273 FOUS30 KWBC 081957 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 357 PM EDT Sat Aug 8 2026 Day 1 Valid 16Z Sat Aug 08 2026 - 12Z Sun Aug 09 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE LOW COUNTRY OF SOUTH CAROLINA AND GEORGIA... ...Low Country... 16Z Update: Weak disturbance is analyzed off the coast of GA with the northern periphery of the inverted trough nosing into southern SC with the radar depiction over Charleston finally showing some activity sneaking ashore. The setup remains conducive for locally heavy rainfall as PWATs remains ~2 standard deviations above normal within the immediate coastal plain of GA and southern SC. The cutoff on the northern edge of the disturbance is sharp, however, as noted via a solid ~0.7" difference in area PWATs between the 12z KJAX (2.42") and KCHS (1.74") soundings this morning associated with the disturbance. Mesoanalysis does show the more prolific tongue of higher PWATs slowly advecting northward into the Charleston area, but the greater PWAT anomalies are still forecast to be closer to Hilton Head down into coastal GA where the heaviest rain is more likely to occur. Latest HREF probs remain more prolific for localized 3+ inch totals between Brunswick up through Savannah and nudging into Hilton Head. Probs fall off as you move north, so it could be a case where Charleston proper misses out on the worst, but it'll still be very close, so maintained the city in the SLGT risk, but just barely as of this latest update. Expectation is for the heaviest rain to occur now until 21z as the disturbance is forecast to shift northeast and the rain chances dwindle heading into the evening. As a result, expectation is for the SLGT risk to likely fall off to a nil during the 01z update, so the primary period of concern will be over the next 4-6 hrs. Kleebauer ...Other Marginals across the Country... 16Z Update: The broad MRGL risk extending from New England into the Lower Mississippi Valley was relatively unchanged as a prominent deep moist airmass with widespread elevated instability will maintain an environment conducive for locally heavy rainfall with rates between 1-2"/hr pretty much anywhere within the aforementioned area. The most notable adjustments were made over southern VA where the MRGL was expanded to include a bit more of the I-95 corridor between Fredericksburg through Richmond, down into Emporia. Heaviest rainfall threat will likely situate west of I-85 in VA, but some of the CAMs are a bit more aggressive towards Richmond, so wanted to include the urban center as this area of VA is more susceptible to flash flood concerns due to the urban footprint. New England remains generally unchanged as well, although did trim a touch more of southeastern MA out of the MRGL given the latest trends of convection expected to be a bit more northwest than originally progged. The Southwest U.S. will continue to see Monsoonal moisture play a role in the daily convective regime with the expansion of higher moisture northward into the Great Basin for today. Latest CAMs are privy for a bit more thunderstorm coverage into NV for this afternoon and early evening before waning after sunset. The MRGL risk was expanded a bit further west to encompass the trend, however, did maintain the nil into Vegas proper as the threat looks to avoid the main urban area, but will be something to watch if any outflows can buck the trend and sneak into the area from the east. Otherwise, the heat remains the talking point for the area with the ridge overhead. Primary concerns will be those areas in southwestern UT, northern AZ, and the Mogollon Rim due to the slot canyons and susceptible grounds for run off. Kleebauer ..Previous Discussion.. Marginal Risks cover nearly the entire Eastern Seaboard, much of the Southeast, and relatively small areas of the Upper Midwest and Southwest. For the East, the same pocket of deep tropical moisture hosting the aforementioned area of disturbed weather off the Atlantic coast of the Southeast will support widely scattered showers and storms from the lower Mississippi Valley up the length of the Appalachians and into the Northeast. This will occur this afternoon at peak heating. Multiple rounds of storms may impact portions of Arkansas and Mississippi as well as into New Hampshire and western Maine today. In both these areas, slightly better forcing along the moisture gradient will support additional storms impacting these areas compared to the rest of the Eastern Seaboard. Both areas also feature drier than normal soils, which should help to temper any flooding to isolated instances. Thus, while Slights were considered, neither were upgraded for this reason. The Marginal in the Midwest was trimmed from the southwest, an unusual direction. Guidance continues to trend drier and further north and east, so the Marginal was adjusted accordingly. A Slight risk upgrade in this area is no longer being considered. Monsoonal moisture will continue to impact portions of the Southwest, following the Mogollon Rim, with broader southwesterly flow allowing a few showers and storms to impact portions of northeastern New Mexico. Any showers and storms will be few and far between, so the flash flooding threat there will be isolated. Wegman Day 2 Valid 12Z Sun Aug 09 2026 - 12Z Mon Aug 10 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE UPPER GREAT LAKES AND UPPER MISSISSIPPI VALLEY... ...Upper Great Lakes & Mississippi Valley... 