FOUS30 KWBC 060813 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 413 AM EDT Sun Sep 6 2026 Day 1 Valid 12Z Sun Sep 06 2026 - 12Z Mon Sep 07 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MID-MISSOURI RIVER VALLEY, GREAT BASIN, AND SOUTHEAST... ...Mid-Missouri River Valley... In collaboration with FSD/Sioux City, SD and DMX/Des Moines, IA forecast offices, a Slight Risk was introduced with this update for southeastern South Dakota, the southwest corner of Minnesota, much of far western Iowa, extreme eastern Nebraska along the Missouri River, and the northwest corner of Missouri. Afternoon convection is expected to break out across eastern South Dakota in response to a ridge riding shortwave in the upper levels encountering a narrow plume of Gulf moisture riding north up the Missouri River Valley, the daytime remnants of an overnight low level jet. Instability will be maximized at this time, with some guidance suggesting values exceeding 4,000 J/kg. To the east of this plume of moisture over much of Minnesota and eastern Iowa, easterly flow from the Great Lakes of significantly cooler and drier air will create a "wall" of sorts that will prevent the convection from tracking too far east. Of course, there is inherent uncertainty as to where this "wall" sets up, but it appears it's most likely across central Minnesota and Iowa. As a result of the presence of this wall preventing much eastward progress, the storms will turn southward following the northbound plume of moisture and instability. With nightfall, the storms from the afternoon are likely to diminish or completely dissipate. Most of the day's rainfall is expected from these afternoon storms. However, the typical strengthening of the nocturnal low level jet will continue advecting plentiful moisture and instability into the area overnight. Guidance is struggling with what forms as a result, but it seems likely that at least some area of light to moderate showers and a few storms will redevelop in the same area of the Missouri River Valley through the overnight, adding rainfall to the same areas hit earlier that afternoon. A small higher-end Slight was also added over the northwest corner of Iowa as this is generally agreed upon in the guidance to be the area where FFG values are most likely to be exceeded. ...Great Basin... Few changes were needed for the Slight Risk area which extends from around Death Valley, CA northeast across southern Nevada and the Las Vegas metro and into southwestern and central Utah. A narrow plume of mostly light to moderate rain will develop along a shear axis and parallel to a developing upper level southwesterly jet streak. The plume rain may extend southwestward as far back as the Los Angeles metro, where a Marginal Risk remains in place. The Slight Risk really highlights a series of areas where FFGs are naturally low, mainly due to highly vulnerable desert areas not used to prolonged periods of rain, even if it's of light to moderate intensity. Since the plume of rain will not be moving much today, just the cells embedded within it, the Slight Risk remains in place. ...Southeast... In coordination with GSP/Greenville-Spartanburg, SC and CAE/Columbia, SC forecast offices, the inherited Slight Risk was expanded to include essentially all of South Carolina and into portions of western North Carolina with this update. Afternoon convection is likely to break out across the Slight Risk area during the afternoon, partially from the advance of a back door cold front over eastern North Carolina, and partially from a deepening longwave trough in the upper levels extending down into north Florida. Scattered to numerous thunderstorms will develop, in the north forced by the back door cold front running into the mountains, and to the south from the upper level trough and local cold pools. As the back door cold front continues moving southwestward down the Piedmont of the Carolinas, it's unlikely to make any progress west into the southern Appalachians. This will keep the front stuck in this region, allowing for training and backbuilding storms to continue into the overnight. The storms will track southward along the frontal interface. Meanwhile from southern SC into north Florida, storm interactions with each other will keep the storms going well into the overnight. Wegman Day 2 Valid 12Z Mon Sep 07 2026 - 12Z Tue Sep 08 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR A PORTION OF NORTHERN MONTANA... ...Northern Montana... A dynamic setup meteorologically will be in place across Montana on Monday. A Plains low tracking northward will support advecting plentiful Gulf moisture westward ahead of the low into western Montana. This will start Sunday night, but as the Plains low strengthens, so too will the moisture advection into Monday morning. Meanwhile a strong pair of upper level shortwaves will advect east across the mountains. The first will be over western Montana at the start of the period, then the second will follow Monday night. The first wave interacting with moisture that will be over 3 sigma above normal for this time of year will provide the bulk of the rainfall into western Montana. This first shortwave will move northeast with the Plains low, but with the second one off to the west, this second one will prolong light to