879 FOUS30 KWBC 111557 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1157 AM EDT Fri Sep 11 2026 Day 1 Valid 16Z Fri Sep 11 2026 - 12Z Sat Sep 12 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN FAR SOUTHERN ARIZONA & NORTHERN MINNESOTA... 16Z update... The latest guidance is depicting an increase in QPF coverage and amounts across the Florida Panhandle for the remainder of this period. Local rainfall rates of 1-2+ inches may be possible along the coast. The Marginal Risk area was expanded eastward to encompass more of the Florida Panhandle. Elsewhere, the highlighted areas of elevated risk for excessive rainfall and flash flooding potential remain in good order with no adjustments needed at this time. Campbell Previous Discussion... ...Southern Arizona... The upper level ridge over southern AZ will direct embedded 500mb vorticity maxima westward along the US/Mexico border on Friday. Much of AZ, southern UT, and western CO will continue to contend with anomalous moisture content with PWs >1.5" throughout much of southern AZ. Latest 00Z REFS/HREF probabilistic guidance does show a section of far southern Arizona particularly at-risk for flash flooding. The latest 00Z HREF shows moderate-to-high chances (50-70%) for 3-hr QPF surpassing 3-hr FFGs around Santa Cruz county Friday afternoon. For these reasons, the Slight Risk remains on track for far south-central AZ. ...Upper Midwest... A 500mb shortwave trough and an accompanying frontal boundary will track through the Upper Midwest at the same time a plume of >90th climatological percentile PWs manifests over the region. Storm motions should be progressive, although some training thunderstorm activity closer to the warm front is becoming inc as low pressure heads for the MN Arrowhead. Rainfall totals could range between 1-2" (locally higher amounts up to 3") in northern MN, while rainfall rates could approach 1.5"/hr as storms track across the region Friday afternoon and evening. ...Lower MS Valley, TN Valley, Central Gulf Coast, and the Southern Appalachians... A 500mb shortwave trough tracking towards the Mid-MS Valley and a sharp 200mb trough axis aloft will generate healthy divergence over the Mid-South, while copious amounts of moisture reside over much of the Southeast. A nearby frontal boundary will trigger widespread thunderstorms that are likely to contain >2"/hr rainfall rates within the more intense storms. Model guidance remains in fairly good agreement on the widely scattered t-storm activity before storms dissipate for most areas by late evening. PWs >2.0" will be common from the central Gulf Coast to the mid-MS Valley, while PWs in the OH/TN Valleys range between 1.75-2.0", which are still above the 90th climatological percentile. Storms are likely to form as far east as the Southern Appalachians and Carolinas, although the flash flood threat is less likely along the Southeast coast. Localized flash flooding is possible, especially in poor drainage areas and within more urbanized communities. Mullinax Day 2 Valid 12Z Sat Sep 12 2026 - 12Z Sun Sep 13 2026 ...THERE ARE SLIGHT RISKS IN PORTIONS OF THE OHIO VALLEY AND MID- ATLANTIC... ...Ohio Valley & Mid-Atlantic... The broad 500mb troughing setup in the Mid-South will induce healthy upper-level ascent over the Ohio Valley and Mid-Atlantic on Saturday. A warm front is forecast to lift north through the Mid-Atlantic on Saturday that will act as a trigger for storms, while daytime heating and height falls support widespread showers and thunderstorms from the Gulf Coast to the northern Mid-Atlantic. PWs will rise above 2.0" as far north as the I-95 corridor from Richmond and D.C. to Baltimore and Philly by Saturday afternoon with increasing instability as the warm sector drifts north. Warm cloud layers are likely to be at least 12,000ft, favoring highly efficient warm rain processes within developing thunderstorms. Where the heaviest thunderstorm activity unfolds remains unclear. However, the preference was to favor a Slight Risk along the I-95 corridor largely due to the greater concentration of metropolitan communities. West of the Appalachians, soils throughout the OH Valley are growing increasingly more sensitive with 1-hr FFGs hovering as low as 1" over southeast OH. It will not take much from this favorable atmospheric and hydrologic environment to support a concern for scattered instance of flash flooding. For these reasons, a Slight Risk was issued for portions of the Upper OH Valley for Saturday. ...Central Plains... An approaching 500mb disturbance and a strengthening southerly low-level jet will interact with a frontal boundary that results in scattered thunderstorms from the central High Plains to the Mid-MS Valley. PWs ranging between 1.5-1.75" are generally around the 90th climatological percentile and ample instability will be supportive of strong thunderstorms producing >2"/hr rainfall rates. Guidance has trended lower in QPF in recent forecast cycles, although the reduction in QPF is largely because model spread in where the axis of heavier storms occur remains unclear. A Marginal Risk remains in place given the favorable atmospheric setup with localized flash flooding possible. ...Florida... Much of the Sunshine State