FOUS30 KWBC 171600 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1200 PM EDT Mon Aug 17 2026 Day 1 Valid 16Z 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... Regional radar mosaic this morning shows convection already blossoming across the Ohio Valley and into Pennsylvania, as well as draped as far southwest as Arkansas along the slowly advancing cold front. Weak impulses moving within broad eastern CONUS troughing will help enhance ascent through this evening, and the 12Z CAMs indicate scattered to widespread thunderstorms will develop through peak heating. Modest bulk shear of 20-25 kts within a plume of 1.75 to 2.0 inch PWs will support clusters of thunderstorms with rainfall rates of 1-2"/hr, briefly even heavier than that, with the most concentrated heaviest rates focused from the Ohio Valley into the Central/Southern Appalachians. This area is also where the FFG is significantly compromised due to recent heavy rain, reaching generally 0.75-1.5"/3hrs. Fortunately, mean 0-6km winds of 10-15 kts with similarly paced propagation vectors aligned perpendicular to the boundary should prevent significant training, although repeating rounds of heavy rain are possible in some areas. The inherited SLGT risk encompasses the most vulnerable areas today, and was tailored only cosmetically from inherited to match the highest FFG exceedance probabilities from both the REFS and the HREF. ...Southwest... Expansive west-to-east oriented mid-level ridge draped from the Low Deserts of California through the Southern Plains will maintain a favorable pattern for at least some monsoonal convection today. While airmass convection will be the primary mode, a potent shortwave noted in GOES-E WV imagery lifting out of West Texas will pivot W/SW through the day, providing at least locally enhanced ascent through southern NM and southeast AZ. Subtly enhanced inflow will help draw PWs to as high as 1-1.25", lesser farther north into the Four Corners, to support rainfall rates for which the HREF, REFS, and UA WRF suggest will reach 0.5-1"/hr. While the CAMs are not extremely aggressive with coverage, mean 0-6km winds of just around 5 kts indicate slow movement for any storms that develop today, posing a flash flood risk over vulnerable terrain features or urban areas. For this reason the MRGL risk remains supported, with a subtle extension south from inherited to capture the advancing shortwave. ...Central Plains to Upper Midwest... Amplifying mid-level trough over Northern Manitoba will shed pieces of energy through its base and into the Northern Plains today. One of these impulses is progged to be potent as it drops NW to SE from the Dakotas into Iowa this evening. This impulse will interact with a surface front, leading to locally accelerated and backed inflow within a modest warm sector to the south. This should result in increasing coverage of thunderstorms from South Dakota into Nebraska this evening, with organization into one or multiple MCS likely overnight within a region of 25-35 kts of bulk shear. As the LLJ backs and amplifies to 25-30 kts, it will draw PWs above 1.5 inches northward, overlapping with a plume of 1000+ J/kg MUCAPE to produce thermodynamics capable of supporting 1-2"/hr rainfall rates (20-40% chance of 1+"/hr from both HREF and REFS guidance). While storm motions may be progressive on 20 kts of 0-6km winds, Corfidi vectors aligned to both this mean wind and the elevated front could support training from west to east, enhancing the potential for excessive rainfall. The CAMs continue to be highly variable in their footprint for tonight, but there appears to be a focus from the Sandhills of NE into western IA, as well as a secondary maxima across the Coteau des Prairies of SD/MN. Any development farther southeast would impinge into lower FFG, but otherwise the exceedance probabilities within the most likely footprint tonight suggests the MRGL risk remains supported, and only minor cosmetic adjustments were made with this issuance. Weiss 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 $$