moderate rain in this region through the rest of the day Monday and into Monday night. When added to any rainfall that falls in this area Sunday night at the start of the storm, Monday's forecast of a broad 1-2 inches of rain over these normally dry and flood-prone areas of western Montana and where terrain will locally enhance the severity of any flash flooding all points to the Slight Risk upgrade being needed for this day 2 period. ...Marginal Risks... The plume of moisture to the east of the low center over northern Minnesota could result in localized flash flooding. However, the highly progressive nature of the storms towards the east should limit the flash flooding potential. Monsoonal moisture continues into portions of the Southwest, sparking a new round of showers and storms Monday afternoon, primarily focused into western New Mexico. Isolated instances of flash flooding are probable. Afternoon showers and storms are also expected into portions of the Southeast again on Monday afternoon. Onshore flow should somewhat limit the instability at the coast. The stalled out front some 50-100 miles north of the Gulf Coast will provide forcing for storms, but abundant cloud cover should limit storm intensity. With normally high FFGs in this area, a Marginal Risk appeared a good balance of these competing factors. Wegman Day 3 Valid 12Z Tue Sep 08 2026 - 12Z Wed Sep 09 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR NORTHERN MONTANA, UPPER MISSISSIPPI VALLEY INTO THE UPPER GREAT LAKES, THE SOUTHWEST, AND PORTIONS OF THE SOUTHEAST... ...Marginal Risks... Lingering moisture from the departing low into Canada could allow for a continuation of flooding that likely starts on Monday into the day on Tuesday. Any new instances of flash flooding on Tuesday should remain isolated. Monsoonal moisture in the Southwest may diminish somewhat from previous days, but enough should still be around for widely scattered shower and thunderstorm development from southern Arizona into southwestern Colorado. An isolated instance of flash flooding or 2 are possible. The trailing cold front behind the low moving into Canada will slow its forward movement from eastern Nebraska into the upper Great Lakes. Afternoon and overnight thunderstorms ahead of the front may backbuild and train in some areas. Isolated flash flooding is possible where backbuilding and training occur over flood-prone areas. Numerous showers and thunderstorms will impact the Gulf Coast and the Florida Peninsula on Tuesday. Due to high FFGs and generally low storm organization, once again isolated instances of flash flooding will be possible, but nothing out of the ordinary is expected. Wegman Day 4 and Day 5 Valid 12Z Wed Sep 09 2026 - 12Z Fri Sep 11 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS CENTRAL TO SOUTHERN CALIFORNIA, THE SOUTHWEST, THE CENTRAL PLAINS TO THE LOWER MISSISSIPPI VALLEY, THE UPPER OH VALLEY/CENTRAL APPALACHIANS AND NORTHERN NY STATE INTO NORTHERN VT... ...Southwest... PW values will remain very anomalous both day 4 and 5 across central to southern California, the southern Great Basin and Southwest from tropical moisture being drawn northeastward by the closed low amplifying days 2 and 3 well off the CA coast. Low confidence in any precip in this anomalous PW axis, 2-3+ standard deviations above the mean, but continued potential for localized heavy rains and isolated runoff issues across these regions. ...Central Plains...Lower Missouri Valley into the Mid Mississippi Valley... A surface frontal is expected to move slowly southward day 4 through the Central Plains, Lower MO Valley, Mid MS Valley region, oriented in a general west to east direction. Favorable right entrance region jet dynamics may enhance uvvs along this front in an axis of 1.75-2.00" PW values, 2-2.5+ standard deviations above the mean, supporting the potential for organized frontal/pre- frontal convection. There may be a period of training of cells parallel to the front as the h85 flow become increasingly westerly and parallel to the boundary. ...Northern New York State into Northern New England... The northern and eastern extension of the front over the Central Plains to Mid MS Valley region is expected to remain progressive day 4 as it pushes from the Great Lakes into the Northeast. There is the potential for organized frontal convection in an axis of PW values 1.75-2" moving eastward across NY State into New England. The previous marginal risk was decrease in size after collaboration with Northeast WFOs. The very progressive nature of the front will be a detriment to any widespread runoff issues, but with saturated/moist adiabatic soundings in a deep layer of the atmosphere, can not rule out an isolated runoff issue. ...Upper Ohio Valley into the Central Appalachians... A marginal risk area depicted day 5 for potential heavy rains along the front dropping south through the Upper OH Valley/Central Appalachians area Thursday. West southwesterly h85 flow in an axis of 1.75-2.00" PW values will support locally heavy rains along the front, with some orographic enhancement of the uvvs into the Central Appalachians. The marginal risk confined to areas of the Upper OH Valley and Central Appalachians that have relatively low FFG. Oravec 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 $$