30% 3 hour flash flood guidance exceedance in central Arizona and 40-60% 3 hour flash flood guidance exceedance across Tucson's CWA. An uptick in moisture transport is expected ahead of the upper trough and subtropical jet. Much of the area will have PW values nearing 1.5 to 1.75 inches which will help support locally heavy rainfall and scattered areas of flash flooding. A broad Marginal Risk remain in effect for the Great Basin and Rockies for diurnally driven convection with above average monsoonal moisture. This will lead to widely scattered to scattered convection across Nevada, New Mexico, and the Colorado Front Range. The pattern would support isolated flash flooding, with favored terrain, urbanized areas, box/slot canyons, arroyos/dry washes, and burn scars being the most prone to flooding. Additionally, areas farther north in the Great Basin and Rockies from Utah to Wyoming and Montana may see some locally heavy rainfall as anomalous moisture streams in from the Pacific ahead of a potent upper-level trough. Model guidance is indicating heavier QPF totals of 1-1.5 inches across northern Utah, eastern Idaho, and western Wyoming. Sensitive terrain would be favored for flash flooding. ...Southeast and Gulf Coast... A stalled frontal boundary along with diurnal heating will support convection capable of producing rainfall totals of 2-3 inches with a 20% chance for exceeding FFG exceedance according to the afternoon HREF. Closer to the Gulf Coast, a combination of a moist, tropical environment and a mid-level low /surface trough may bring locally heavy rainfall and some flash flood concern as well, therefore the Marginal Risk area was maintained for this period. ...Northern Plains into the Upper Midwest... Model guidance continues to indicate rounds of storms that form overnight in the Plains to head eastward towards Minnesota and Wisconsin. Generally, anomalous moisture and sufficient instability with lift from a shortwave should bring a general 1-2 inches of rain with embedded locally heavy rain rates that may exceed flash flood guidance. Renewed convection is possible ahead of an upper-level trough Saturday afternoon and evening across eastern Montana and western North Dakota that may also bring a localized flash flood threat. Campbell Day 2 Valid 12Z Sun Aug 30 2026 - 12Z Mon Aug 31 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF ARIZONA AND SOUTHERN UTAH... ...Southwest/Great Basin/Intermountain West... A persistent Monsoon environment characterized by PWATS in the 1.5-1.75 inch range and CAPE around 1000-1500 J/Kg, as well as increasing synoptic support as an upper-trough enters the Southwest, will support increasing thunderstorm coverage and periods of heavy rainfall from Arizona to southern Utah. During this time portions of Arizona and southern Utah will have increasing coverage of showers and thunderstorms and thus have a Slight Risk for excessive rainfall in effect. Elsewhere, a Marginal Risk of excessive rainfall remains for above average moisture fueling diurnally driven widely scattered convection across portions of the Rockies, and parts of the Great Basin. 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...with favored terrain, urbanized areas, box/slot canyons, arroyos/dry washes, and burn scars being the most prone to flooding. ..Southeast... During this period an upper level disturbance will be retrograding near the central Gulf in proximity to a dying frontal boundary. With precipitable water values of 2"+ expected, hourly rain amounts to 3" are possible where cells merge or train, thus maintaining an elevated threat for isolated flash flooding. The Marginal Risk was maintained across the Gulf Coast/Southeast. ...Upper Midwest/Northern Plains/Great Lakes... Upper level ridge axis will continue to shift to the east during this period which will increase monsoonal moisture reaching to the Plains and Midwest. Precipitable water values are expected to be in the 1.5-2" range. Where cells train or merge, hourly rain amounts to 2.5" are possible. Marginal Risk areas remain in effect. Campbell Day 3 Valid 12Z Mon Aug 31 2026 - 12Z Tue Sep 1 2026 ...