FOUS30 KWBC 080730 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 330 AM EDT Sat Aug 8 2026 Day 1 Valid 12Z 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... An area of disturbed weather moving up the Atlantic coast off of north Florida is expected to impact the Low Country of South Carolina and Georgia as soon as in the next few hours. Deep tropical moisture with PWATs exceeding 2 inches accompany the associated showers and thunderstorms. Thus, the storms will be quite capable of very heavy rainfall in the affected areas. The big question in the guidance is how far inland this activity will reach. Much of the guidance suggests it will impact just the immediate coastal areas from Charleston south through Hilton Head and Savannah. Others are struggling to even show that...keeping most of the associated rainfall this morning offshore. Given this discrepancy, the inherited Slight was maintained, albeit shrunk to the immediate coast from about halfway down the South Carolina coast to halfway down the Georgia coast. Urban and small stream flooding in Charleston and Savannah will be most likely, as opposed to flooding in the areas in between. Flooding impacts will be worsened at high tide. Fortunately, high tide is around 4-5am, and the greatest likelihood of showers and storms impacting the Slight Risk area will be in the late morning around low tide, which should mitigate the flooding threat significantly. ...Other Marginals across the Country... 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... 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 and Detroit are the most likely impacts. ...Marginals Elsewhere... 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... 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. ...Southeastern Arizona... 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 EASTERN CORN BELT & UPPER OHIO VALLEY... ...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 $$