FOUS30 KWBC 170805 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 405 AM EDT Mon Aug 17 2026 Day 1 Valid 12Z Mon Aug 17 2026 - 12Z Tue Aug 18 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE OHIO VALLEY AND THE CENTRAL APPALACHIANS... ...OH/TN valley into the Appalachians... Cold front currently analyzed across the western edge of the Ohio Valley will continue pushing east-southeast through the overnight hours tonight and over the course of the D1 time frame. Multiple small mid-level perturbations will zip through the mean flow aloft at the same juncture leading to periods of localized ascent within the proximity of these little disturbances. A small cluster of convection currently situated across southern IL will translate to the east, a relevant depiction of at least one of the shortwaves migrating across the Ohio Valley with sights downstream on the eastern half of the Ohio Valley into the Central Appalachians. This particular zone extending down the Central Appalachians from WV down through eastern KY and neighboring areas of southwest VA and northwest NC are the most noteworthy for potential impacts due to the antecedent moist soils remaining from what was a tumultuous period of heavy rainfall events that greatly impacted Appalachia with widespread flash flooding and heightened riverine responses. Conditions remain favorable for flood prospects, noted via an abnormally low FFG threshold for all 1/3/6 hour periods. Latest 00z HREF 3hr FFG exceedance probabilities for later this afternoon across eastern OH into WV are >70% for a large area, mainly along and west of I-79 within that area that has seen 4+ inches of rainfall in the past 5-6 days. Flash flooding occurred once again this past evening from another round of organized convection over the area, so the hydrologic state located within that region of the CONUS remains extremely sensitive and will not take much to breach those low FFG thresholds currently in place. Considering the environmental priming anticipated by the afternoon with ample destabilization and sufficient low to mid-level forcing thanks to the incoming cold front and shortwave advection, yet another round of thunderstorms capable of rates between 1-2"/hr will impact the aforementioned region leading to another instance of scattered to perhaps widespread flash flood prospects. Timing is likely to be cell initiation by the early afternoon hours, lingering through sunset before conditions finally improve with the frontal passage and drier air advecting into the area, at last. A SLGT risk was maintained in the zones referenced above with the highest threat likely over southeast OH into WV, and the terrain located across eastern KY into southwest VA and northwestern NC. ...Southwest... Upper ridging centered over the Southern Plains and Lower Mississippi Valley will lead to the Southwestern U.S. positioned on the western flank of the anti-cyclonic flow for yet another day leading to another round of scattered thunderstorm activity over the sensitive terrain within eastern AZ through much of NM. Signals within the CAMs are consistent in the scattered convective coverage with focal points likely over the terrain extending from the NM Bootheel up through the Southern Rockies, including areas of the Sangre de Cristos and Sacramento Mtns. Locally heavy rainfall capable of a quick 1-2" will be the theme of any higher impact scenarios with the convective pattern, so the threat will remain more within the MRGL risk category when assessing the magnitude and expected coverage of the convection. As a result, the previous MRGL risk was maintained with little to no change as models remain consistent in their interpretations of the setup for today. ...Central Plains to Upper Midwest... Troughing across Canada into the Northern tier of the CONUS will promote the advancement of the next disturbance over the Plains and Upper Midwest with a trailing cold front likely to shove southeast over the next several days. The advancement of the first front in question will allow for an initiation of thunderstorm activity over the Northern High Plains thanks to the aforementioned front, as well as a quick-moving mid-level shortwave ejecting out of the Rockies by later today. Models have been all over the place with regards to the final convective output, but CAMs consensus has grown for a more consolidated convective cluster maturing over the NE Sandhills, migrating southeast into the eastern half of NE by the overnight period with some enhancement provided by a modest LLJ structure situated over the Plains. Current probs and general blended mean output of QPF from the latest HREF aren't exactly spectacular, but some deterministic depictions are much more prolific, enough to warrant some consideration for a flash flood threat outside the Sandhills. The threat extends into