20Z Update: The overall pattern remains favorable for a multi- round convective episode during the D2 period with the area of greatest focus likely to materialize over northern IL and southern WI and points east with the Chicago area the most notable area of potential impacts. There's a growing concern for a more locally significant flash flood prospect for the Chicago metro, including adjacent suburbs with the nocturnal convective maturation that's expected on Sunday night into early Monday morning. Warm front positioning and a strengthening 850mb LLJ will be key components in the threat over the area, one that is more susceptible to flash flooding thanks to the broad urban sprawl that extends from southern Lake Michigan and encompasses the entire area spanning northeast IL through northwestern IN. 12z HREF neighborhood probabilities for >3" are upwards of 60-80% currently for the above region, including the center of Chicago proper where high run off capabilities are notorious for local flooding hazards. There are even some notable >5" probs of 40-60% within the area spanning the western and northern Chicago suburbs down through the center of Chicago proper, a testament to the potential in this setup, and a general agreement from the CAMs on the positioning of the heavier rainfall. Ensemble mean QPF is greater than 2" over much of the Chicago metro, a signal that typically favors at least scattered flash flooding with perhaps widespread flash flood concerns if everything comes together perfectly. The back-building scenario with the setup is the reason for such a strong mean QPF signature, correlating well with the strong low-level convergence regime and locally backed flow between sfc-700mb positioned near the warm front. Ultimately, that frontal positioning will be pivotal in where the heaviest rainfall will materialize, and current indications is it will be within that zone bisecting northern IL into northern IN by the second half of the period. In this case, there is enough of a signal and general consensus on a high-end SLGT risk forecast for that zone with emphasis on northeast IL and northwest IN given the elevated probabilities for >3" of rainfall. Kleebauer ..Previous Discussion.. A series of disturbances moving eastward along the southern periphery of a large upper low well into Canada will result in multiple rounds of storms on Sunday across the Slight Risk area. The first will be the remnants of an MCS that will start the day in far southeastern Minnesota, and will then track south and east along the Wisconsin/Illinois border. The storms in general should be pretty fast moving, so it's unlikely too many flooding impacts will be seen from this feature alone. However, what it will do is prime the soils by moistening them up for the later round. The second round will form at the nose of a developing low level jet Sunday night. The storms will follow the greatest instability, which will mean the storms will track southward. The should be significantly slower moving, and much more widespread compared to the first round earlier in the day. The widely scattered instances of flash flooding associated with the Slight will be mostly confined to this Sunday night period. Urban and small stream impacts in and around Chicago and possibly Milwaukee are the most likely impacts. Wegman ...Marginals Elsewhere... 20Z Update: Only minor adjustments made to the remaining MRGL risks across both the Eastern and Southwestern U.S. Primary change was a narrow expansion of the MRGL to include the VA Tidewater and southern VA as guidance insists on another round of convection with rates between 1-2"/hr impacting these areas by Sunday afternoon and early evening before dwindling with the loss of diurnal instability after sunset. Southwestern U.S. remains on track for prevalent monsoonal moisture making its presence known across AZ, extending northward through the Great Basin. Kleebauer ..Previous Discussion.. As on previous days, deep tropical moisture across the Southeast will favor another round of afternoon showers and storms from northern Mississippi into the southern Appalachians. Flash flooding will likely be confined to areas where the storms are most persistent (northern Mississippi), or where terrain helps funnel the rainwater (southern Appalachians). Generally the storms, while scattered, should be disorganized, so a Marginal was maintained for now, though targeted Slights are certainly possible in the coming days. The monsoon will continue across the 4 Corners region on Sunday. While a bit more widespread over the Southwest, most organized convection will be very limited, so few changes to the inherited Marginal Risk were needed. Wegman Day 3 Valid 12Z Mon Aug 10 2026 - 12Z Tue Aug 11 2026 ...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL OVER PORTIONS OF THE GREAT LAKES AND SOUTHEASTERN ARIZONA... ...Great Lakes... 20Z Update: The setup for D3 remains pretty consistent in run-to- run continuity with the highlight of heavy rainfall extending over the southern Great Lakes and northern Ohio Valley into western PA and the adjacent WV Highlands. A lot of the setup is contingent on the behavior of any complexes that materialize upstream over the Midwest and migrate east through the forecast. Looks like activity will be prone to producing rates upwards of 2"/hr, locally higher given a robust moist and unstable airmass present on the northern periphery of a broad ridge to the south, and the southern inflection of a sprawling ULL over Canada. In any case, the setup is textbook for an area of above normal convective chances that will take advantage of both a favorable thermodynamic and kinematic environment as the positioning of the jet will favor large scale ascent across much of the Great Lakes and Ohio Valley into the Appalachian front. As of now, the only change from the previous forecast was to expand the SLGT a bit further south into WV and east to bank up against the western slopes of the Appalachians in PA. Will be monitoring this setup closely as a high-end SLGT is not out of the realm of possibility considering all of the above factors, as well as the addition of terrain influences on the eastern extent of the current SLGT. Kleebauer ..Previous Discussion.. A large upper level low well over Canada will spin a strong shortwave across the Great Lakes on Monday. Deep tropical moisture will be drawn northward from the reservoir of moisture over the Southeast, allowing for widespread showers and thunderstorms, generally from Chicago east to portions of the Central Appalachians. The inherited Slight was expanded to the east, to includes much of eastern Ohio, western Pennsylvania, including Pittsburgh, and far western New York, including Buffalo with this update. Wegman ...Southeastern Arizona... 