254 FOUS30 KWBC 080829 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 429 AM EDT Wed Jul 8 2026 Day 1 Valid 01Z Wed Jul 08 2026 - 12Z Wed Jul 08 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CENTRAL PLAINS THROUGH THE UPPER GREAT LAKES... ...Central Plains to the Upper Great Lakes... Embedded upper-level shortwaves rounding an upper-high over the southwestern U.S. will lead to increasingly northwesterly flow over a quasi-stationary frontal boundary across the Upper Great Lakes/Midwest. An overnight MCS may still be ongoing through the start of the day 1 period across the Michigan UP/northern Wisconsin west-southwest through southern Minnesota, with the outflow from these storms likely helping to delineate this as the eventual position of the frontal boundary later this afternoon. This will favor renewed development of storms along and just to the south of the boundary from the UP/northern LP of Michigan southwest through central Wisconsin and into southern Minnesota/northern Iowa. Seasonably high PWAT values 1.5-2 standard deviations above the mean will support rain rates of 1-2"+ per hour. The latest hi-res guidance shows the potential for locally heavy rainfall totals of 3"+, backed by moderate (40-60%) HREF probabilities of totals exceeding 3", supporting the risk for scattered flash flooding. The Slight Risk has been extended further to southwest where heavy rainfall is also expected ahead of a developing surface wave along the cold front over Nebraska. This will help focus convergence along the front promoting more widespread storm development with storm clustering/cold pool mergers in the presence of northwesterly flow leading to the potential for upscale growth into an organized convective system. Seasonably high moisture flowing northward on the west side of surface high pressure to the east will pool ahead of the boundary with daytime surface heating leading to moderate to strong instability supporting heavy rainfall rates of 1-2"+. A developing low level jet over the central Plains will also help to maintain storms into the evening hours. Similar to the Upper Great Lakes/Midwest, the latest hi-res guidance supports the potential for locally heavy rainfall of 3"+ and scattered instances of flash flooding There may be a relative minima between these two regions that would not quite rise to the level of a Slight Risk. However, there's enough agreement amongst the guidance that this region would be limited if present at all so felt a full extension of the Slight Risk southwestward was warranted rather than introducing a separate risk area. ...Central/Northern High Plains... A cold front pushing into the central/northern High Plains will provide a focus for convergence as well as moist, post-frontal upslope flow to support scattered afternoon thunderstorms off the higher terrain. Locally higher PWAT values of 1"+ will support heavier downpours with an isolated risk for flash flooding. ...Eastern Texas/Lower Mississippi Valley to the Tennessee/Ohio Valley and the Mid-Atlantic... A Marginal Risk was maintained for this region. A shortwave moving across Kentucky/Tennessee and a lingering front should both act as a focus for convective development. Additional day time storms are likely further east along the front into the southern Mid- Alantic/North Carolina as well as southwest along the front through the Lower Mississippi Valley and then south through the northwest Gulf Coast. Not expecting anything too organized or widespread, but enough moisture and instability will be around to support locally heavy rainfall rates. Putnam Day 2 Valid 12Z Wed Jul 08 2026 - 12Z Thu Jul 09 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE MIDDLE MISSISSIPPI AND LOWER OHIO VALLEYS... ...Middle Mississippi and Lower Ohio Valleys... Embedded shortwaves rounding the upper-ridge will eventually lead to deepening troughing across the the Midwest and adjacent areas on Thursday. The southward shift of stronger flow aloft overtop a southward progressing cold front will bring broadening scattered thunderstorm chances from the central Plains east through the Midwest and into the Lower Great Lakes/Interior Northeast. Seasonably high moisture and instability ahead of the front will support the threat for locally heavy downpours and isolated flash flooding. To the south, a lingering frontal boundary will lift back northward as a warm front towards the Middle Mississippi/Lower Ohio Valleys ahead of the approaching cold front, helping to focus storm development through this corridor. Embedded waves locally slowing the progression of the cold front as well as generally mean westerly flow over the west-to-east oriented boundary will promote potential repeated/training storms, increasing rainfall totals. The most recent deterministic global and hi-res guidance have shown an increasing potential for multi-inch rainfall totals, with the AI guidance also favoring a heavier rainfall axis through this corridor. For these reasons, a