southern MN when assessing the mean QPF alignment, but the best ascent seems fixed just a bit south. Still, some threat of heavier convection developing within that area between northeastern NE into southern MN when looking at the ensemble mean. Enough mid-level vorticity will be advecting through the area with the trough, so its plausible heavier rain sets up a bit north of the current maxima. Previous MRGL risk was generally maintained for the threat as forward propagation speeds of any convection should limit more substantial impacts resulting in more isolated to widely scattered flash flood concerns for the D1. Kleebauer Day 2 Valid 12Z Tue Aug 18 2026 - 12Z Wed Aug 19 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE MIDWEST AND MISSOURI VALLEY... Longwave pattern across the Northern CONUS into Canada will lead to continued trough amplification down into the northern tier with multiple shortwaves migrating under the trough base. Tuesday will see one of the more prominent waves eject out of the Northern Rockies with an anticipated downstream amplification as it motions into the Northern Plains. Mid and upper level height falls with an attendant vigorous shortwave will translate eastward on Tuesday with sights on the eastern Northern Plains with heavy convective prospects extending from eastern NE down into the Missouri Valley and adjacent Corn Belt (Iowa). A secondary cold front will accompany the disturbance a strong surface ridge follows in wake of the trough swinging through the area. A combination of diffluent flow aloft and ample mid-level vorticity advection into a well- positioned theta_E ridge will lead to convective initiation across the Sandhills, once again but with a greater focus downstream given the vigor of the shortwave and sharper cold front. Ample instability and PWAT anomalies between +1 and +1.5 standard deviations above normal will compliment the upper forcing necessary to promote a period of heavy thunderstorm activity where convergence is maximized near the surface and diffluent flow aloft works in tandem. Add in a modest LLJ developing across the Central Plains with a nose into the Missouri River Valley and we'll see a period of enhanced convective impacts that will materialize Tuesday evening, carrying through the overnight period across the intersection of MO/IA/KS/NE. ML outputs have been consistently favoring this area over the past several model iterations which historically provides a great enough consensus to warrant more confidence in the forecast. Local totals between 2-4" are forecast in the more heavily impacted areas when assessing the latest ensemble probs and deterministic forecasts that extend out through the D2 period. The CAMs window ends 00z Wednesday, a bit before the more appreciable impacts occur, so the next forecast update for the threat should provide more detail on the threat itself, including more up to date probabilities from the hi-res suite. In any case, there was enough confidence to maintain the previous SLGT risk forecast with some modifications on the southwest flank of the risk to account for the latest trends in available guidance, as well as matching some of the consensus within the ML fields. Further downstream into the western Great Lakes, additional mid- level vorticity will stream into the region with some of the global deterministic more emphatic in the potential for heavy rainfall over portions of southern WI into northern IL, including both the Chicago and Milwaukee metros. Current CAMs are a mix with regards to the potential with some looking more aggressive than others at the end of their runs. In any case, this will be an area to monitor for possibly an expansion of the SLGT to encompass those areas of southern WI and northern IL, especially in those spots that are still recovering from the onslaught that occurred this past week. For now, a MRGL risk is sufficient to cover for the threat, but bears watching the next few forecast cycles. Kleebauer Day 3 Valid 12Z Wed Aug 19 2026 - 12Z Thu Aug 20 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE MID-MISSISSIPPI AND OHIO VALLEY'S... ...Mid-Mississippi and Ohio Valley... Shortwave amplification from the previous period will continue its approach downstream into the Mid-Mississippi Valley with an increasing threat of organized heavy rainfall in-of the confluence area of MO/KY/IL/TN. Both numerical and ML guidance have been upping forecast totals over the area over the past few days as consensus grows for an upscale growth of convection along and ahead of a cold front continuing its approach from the northwest. Increasing low-level convergence along with a strong mid-level shortwave will enhance ascent within that area of the country leading to prospects for locally heavy rainfall capable of scattered flash flood signals as we move into Wednesday. Some of