will remain mired in an environment containing copious amounts of moisture that will continue to support scattered thunderstorms containing torrential downpours. PWs between 2.0-2.25" will be common and storms will be capable of causing localized flash flooding, especially in poor drainage areas and in the more urbanized sections of Florida. ...Rockies & Southwest... A strong 500mb closed-low with heights below the 10th climatological percentile will work with an increasingly anomalous PW field to produce scattered thunderstorms with efficient rainfall over the Northern Rockies. Farther south, the pool of anomalous PWs that supported daily rounds of excessive rainfall in the Southwest through the week will stick around and cause additional thunderstorms that produce locally excessive rainfall rates. Instability will be lacking over the northern Rockies, but soils are more saturated following recent rainfall and may provide a slightly more favorable environment for localized flash flooding. Mullinax Day 3 Valid 12Z Sun Sep 13 2026 - 12Z Mon Sep 14 2026 ...THERE IS A SLIGHT RISK OVER PORTIONS OF THE MIDWEST... ...Midwest... To start Day 3 (Sunday) a frontal passage that is tied to a fast moving 500mb shortwave trough heading for the Great Lakes will trigger rounds of showers and thunderstorms from South Dakota and the Nebraska Sand Hills to as far south as northern KS/MO and as far east as the MS River. As the first shortwave trough exits east, a strong 500mb closed low will head through the Northern Rockies, prompting the development of low pressure in the northern High Plains. This area of low pressure will work in tandem with high pressure over the Great Lakes Sunday night to generate a 30-40kt LLJ over the Central Plains. The LLJ will act to advect rich moisture and warmer theta-e air into the Midwest with the frontal boundary that passed through early Sunday morning lifting as a warm front Sunday night and into early Monday morning. Latest ECMWF guidance shows a pool of anomalous PWs (above the 97.5 climatological percentile) sitting over the Central Plains on Sunday that lifts north into the Midwest Sunday night. With the LLJ intersecting the frontal boundary, and elevated instability available, additional strong thunderstorms are likely to form late Sunday night and into early Monday morning. While exact placement of the heaviest rainfall still differs, most guidance is in agreement that the Midwest will be the focus for potential flash flooding on Sunday and into Sunday night. The latest WPC QPF calls for 1-2" of rainfall in central IA with locally higher amounts possible. ...Southern Rockies & Four Corners... The troughing pattern in the Northwest and the mid-upper level ridge axis over the Southern Plains will direct additional monsoonal moisture into the Southwest and Southern Rockies. ECMWF percentiles suggest PWs will rise to above the 97.5 climatological percentile Sunday evening as IVT values >300 kg/m/s direct moisture into the regions. Some locations continue to sport saturated soils, thus making these areas more prone to potential flash flooding. While confidence in higher rainfall rates is lower at the moment, the rgion's sensitive soils and potential for excessive rainfall rates >1/0"/hr support a Marginal Risk as of this forecast cycle. Mullinax Day 4 and Day 5 Valid 12Z Mon Sep 14 2026 - 12Z Wed Sep 16 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR EASTERN PORTIONS OF THE MISSOURI VALLEY INTO PARTS OF THE CORN BELT AND UPPER MIDWEST... ...Day 4... Early next week will likely see a growing concern for heavy rainfall and widespread convective episodes beginning across the Central and North-Central U.S. with a longwave trough/ridge evolution likely to induce a prolonged threat for flash flooding. The initial setup begins with two distinct features of interest, one of which will become an indirect player in the anticipated threat. The first feature is a strong mid-level trough ejecting through the Northern Plains, making headway into the Northern Great Lakes and adjacent Ontario Province with a subsequent surface low and cold front materializing as a result. The cold front will be the surface forcing mechanism that will aid in convective foci, as well as a delineation point for where the heavier precipitation prospects will evolve. The cold front will lose latitude back over the Plains with the current evolution likely to take the frontal boundary down to at least Kansas latitude before incurring some resistance due to the evolution occurring downstream. That of which is our second feature of note; a well-defined ridge axis that will control the ambient weather pattern for the Southeastern U.S. up through the Southern Mid Atlantic and Tennessee Valley for at least the first half of next week. This ridge will be a road block for any frontal progression upstream as the main ascent driver in the shortwave trough will continuing ejecting northeast, leaving behind the front and the primary driving mechanism for the surface feature in question. Monday afternoon and evening will see an uptick in convective activity from the Southwestern U.S. up through the Central Plains to Upper Midwest as a steady moisture advection pattern materializes