FOUS30 KWBC 301911 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 311 PM 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 MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF UTAH... ...Desert Southwest and Four Corners... After coordination with WFO SLC, an upgrade to a MDT risk has been issued for portions of Utah. Convection is expected to once again be widespread on Monday as the region remains entrenched between an approaching trough from the west and an anomalous ridge over the Mississippi Valley. Within this pinched flow, 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 RRQ 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 AZ/UT and western CO. 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 UT/CO, 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. With this activity occurring atop a naturally vulnerable area, aided by anticipated heavy rain on D1/Sunday and noting impacts from heavy rain that occurred Saturday, adjustments to the inherited risk areas were significant and coordinated. This included the MDT for Utah, an expansion of the internal 25% area surrounding the MDT, and a secondary 25% area over far southern AZ. 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... Complex dual-frontal structure will provide the impetus for convective development Monday as weak impulses crest the ridge and dig E/SE along these boundaries. Mean 0-6km winds of 20-25 kts suggest convection will be progressive to the E/SE, but ample 30-40 kts of bulk shear will support organization into clusters to enhance rain rates to 1-2"/hr (thanks to elevated PWs) offsetting at least somewhat the faster motion of these storms. FFG exceedance is modest (less than 15%) so the inherited MRGL risk remains with only cosmetic adjustments. ...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). This system will drift westward beneath the anomalous ridge over the MS/TN VLY, and approach the Upper Texas coast by the end of the forecast period /Tuesday morning/. 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. However, the overlap of elevated MUCAPE/PW will support heavy rain within convection that occurs along the Gulf breeze, with some enhancement possible due to convergence north of AL97. The MRGL risk was maintained and modified for updated guidance. Weiss Day 3 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... Invest AL97 will be drifting westward along the Central Gulf Coast Tuesday morning, with the guidance suggesting it will move onshore the Upper Texas Coast early Tuesday morning. Despite the uncertainty into both timing and intensity, there is an increasing threat for heavy rainfall as this system approaches the coast and then moves onshore. With PWs expected to be above 2.25", and 850mb inflow increasing onshore to intensify ascent, it is becoming more likely that several inches of rainfall will occur across this region on Tuesday. At this point it is still too early to discuss specific amounts and locations, but the SLGT risk was coordinated with WFO HGX and WFO LCH to highlight the most likely area for at least 3" of rain, with some deterministic guidance suggesting much higher amounts are possible. ...Portions of the Southwest... The extremely active period across the Southwest wanes drastically on Tuesday as the mid-level trough axis pushes onto the CA coast driving 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. There will continue to be a modest threat for generally fast moving cells from far southeast AZ through NM and into CO, but in general the risk is much lower Tuesday and will be confined to any heavier rain rates that move across vulnerable soils/terrain or regions saturated from prior rainfall. The inherited MRGL risk was maintained and cosmetically adjusted for a continued isolated threat. ...Great Lakes to New England... Pinched flow around the periphery of a ridge centered over the TN VLY will serve as a corridor through which mid-level impulses will track WNW to ESE from the Great Lakes through the Northeast. At the same time, a stationary front will waver within this pinched flow, upon which these shortwaves will act to enhance 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 guidance suggests that there may be 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. At this point the MRGL risk remains, but should any repeating rounds cause the heaviest rain to fall atop the more vulnerable soils around the Tri-State area of CT/NY/NJ, an upgrade to a SLGT risk may be needed. ...Gulf Coast of the Florida Peninsula... 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 Tuesday, 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, prompting the addition of this MRGL risk area. Weiss Day 4 and Day 5 Valid 12Z Wed Sep 02 2026 - 12Z Fri Sep 04 2026 ...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PARTS OF THE NORTHERN MID-ATLANTIC STATES WEDNESDAY/WEDNESDAY NIGHT... ...Northern Mid Atlantic & Southern New England... The pattern will remain quite unsettled Wednesday and Thursday, as numerous showers and thunderstorms ahead of mesoscale/convectively enhanced shortwaves move across the area on the northern side of a warm core ridge. The activity should wind down by Friday as the surface front pushes south of the area. Thermodynamics will be favorable, especially considering the precipitable water values of 1.75-2" available for portions of Wednesday and Thursday. The guidance heavy rain signal shows rather good agreement for Wednesday/Wednesday night considering the forecast range, so added a Slight Risk to portions of NY, PA, & CT. Hourly rain amounts to 2.5" with local totals to 5" are possible in this environment. As the wet ECMWF trended down this cycle, removed the Marginal Risk area from much of ME Thursday into Thursday night, which was coordinated with CAR/the Caribou ME forecast office. ...Northern Plains and Midwest... Lingering MCS activity in possible early on Day 4 across the Upper Midwest, though it is unclear if it would lie close to the IA/MN border or across southern MI, based on the 12z guidance. Warm air advection should be occurring in earnest Thursday into Thursday night, allowing precipitable water values and a low-level focus to waver across the region mid-week. 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. Until we get a stronger and more consistent heavy rainfall signal from the guidance, the excessive rainfall category will remain at Marginal both days. ...Southeast TX... A convective low in the Gulf of Mexico is expected to move into southeast TX early to mid-week. 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. Marginal Risk areas have been added to southeast TX for both days 4-5 to cover this system's heavy rainfall prospects, and later shifts can determine if we need to step up to a Slight Risk or higher, which is a real possibility/concern. It should be wet in its vicinity whether or not it develops into a tropical cyclone. See NHC Tropical Weather Outlooks for more information on its chances of TD/TS formation. ...In and near FL... 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. Marginal Risks of excessive rainfall have been drawn for both days, 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. 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 $$