the guidance is more keen on enhancing the storm threat further southeast into TN as outflows propel the convective cluster further into the backed flow orienting into the Tennessee Valley. In any case, QPF maxima over the confluence areas of the rivers is the current consensus with both the numerical ensembles and ML guidance outputs leading to greater confidence in a SLGT risk addition for the D3 time frame. Will continue to monitor the threat closely as small shifts could spell more significant impacts to some major population centers located within that area. Until then, the SLGT remains sufficient for the current outlook. ...Southwest into the Front Range... Persistent upper ridge positioned over the Southern Plains will allow for another burst of convective potential across the Southwestern U.S, extending northeastward into the Front Range and western High Plains. Multiple shortwaves embedded in the overall flow across the deserts up through the Southern Rockies into the Front Range will provide scattered thunderstorm activity and potential for localized flash flooding scenarios. The most notable shift from previous periods is the expansion of the threat back into the Mogollon Rim with some concern for outflow driven convection to bring back flash flooding threats into the adjacent deserts adjacent to the terrain. Mixed signals so far in terms of magnitude, but the threat is certainly within the bounds for at least a MRGL risk which agrees with the current QPF outputs and isolated to widely scattered nature of the convection forecast. The previous MRGL risk was relatively unchanged in this update, however some small modifications we made to extend a touch further west into the desert areas of southwestern AZ. Kleebauer Day 4 and Day 5 Valid 12Z Thu Aug 20 2026 - 12Z Sat Aug 22 2026 ...THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHWEST, OHIO VALLEY, AND MID-ATLANTIC... ...Ohio Valley into Mid-Atlantic... Large scale mid to upper level troughing moving into the eastern U.S. will drive an active weather pattern across portions of the OH Valley and Mid-Atlantic Thursday into Friday. On Thursday (day 4), an embedded shortwave should track across the OH Valley into the Mid-Atlantic, initiating convection from portions of AR and MO northeastward into WV. While some recovery is expected by this time, recent heavy rainfall has left soils saturated over portions of this corridor, and thus hydrologic sensitivity will remain heightened and an isolated flash flood threat should exist. Further east, a stronger model QPF signal exists across the eastern Mid-Atlantic, spanning from VA into NJ. In addition to a surface frontal boundary acting as a low level focus, upper level jet dynamics within the favorable right entrance region will enhance large scale lift. PW values exceeding 2" overlapping strong instability will support heavy rainfall rates Thu into Thu night. An embedded Slight risk may eventually be required somewhere from eastern VA into southeast NJ...however lingering uncertainty regarding mesoscale positioning and the climatologically higher FFG across the coastal plain favor keeping the risk at Marginal for now. By Friday (day 5), the mid/upper trough will deepen over the East, resulting in a stalling surface front. With anomalously high PWs and moderate to strong instability remaining in place, the environmental setup will remain conducive for additional heavy rainfall and possible flash flooding. Model spread increases regarding the timing and strength of individual shortwaves traversing the deepening trough which will impact where the better ingredient overlap will be. The GFS and ECMWF are in relatively good agreement bringing a heavy rainfall threat from the Carolinas into MD, whereas the AIFS and AIGFS shunt more of the deeper moisture offshore, keeping the heaviest rain either offshore or along coastal areas. While a Slight risk could eventually be needed, will hold at Marginal until model guidance better converges on the exact spatial overlap of forcing and moisture. ...Southwest... A Marginal risk remains in place across portions of the Southwest for both Thursday and Friday, with the risk expanding slightly northwestward on Friday relative to Thursday. A strong upper level ridge will remain anchored over the region, trapping embedded shortwave energy within the light flow. Plentiful monsoonal moisture and instability will remain in play, making localized flash flooding likely both days. Because convective forcing will be primarily terrain induced, overall storm coverage should stay below Slight risk thresholds. However trends will continue to be monitored, as any increase in convective coverage/organization within the favorable CAPE/PW footprint could support a more concentrated flash flood risk. Chenard 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 $$