around the western and northern periphery of our broad upper ridge field centered east of the Continental Divide. Anticipating PWAT anomalies between +1 to +2 deviations, at minimum, to be positioned over the aforementioned areas with embedded shortwave perturbations rounding the western flank of the ridge, eventually into the northern tier of the CONUS. Mean winds aloft will become more parallel with the flow occurring in the boundary layer leading to a heightened threat of training convection across the Plains to Great Lakes area with the primary concern materializing over the Missouri Valley into the Central Midwest where the additional forcing brought upon by the stronger mid-level shortwave ejection will aid in the heavier convective footprint. PQPF from the current forecast and 90th percentile QPF outputs are right within the 2-3" marker at this current lead, a testament to the potential as historical precedence indicates these types of outputs at lead tend to trend closer to more impactful scenarios, so long as the synoptic evolution doesn't deviate from consensus. Latest First Guess Fields are keen on a broad SLGT risk across the Midwestern U.S. with an extension back into IA and points northeast into the U.P. ML models and a few global outputs extend that threat back further to the west to include more of the Missouri Valley through eastern NE, perhaps even into northern KS. Utilizing some of the recent probs >50% for at least 1" precip on ensembles, the SLGT risk from previous forecast was extended back into that above zone with the SLGT now situated from eastern NE up through the Corn Belt, extending northeast through portions of MN/WI and the Michigan U.P. This threat will flow into the following day, but this is the beginning of an active period of heavy rainfall potential with prospects for future upgrades, pending the behavior of the evolving synoptic pattern, both aloft (trough/ridge strength and orientation) and at the surface (low pressure and cold front positioning). MRGL risk extends back into the Southwest as scattered thunderstorm activity will likely maintain a foothold on the terrain of AZ/CO/NM thanks to the continued moisture advection regime and suitable thermodynamic environment in place. ...Day 5... The forecast synoptic pattern heading into Tuesday is trending to continue a very active stretch for the Central U.S. as a frontal boundary will struggle to lose much latitude over the course of the first half of next week leading to what could be a focal point for convection for multiple days beginning with the previous D4 time frame. The longwave pattern will remain relatively stable east of the Mississippi with a strong mid and upper ridge axis located across the southeastern quarter of the U.S. with a formidable area of surface high pressure dominating the weather pattern for these areas. Across the Western CONUS, multiple shortwave troughs will lead to a general trough axis and prevailing southwesterly flow extending from the Southwest U.S. through the center of the nation with an expected quasi-stationary frontal placement bisecting areas from the Central High Plains to points east-northeast through the Missouri Valley and Corn Belt with the eastern inflection back into the Lower Great Lakes. This setup will be coincident with a bit of a Maddox signature which typically precludes more appreciable heavy rain prospects given the limited forward propagation expected of a surface front with mean flow generally parallel to the surface feature leading to training and back- building scenarios within proximity of the front. Global deterministic implies multiple smaller shortwave perturbations ejecting northeast through the main axis of diffluence situated over the above regions. This will only act as additional regional ascent to aid in convective genesis, and creating a repeated pattern of heavy rain prospects as the setup evolves. Latest ensemble mean QPF is relatively robust at 5-day leads with 2-3" mean QPF with some members already depicting closer to 4-6" over areas of northeastern KS up through northwest MO and IA. These are noteworthy as they reflect the potential for a setup of this nature, one that tends to infer a more enhanced impact where repeated convection occurs. Given the latest signals in the means and the associated probs from the latest ECAIFS-ENS and ECENS, a broad SLGT risk was added to the new D5 encompassing the Central Plains and points northeast into the Lower Great Lakes (southern WI and northern IL). This is a setup that will need to be monitored closely given the potential for significant rainfall totals over a multi-day period where antecedent soils may become primed and then impacted shortly after leading to a rising flash flood threat and potential hydrologic responses. Another period of scattered convective potential across the Southwest U.S. will lead to yet another MRGL risk in place for the D5, but will also have to keep an eye for any short term upgrades given the propensity for repeated action, as well as the introduction of better kinematics as these smaller shortwaves make their way around the western periphery of the ridge. Kleebauer 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 $$