Slight Risk has been introduced with this outlook. ...Mid-Atlantic... Further east, some of the 00Z hi-res guidance now stretching at least partially into the day 2 period is rather bullish on locally heavy rainfall across the Mid-Atlantic with totals of 2-3"+. An embedded upper-level shortwave and accompanying surface wave along the lingering southern frontal boundary could lead to more widespread thunderstorms through this region with plentiful moisture to support heavy rainfall. For now, have extended the Marginal Risk eastward to cover this region, but another embedded Slight Risk may be needed with more confidence in this scenario. ...Central High Plains... A familiar set-up will remain in place Thursday as a frontal boundary becomes quasi-stationary through the region. Moist/post- frontal upslope flow and convergence along the boundary will continue to support storms with locally heavy downpours and an isolated risk for flash flooding. Putnam Day 3 Valid 12Z Thu Jul 09 2026 - 12Z Fri Jul 10 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE MIDDLE MISSISSIPPI/OHIO/TENNESSEE VALLEYS AND CENTRAL APPALACHIANS... ...Middle Mississippi Valley to the Central Appalachians... A similar set up to day 2/Thursday will be in place broadly from the central Plains east through the Middle Mississippi/Ohio Valleys to the Appalachians and Mid-Atlantic as embedded shortwaves help to maintain gradual upper-troughing and stronger flow over the region. Moisture will continue to pool ahead of the slowly progressing cold front with PWAT values reaching as high as 2"+ through the Middle Mississippi/Ohio/Tennessee Valleys, continuing the threat for high rain rates with storms. The slow progression of the wavy frontal boundary will also keep the focus for storms over a similar area to day 2, particularly across the Middle Mississippi to Ohio Valleys, leading to increasingly wet antecedent conditions and risk for flash flooding with additional rainfall. The southward progression of the front will also bring storms further into the central Appalachians and Tennessee Valley. The deterministic guidance continues to show areal average rainfall of 1-2" with locally higher amounts, and the AI guidance also lines up favorably through this corridor. A higher categorical risk area may eventually be needed as confidence grows in the heavy rainfall potential Thursday and overlap of this region into Friday. ...Central/Southern High Plains... The lingering front through the central High Plains Thursday will begin to press further southward on Friday with the accompanying storm risk shifting southward into the southern High Plains. Continued convergence/moisture pooling along the boundary will bring another round of afternoon thunderstorms capable of locally heavy rainfall leading to isolated flash flooding. Putnam Day 4 and Day 5 Valid 12Z Sat Jul 11 2026 - 12Z Mon Jul 13 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE OHIO/MID-MISSISSIPPI VALLEYS AND CENTRAL APPALACHIANS ON SATURDAY... ...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE PLAINS/MID-MISSISSIPPI VALLEY EASTWARD TO THE MID- ATLANTIC/SOUTHEAST... ...Ohio and Mid-Mississippi Valley's... Embedded shortwaves rounding the upper-ridge will deepen troughing across the the Midwest and adjacent areas by Thursday. The southward shift of stronger flow aloft will bring increasingly widespread storm chances from the central Plains east through the Mississippi Valley into the Ohio/Tennessee Valleys and central Appalachians. Seasonably high instability and moisture pooling along a southward progressing, wavy cold front will lead to multiple rounds of heavy rain- producing storms later this week and into the weekend. Mean flow generally parallel to the east-west oriented front will support clustering/training storms increasing the threat for longer- duration rainfall and higher totals. The frontal progression will slow at times which will help setup training of storms. Widespread 1-2 inches will be possible for the Ohio/Tennessee Valleys into the central Appalachians, where a Slight Risk has been maintained. A similar setup for heavy rainfall will be in place for Sunday across the Tennessee Valley to the Mid- Atlantic/Southeast. A Marginal Risk area is in effect for D5 spanning from eastern Kansas/Oklahoma to the Mid- atlantic/Southeast. ...Southwest U.S... Monsoonal moisture will ramp up convection across the terrain of southeastern and central AZ, mainly following the Huachucas up through the Mogollon Rim in east-central AZ. PW values will surge north along prevailing southerly flow on the western side of the broad upper ridge centered over the Four Corners. QPF signatures are textbook in monsoonal convection billowing up across the terrain at lead which is a good indicator of at least the isolated threat of flash flooding to kick off the Monsoon season across the Southwest. A Marginal Risk was maintained for Day 4. Putnam/Campbell 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 $$