FOUS30 KWBC 300822 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 422 AM EDT Sun Aug 30 2026 Day 1 Valid 12Z Sun Aug 30 2026 - 12Z Mon Aug 31 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN AND NEAR PORTIONS OF UTAH & ARIZONA... ...The West... A Slight Risk of excessive rainfall continues in and near central & northern Utah as well as central & southern Arizona for this period. A deep positively tilted trough swinging across Intermountain West today has lured anomalous moisture into the region. Scattered shower and thunderstorms have formed and there have been signs of cell training and occasional backbuilding. The 18z HREF and 12z REFS guidance suggest that the highest potential of heavy rainfall beyond 01z is where the Slight Risk remains. Although cell coverage should reduce from here on out, convection could bubble overnight into early Sunday morning as shortwaves interact with residual MU CAPE, which in spots remains 2000+ J/kg. Slot/box canyons, burn scars, dry washes/arroyos and urban areas are most at risk. ...Northern Plains into the Northeast... Shortwave energy tracking through the northern tier will trigger scattered convection with localized areas of backbuilding posing and isolated flash flooding threat near an area of 850 hPa confluence and precipitable water values of 1.5-2". There is a decent signal for 2-3" of rainfall, which could fall quickly near the central border of the Dakotas, as quickly as within a couple of hours where cells merge or manage to train. Localized Slight Risk impacts cannot be ruled out should this heavy rainfall impact urban areas. A Marginal Risk area spans from northeast Montana to southern portions of the Northeast. ...Southeast... Heavy rainfall possible across the Gulf Coast, Florida and for the coastal Carolinas with the onshore. PW values across the region are in the 2-2.5" range and instability from the warm waters offshore brings the potential of 3" in an hour where cells can manage to merge, train, or backbuild. Accumulations and/or rainfall rates of this amount would be most problematic in urban areas, given the moderate to high flash flood guidance values regionally. The Marginal Risk area remains in effect. Campbell/Roth Day 2 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... A broad Slight Risk for excessive rainfall and elevated threat for flash flooding remains in effect from Arizona northward to Utah and Colorado. During this period a series of upper level disturbances will be traveling through the deep longwave trough over the Pacific Northwest. Meanwhile Pacific moisture will be pooled over the region along a steady influx leading to anomalous moisture of 2-3 sigma above normal. This environment will be supportive of 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. The degree of moisture expected could lead to hourly rain amounts to 2" wherever cells sit, merge, or train especially near favored terrain, urbanized areas, box/slot canyons, arroyos/dry washes, and burn scars being the most prone to flooding. ...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 from the Minnesota Arrowhead to portions of the Northeast. ...Eastern Gulf Coast... The Marginal Risk for excessive rainfall remains in effect. 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. Campbell/Mullinax/Wegman Day 3 Valid 12Z Tue Sep 01 2026 - 12Z Wed Sep 02 2026 ...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PARTS OF THE SOUTHWEST & NORTHERN TIER OF THE COUNTRY... ...Southwest... Above average PW values expected to return to portions of Arizona as the upper level trough strengthens over the West during this period. 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. Therefore a Marginal Risk was maintained for portions of Arizona, New Mexico, West Texas and southeast Colorado. ...Northern Plains, Southern New England and northern Mid- Atlantic... A shortwave passing within the flow will trigger scattered convection across the northern tier states capable of producing hourly rates up to 2 inches/hour should cell training and mergers occur. Through this period PW values will be near 1.25-1.75 inches as Gulf moisture advects northward within the low-level flow. 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. A Marginal Risk spans from eastern Montana/Wyoming to southern New England. There may be a need for an upgrade in the future particularly across portions of the Megalopolis between Philadelphia and New York City should guidance trend higher. Campbell/Roth Day 4 and Day 5 Valid 12Z Wed Sep 02 2026 - 12Z Fri Sep 04 2026 ...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PARTS OF THE UPPER MIDWEST, GREAT LAKES, AND NORTHEAST... ..Northern Plains and Midwest... Lingering MCS activity on Day 4 (Wednesday), especially early, will be most likely across southern WI into southern portions of Lower MI, per the latest global ensemble 12-24hr QPF exceedance probabilities. PWAT anomalies remain elevated (1.5 to 2 standard deviations above normal for early September), however the spread in the deterministic QPF is still quite high, with the heavy rainfall signals in the guidance more sparse than otherwise. The models show shortwave energy (MCV) moving steadily east of the Upper Midwest and western GL by Wednesday afternoon, followed by shortwave ridging. Until we get a better (more consistent) heavy rainfall signal from the guidance, for now the ERO will remain at Marginal. By Day 5 (Thursday-Thursday night), strong right-entrance region upper jet forcing just behind the upper ridge axis (and attendant lobe of mid level vorticity) will traverse the Upper MS Valley. Expect convection to initiate and become better organized across the Northern Plains during the afternoon, then likely grow upscale into an MCS Thursday night upon reaching the Upper MS Valley/Upper Midwest. At this point, quite a bit of north-south variance in the models in terms of the axis of heaviest QPF, along with differences in the orientation (i.e. NW to SE vs. more WNW to ESE). Given the guidance spread and the transient nature of the shortwave energy, for now we have opted for a Marginal Risk in the ERO, though given the highly favorable thermodynamic parameters, an upgrade to Slight somewhere within the broad Marginal will be possible as the event draws closer. ...Upper Mid Atlantic and Northeast... The pattern will remain quite unsettled Wednesday and Thursday, as numerous showers and thunderstorms ahead of mesoscale/convectively enhanced shortwaves move across the area. This as the closed mid/upper low pivots slowly across Quebec and the northern stream trough impinges on the upper ridge. The activity should wind down by Friday as the surface front pushes south of the area. Thermodynamics will be favorable, especially the deep-layer moisture (PWAT anomalies 1.5 to 2 standard deviations above normal). However there is some uncertainty as to how unstable the environment will get across northern New England. For now, we continued with the Marginal Risk on Day 4 (from yesterday's Day 5 ERO), opting to hold off on a Slight given the model spread with the QPFs, along with relatively low 24hr probabilities of QPF>2" per the GEFS/ECENS/CMC. 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 $$