FOUS30 KWBC 070057 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 857 PM EDT Sun Sep 6 2026 Day 1 Valid 01Z Mon Sep 07 2026 - 12Z Mon Sep 07 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF IOWA... ...Corn Belt/Upper Midwest... Afternoon convection persists this evening over Iowa northward into Minnesota, with 1-1.5"/hr rates but over some areas that have seen several inches of rain this afternoon. Removed the targeted Moderate risk given the overall downward trend in coverage/intensity but maintained a Slight risk as the slow movement could allow for higher amounts over saturated areas. Latest guidance shows rainfall lightening after a few more hours and we have kept the Marginal in tact over the broader region given the potential for some heavier rain, especially early, to move over areas that saw heavier rain earlier in the day, including central Minnesota. ...Northern MT... Maintained the Marginal risk for most of Montana in advance of the upper trough over WA/OR. Well above normal moisture remains in place, and still expect an uptick in rain coverage overnight as the upper trough approaches and upper diffluence expands atop surface low pressure in southeastern MT after 06Z. ...Great Basin & New Mexico... Generally lighter rain with little convection remains over Utah. Removed the Slight risk given lower rates but will maintain the Marginal risk for at least the near term as some convection has fired over the region as the upper jet streak pushes through. Over New Mexico, convection is on the downturn but will maintain the Marginal risk for the remaining potential. ...Southeast... Removed the Slight Risk over the Southeast as convection has weakened from earlier this afternoon, but still persists and sinks southward. Some 1-2.5"/hr rates have been observed but should generally be below FFG values that are >=2.5"/hr. Fracasso 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... 1930Z Update... No upgrades were made this update with adjustments made primarily through reviewing the latest WPC QPF and the 12Z HREF/REFS probabilistic guidance. The biggest change was to expand the Marginal Risk from central MN all the way to the MO River and northeast NE. This includes portions of northwest IA, where soils will be more sensitive following the excessive rainfall that unfolded on Sunday. Latest guidance shows a frontal boundary drifting east towards western IA could be the focus for additional thunderstorms on Labor Day. 12Z HREF probabilities currently show low chances (15-30%) for pockets of rainfall totals >3", which on top of saturated soils could pose a localized flash flood threat. Elsewhere, the Marginal Risks in CO and the interior Southeast were scaled back given recent drier trends in model guidance. The inherited Slight Risk in northern MT was adjusted to account for the latest WPC QPF, but no risk upgrades were made. Mullinax ---Previous Discussion--- ...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... 1930Z Update... After reviewing 12Z guidance and the latest WPC QPF, only minor adjustments were made to the Marginal Risks to account for small changes to the QPF footprints. No upgrades were made this forecast cycle. Mullinax ---Previous Discussion--- ...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. Trimmed a bit on the northern edge to remove southern NV per a lack of any QPF in nearly all guidance. ...Central Plains...Lower Missouri Valley into the Mid Mississippi Valley... A surface front 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. This could extend into Ohio (where FFG values decrease) per some 12Z guidance. ...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 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, cannot rule out an isolated runoff issue. ...Upper Ohio Valley into the Central Appalachians... A marginal risk area is 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. With a broader footprint of higher QPF in the 12Z guidance, expanded the marginal risk to the southwest. Note that lower FFG values (~1"/hr) exist currently over most of WV/southeast OH/southwest PA. Fracasso/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 $$