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS SOUTHERN AND CENTRAL ARIZONA... ...Southwest/Four Corners/Eastern Great Basin/Rockies... A surge of Pacific moisture will be directed to the Southwest that will reach/exceed 90th climatological IVT values over the Desert Southwest. Meanwhile, multiple days of accumulating rainfall will likely increase soil saturation, particularly for Arizona. The unusually strong synoptic- scale forcing, increasing IVT and lingering 90th climatological percentile PWs, and increasingly sensitive soils will continue to support a Slight Risk for excessive rainfall and flash flooding for this part of the region. A broad Marginal Risk spans from Arizona/New Mexico northward to Utah and Colorado. ...Upper Midwest/Lower Great Lakes/Northeast... Organized showers and thunderstorms expected thanks to upper-level forcing for ascent supporting local enhancement. PW values above the 90th climatological percentile will revolve around the northern flank of the ridge and reach the Northeast by evening. The environment will be supportive for at least localized areas of excessive rainfall across much of New York, northern Pennsylvania, and as far east as the I-95 corridor from northern New Jersey to southern New England. A Marginal Risk remains in effect for portions of the Northeast. ...Eastern Gulf Coast... Shortwave energy moving westward underneath the closed 500 mb High will be the focus for potentially more numerous (organized) convective clusters during peak diurnal heating, especially when interacting with the Atlantic and Gulf sea breezes. A Marginal Risk remains in effect for the Gulf Coast/Florida. Campbell/Mullinax Day 4 and Day 5 Valid 12Z Tue Sep 01 2026 - 12Z Thu Sep 03 2026 ...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PARTS OF THE WEST AND FROM THE NORTHERN PLAINS AND MIDWEST TO THE NORTHEAST... ...Southwest/Four Corners/Easter Great Basin/Rockies... For Day 4 (Tue-Tue night), have pared the elongated, largely N-S Marginal Risk area. Favorable cool-season dynamical forcing ahead of the full-latitude trough along the West Coast (upper level difluence/DPVA and thus deep-layer Qs vector convergence) will favor more widespread showers and embedded thunderstorms over the northern portions of the outlook area (ID-MT and western WY), however the degree of deep-layer instability and thus the short- term rainfall rates remain questionable to support anything more than a Marginal (isolated) flash flood risk. Across the Southwest into southern Great Basin (AZ-NM into UT), somewhat more favorable CAPE and PWAT profiles (PWAT anomalies closer to 1 standard deviation above normal) will make for better thermodynamic support, however large-scale forcing southeast of the upper jet streak traversing the Central Great Basin will be much weaker. Therefore the risk here remains Marginal as well. By Day 5, the Marginal Risk shrinks a bit towards the Four Corners area, given the weakening dynamical support/flat upper ridging. Model QPFs (including AI guidance) aren't overly impressive nor clustered very well, even across the Southwest where the moisture anomalies remain most impressive. ...Northern Plains and Midwest... On Day 4, an upper level jet streak traversing far northern MN into southern Manitoba will help fire convection across eastern portions of the Northern Plains within the right-entrance region Tuesday afternoon-evening. While the spread in the QPF guidance is still quite high, kinematic and thermodynamic profiles do support some upscale growth (potential MCS) Tuesday night farther downstream across the Midwest as the resultant meso/convective enhanced shortwave flattens the upper ridge within this corridor. Until we get a better (more consistent) heavy rainfall signal from the guidance, for now the ERO will remain at Marginal. ...Upper Mid Atlantic and Northeast... Both Days 4 and 5 (Tuesday through Wednesday night) look to provide multiple opportunities for at least locally heavy rainfall given periodic transient upper jet streaks passing to the north. The ensuing favorable right-entrance region upper forcing and resultant convectively-enhanced shortwaves rippling eastward along the outer perimeter of the upper ridge will provide at least a couple of opportunities for more organized convection (one Tue and the other later Wed-Wed night). The areal spread in the model and ensemble mean QPFs at this point is still rather high, even on Day 5 (Wed- Wed night) when the globals show a better signal for heavier rainfall across Upstate NY into New England. For this reason, for now have opted to not hoist a Slight Risk in the Day 5 ERO across the Northeast, however suspect at some point one will be needed somewhere within the broad Marginal Risk area, pending on the guidance trends. Hurley 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 $$