FOUS30 KWBC 301557 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1157 AM EDT Sun Aug 30 2026 Day 1 Valid 16Z Sun Aug 30 2026 - 12Z Mon Aug 31 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHWEST AND FOUR CORNERS... ...The West... An active monsoon day is still expected today as the region becomes increasingly pinched between an anomalous mid-level ridge centered over the MS VLY (500mb heights above the 97th percentile) and an amplifying trough over the Eastern Pacific. The pinched flow between these two features will manifest as intensifying 850-500mb SW winds, which will draw impressive thermodynamics northward from Baja, Mexico through the Central and Northern Rockies. The accompanying IVT plume is progged to exceed the 99.5th percentile according to NAEFS, driving PWs to above 1 inch as far north as the Four Corners, overlapping MUCAPE of more than 1000 J/kg, with 500+ MUCAPE arcing all the way into southern WY. Additionally, embedded shortwaves within this flow will help focus ascent which will already be robust through the RRQ of a developing downstream jet streak. Together, this all suggests widespread thunderstorm activity today from Arizona/New Mexico northward into Utah/Colorado, with some lingering activity this morning even stretching into Wyoming (although that region should wane by the aftn as drier air advects eastward). Sufficient shear of 30-40 kts combined with widespread cells will likely lead to clusters of storms either through organization or mergers/collisions, through which rainfall rates could top 1"/hr. Although 0-6km mean winds are likely to remain progressive at 15-25 kts, parallel propagation vectors suggests repeating storms to offset the rapid motions, suggesting the inherited SLGT risk remains needed as FFG exceedance reaches 20-30%. The greatest risk will be across vulnerable soils and terrain features, as well as urban areas this aftn/eve. Tonight, the guidance indicates a stronger vort max may lift within the flow and result in an impressive cluster of storms moving northward through Arizona. This may produce a more widespread excessive rainfall risk tonight that could impact areas that are generally less vulnerable, but that development is uncertain enough that no changes to the ERO are needed. ...Northern Plains into the Northeast... An elongated frontal boundary aligned generally west to east from North Dakota, through the Great Lakes, and into the northern Mid- Atlantic will serve as a boundary for convective development today. While convection will likely be scattered, and generally focused where shortwaves tracking within the flow overlap the low-level convergence, sufficient 0-6km bulk shear of 25-40 kts will help organize storms into clusters, through which rainfall rates could reach 1-2"/hr. While there is considerable uncertainty into the placement of clusters today (as well as the overall coverage), any storms that due develop will occur within favorable thermodynamics to support heavy rain, with some training likely through aligned Corfidi vectors/mean winds. For this reason the inherited MRGL risk was modified only slightly account for new guidance. ...Southeast/Gulf Coast... Slow moving and weak upper low drifting westward along the Central Gulf Coast combined with a weakening TUTT positioned east of Florida will combine to produce another active day of slow moving thunderstorms from the Gulf Coast through the Southeast Coastal Plain. A plume of impressive PWs exceeding 2 inches will remain entrenched across this region, which when combined with ample MUCAPE of 750-1500 J/kg and 14,000+ ft of warm cloud depth will support 2+"/hr rain rates (20-40% chance). Although shear will be minimal, suggesting primarily pulse type storms, very slow movement (0-6km mean winds just 5 kts) and weak/anti-parallel Corfidi vectors indicate a likelihood for some training or mergers to enhance the rainfall potential. Although this region continues to have high FFG, 0-40cm soil moisture has increased due to recent heavy rain, making portions of the area slightly more vulnerable to impacts. While the inherited MRGL risk remains, some adjustments were made, mostly to expand it north along the eastern Carolinas, along with other small cosmetic changes to account for the latest HREF guidance. Weiss 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 $$