FOUS30 KWBC 291947 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 347 PM 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 THE INTERMOUNTAIN WEST... ...2030Z Update... ...Intermountain West... In coordination with all the forecast offices around the Four Corners, a large expansion of the inherited Slight Risk was made with this update. A deep longwave trough over the Pacific Northwest will guide several upper level disturbances northeastward along its periphery. Anomalous moisture of 2-3 sigma above normal over much of the area will support scattered showers and thunderstorms developing with the monsoon. Much of this area is very prone to flash flooding should heavy enough rains occur, which is likely given the history of afternoon showers and storms in this same region under the same pattern. Thus, given the storms are expected to once again be widely scattered to scattered, much like they are in many areas currently, the Slight Risk upgrade was made. ...Elsewhere... A few Marginal Risks in the northern Plains, Great Lakes to Mid- Atlantic, and along the Gulf Coast. Storms in the Plains and Great Lakes will be ridge-riders. They'll be generally fast-moving, but capable of heavy rain with decent probability of backbuilding and training to warrant an isolated flash flood risk. Along the Gulf Coast and Florida, downright tropical amounts of atmospheric moisture will fuel numerous storms once again, with a semi-tropical low in the northern Gulf near coastal Louisiana helping focus the convection. In Florida, daily sea breezes appear to favor either coast for potential slow-moving storms. Wegman ...Previous Discussion... ...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 01 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE INTERMOUNTAIN WEST... ...Intermountain West... In coordination with the various forecast offices, a large expansion of the previous Slight Risk area to include the 4 Corners region was made with this update. Total rainfall across this region is expected to be far more significant on Monday than on Day 2/Sunday. An upper level shortwave, trapped at the base of the trough over southern California on Sunday night ejects out into the Intermountain West on Monday. This could be paralleled with a second shortwave a bit to its east. The result will be much better forcing especially from the upper levels on Monday as compared to Sunday. Instability will likely be the primary limiting factor, as prior days' storms are likely to have left decent cloud cover that will need to be eroded to allow for maximum instability Monday afternoon. With the fast flow aloft, this could happen to some degree. Given both prior days' rains and the flashiness of those basins, expect more scattered instances of flash flooding, some of which may be considerable. ...Elsewhere... Marginal Risks remain across portions of the Northeast, with an expansion over the UP of Michigan and small neighboring areas, as well as along the Gulf Coast. No major changes to either area. Ridge riding shortwaves will still contribute to areas of heavy rain in the Northeast which could cause localized flash flooding, particularly in urban and low-lying areas. Along the Gulf Coast, much heavier rains with far more abundant moisture will make for another day of heavy rains from the Florida Peninsula back through New Orleans. High FFGs should keep any flash flooding restricted to urban and low-lying areas. Wegman ...Previous Discussion... ...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 SOUTHWEST & NORTHERN TIER OF THE COUNTRY... ...Southwest... A strengthening upper level trough across portions of the West and associated front extending across the West/Great Basin/Rockies eventually lures precipitable water values back above average late Tuesday night through Wednesday/Wednesday night across portions of southern AZ, while they remain ample enough for heavy rain concerns across portions of NM which is far enough away from the warm core ridge drifting east from the Lower MS Valley into the Southeast for mid-level capping to not be a significant concern. Hourly rain amounts up to 2" are possible given the moisture and instability expected, which would be most problematic in burn scars, slot/box canyons, dry washes/arroyos, and urban areas. The Marginal Risk level still looks most reasonable, though the areal extent was quite reduced across the Southwest/Great Basin both Tuesday and Wednesday given the synoptic pattern and precipitable water values forecast from the 12z guidance. ...Northern Plains... There's a heavy rain signal in the guidance that is strongest across SD/southern NM Tuesday and in the vicinity of ND on Wednesday. Its cause is a shortwave passage on Tuesday and the attempted emergence of a strong and possibly negatively tilted vorticity lobe out of the Western Trough into MT Wednesday night into Thursday morning. The 12z model QPF signal still favors the Marginal Risk category over anything higher. Precipitable water values of 1.25-1.75" are seen at times across the region during a couple periods of return flow from the direction of the Gulf on Tuesday and again late Wednesday night/early Thursday, which would allow for hourly amounts to 2" should enough cell coverage occur to allow for brief periods of cell training and cell mergers. ...Northern Mid-Atlantic and Southern New England... A moist west to west-northwest low-level flow imports precipitable water values of 1.5-2" across the region on the northeast side of a warm core ridge as upper level disturbances near a front lead to heavy rainfall concerns. Hourly rain amounts to 2.5" with local totals to 5" are possible given the ingredients expected, with Wednesday the wetter day across the region. There's enough dispersion in the guidance to preclude a Slight Risk on Wednesday/ Wednesday night, for now, but if the guidance becomes more agreeable, a higher risk than Marginal would become necessary, particularly across portions of the Megalopolis between Philadelphia and New York City. 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 $$