20Z Update: A favorable setup for heavy rainfall across southeastern AZ exists within a majority of guidance with ensemble mean QPF pushing >1" over much of the terrain focused within eastern Pima, Santa Cruz, and Cochise counties along and south of I-10. The previous high-end SLGT forecast continues to have merit with some of the deterministic outputs pushing local totals up near 3", a threshold that would undoubtedly cause locally significant flash flood concerns within the aforementioned areas. Areas like the Huachuca mountains over to I-19 are the most likely areas to see significant rainfall in the setup. As we move closer to CAMs range, it will be an area to monitor for potential further upgrades if the signal becomes more pronounced in the general magnitude of the rainfall forecast. For now, the SLGT remains with the highlighted area across the southern tier the noted high-end SLGT risk. Kleebauer ..Previous Discussion.. Deep tropical moisture moving up the Gulf of California will intrude into southeastern Arizona on Monday, with PWATs exceeding 1.5 inches in many areas. A thermal low due to the strong high aloft will support widespread shower and thunderstorm activity, particularly along the Mexican border and up I-19 from Nogales through Tucson. This corridor is both highly flood prone, has seen numerous days of widely scattered flash flooding in recent days, and is right in line for additional widespread convection Monday afternoon. There is considerable uncertainty moving west up I-10 beyond Tucson, so the introduced Slight is primarily for this most vulnerable corridor, where a higher-end Slight in in place. Wegman Day 4 and Day 5 Valid 12Z Tue Aug 11 2026 - 12Z Thu Aug 13 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE OHIO VALLEY... 20z Update: Still on track for an excessive rainfall threat Tuesday from portions of IL/IN into WV. Some uncertainty on the exact latitude of the threat, and not seeing any strong trend in todays models. Thus only made a small southward expansion of the Slight risk to account for the consistent AIFS, which has been on the southern end of the model envelope. We were able to cut back on the Marginal risk over the Northeast. Only the GFS keeps robust convection across this region...with the ECMWF/ECENS/CMC/AIGFS all showing drier air over the northeast and the better convective threat farther southwest. On day 5 (Wednesday) a heavy rainfall threat will continue on the periphery of the southern U.S. ridge, with convection from the Southwest into the central/northern Plains, and then extending southeastward into portions of the OH and TN valley. The inherited Slight risk over WV was made a bit larger to include some of eastern KY and southern OH. Overall confidence in this area is low, as seeing divergence in model QPF solutions. Think there will be an excessive rainfall threat, but could easily see this Slight risk area shift in location and/or size over the coming days. Still seeing a model signal for organized convection over portions of SD/NE, but enough uncertainty on those details to keep a Marginal for now. The Southwest still look to be more active Wednesday, but spatial confidence too low to introduce any slight risk area at this time. Chenard ...Previous Discussion... ...Great Lakes/Northern and Central Plains/Midwest/Upper Ohio Valley... Early to mid next week, broad cyclonic flow expected to persist across the northern tier of the Lower 48 from the Northern Plains/Upper MS Valley, eastward through the Midwest/Great Lakes and into the Northeast, to the south of a closed mid-level low moving from Hudson Bay across Quebec. A frontal boundary will lie along the southern periphery of this broad trough. Embedded shortwaves in this broad cyclonic flow will accentuate lift in the vicinity of this front where PW values are expected to rise to ~1.75" ahead of each set of shortwaves and low- level confluence is expected. When looking at the ingredients, organized convection looks most likely south of the Great Lakes within eastern portions of the Corn Belt Tuesday into Tuesday night and across the Upper Ohio Valley Wednesday into Wednesday night, where hourly rain amounts to 2.5" are possible. There is significant disagreement on Wednesday/Wednesday night on where a heavy rain axis would be across the eastern Corn Belt/Midwest, so for the time being, no Slight Risk has been drawn/assigned. Recent guidance nudged north, so the Slight Risk area on Tuesday was shifted similarly. For Wednesday/Wednesday night, a Slight Risk area was introduced for WV where the guidance has relatively good agreement on the potential for heavy rainfall within areas of significant relief. ..Southwest/Colorado... Broad Marginal Risk areas exist early to mid next week across portions of the Southwest, Rockies, and Great Basin. Ridging/warm 700 hPa temperatures shifts north and weakens/lowers somewhat across the Intermountain West, which opens the door to broader convective development in the Southwest at some point, potentially next Wednesday, where above average precipitable water values persist over the next five days with minimal day to day variation. While there continues to be low confidence with the details, locally heavy rains with the potential of 2" an hour amounts could become more expansive Wednesday. A Slight Risk cannot be ruled out across portions of this area as we get closer to the days in question. As usual, urban areas, arroyos/dry washes, box/slot canyons, and burn scars would be most at risk for flash flooding. 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 $$