FOUS30 KWBC 190053 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 853 PM EDT Tue Aug 18 2026 Day 1 Valid 01Z Wed Aug 19 2026 - 12Z Wed Aug 19 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE MISSOURI VALLEY... ...01Z Update... No significant changes were needed. The Slight Risk area across much of Missouri remains in lockstep with where the guidance for the overnight period suggests there will be additional heavy rainfall. The Marginal Risk area was expanded in portions of Nebraska and South Dakota for the disturbance exiting out of Montana later this evening interacting with increased moisture across NE/SD. Ongoing convection across western Michigan will continued to track across the remainder of the lower Peninsula, so the Marginal Risk there was expanded to include the Huron and Erie shores, including metro Detroit. Wegman Day 2 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 VALLEYS... ...2030 UTC Update... Minimal areal adjustments were made to the inherited Slight Risk across portions of the Middle Mississippi and Lower Ohio Valleys based on the now available CAM guidance through the full day 2 period. Despite some expected spatial disparity at this time range, all of the CAM guidance is generally focused on this region with locally heavy rainfall amounts upwards of 3-5" supported by the synoptic setup outlined below. Both the HREF and RRFS also highlight this region with moderate to high probabilities of amounts greater than 2". The only notable adjustment was an extension of the broader surrounding Marginal Risk northwestward back into the Middle Missouri Valley. A passing upper-level shortwave and pooling moisture ahead of a frontal boundary will lead to renewed thunderstorm development into the early afternoon hours. Most of the now available CAM guidance supports some locally heavier downpours of 2"+ which could lead to some isolated flash flooding concerns. The forecast for the southwestern U.S. also remains on track as greater instability and embedded shortwaves within the overall flow through the upper-ridge lead to increased Monsoon-related activity compared to day 1 supportive of an isolated flash flood threat. Putnam ...Previous Discussion... ...Mid-Mississippi and Ohio Valley... Amplifying shortwave trough from the previous period will continue its progression southeastward with sights on the Mid-Mississippi and western Ohio Valley's as we step through Wednesday. Enough trailing energy will feed into the backside of the trough allowing for a bit more of a robust response compared to what transpires upstream the prior evening. First round of convection will the remnants of the overnight MCS with expected decay of the complex of thunderstorms as we step through Wednesday morning. Cold front will also continue its progression south and southeastward through the Ohio and Missouri Valley during the period with a bit more latitudinal loss further west compared to downstream. By Wednesday afternoon, we'll see the introduction of the shortwave into the center of the CONUS lead to broader amplification of the longwave trough positioned east of the Mississippi allowing for greater large scale ascent focused over the western Ohio Valley back into the confluent area of the Ohio and Mississippi River between the MO/TN/IL area. This has been consistently depicted over the past series of forecast runs with the end of the current 00z CAMs suite also noting this particular zone as the more favorable area of organized convection with multiple rounds anticipated. After 00z, nocturnal LLJ will kick in across the Mississippi Valley with the nose of the jet acting as a low-level convergent feature and aiding in an increasing regional bulk shear to provide a necessary element to organize and sustain convective development. Forecast soundings via the NAMNest and ECMWF are consistent with robust ascent within the mid-levels thanks to the amplifying trough, as well as the added low-level convergence with the arrival of the cold front and nose of the LLJ into the region. This signature typically correlates to a focal point for heavy convection with back-building potential until fropa is complete leading to some heavier rainfall lingering for several hours before the threat subsides. Considering PWATs between 2-2.25" centered across the tri-state region outlined above, and the added ascent factors in both the boundary layer and mid-levels (sfc-500mb), there will likely be an area that sees a period of 1-2"/hr rainfall for at least 3-6 hrs. leading to flash flood prospects carrying overnight into early Thursday morning. Consistency in the signal has allowed for a maintenance of the previous SLGT risk, however some adjustment was made to convey more certainty across the area between southeastern MO into southern IL and western KY/TN. Some areas are liable to see 2-4" of rainfall with a max of up to 6" plausible considering the environment at large. Other areas of convection will line up along the front as it progresses slowly southeast through the rest of the Ohio Valley, entering areas that are still in recovery from the onslaught of heavy rainfall the past 7 days. These storms are not anticipated to be nearly as prolific as the past several days, and even not as prolific as what will transpire upstream. That said, antecedent soil moisture anomalies approaching the 95th percentile over a large area, plus the threat of slow moving convection along the front is more than sufficient for a MRGL risk carrying through parts of KY/IN/OH with the eastern extent into portions of WV. This whole region remain sensitive, so the forecast will be monitored closely for any targeted upgrades if trends seem fit. Kleebauer ...Southwest into the Central High Plains... The previous forecast remains on track with the global deterministic and newest 00z CAMs suite in agreement on scattered thunderstorm activity likely to transpire Wednesday afternoon, carrying into the evening hours with areas of the Front Range the most notable for seeing more organized convection thanks to added shear and regional ascent attributed to a shortwave proxy off the Central Rockies. Synoptically, the evolution of the setup across the Southwest and Front Range begins with an upper ridge centered over the Four Corners, re- centering and amplifying while an inverted trough pushes west on Wednesday. This will promote a more favorable setup for monsoonal convection as a surge of thermodynamics (PWs above 1.25" overlapping MUCAPE of 1000-2000 J/kg) lifting northward behind the inverted trough. At the same time, weak shortwaves embedded within the flow and rotating around the ridge will produce areas of locally enhanced ascent, which into the improving thermodynamics suggests more widespread convective activity is likely on Wednesday. While a more substantial risk for greater coverage and intensity exists just beyond D2, a large MRGL risk for excessive rainfall appears supported from the Deserts of Arizona northeast into the Central High Plains of CO/KS. With rainfall rates of 0.5-1"/hr likely at times, and mean storm motions of just 5-10 kts, (higher in CO but with stronger bulk shear for organization) this will support at least isolated instances of flash flooding, especially over vulnerable terrain, urban areas, or recently saturated soils. A MRGL risk remains in place for much of the Desert Southwest into portions of the Central Rockies through the Colorado Front Range. Weiss/Kleebauer Day 3 Valid 12Z Thu Aug 20 2026 - 12Z Fri Aug 21 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE OHIO VALLEY, MID ATLANTIC, AND SOUTHWESTERN U.S... ...2030Z Update... Minimal adjustments were made to the inherited Marginal Risk areas over the Ohio Valley to Mid-Atlantic and southwestern U.S. as the prior forecast remains on track. The one area of potentially greater intrigue/upgrade potential as mentioned in the prior discussion is throughout portions of the coastal Mid-Atlantic. The global guidance continues to maintain a more focused corridor of heavier rainfall with storms developing along a quasi-stationary frontal boundary expected in vicinity of the Virginia Tidewater region northeast through the DelMarVa Peninsula and into New Jersey. Forecast amounts remain similar in the latest 12Z guidance with areal average QPF above 1" and localized heavier amounts in the 2-3" range. However, there remain a few factors that lead to enough uncertainty that a Marginal Risk will be maintained for now. First, much of this region has relatively higher FFGs given the proximity to Chesapeake Bay/the Atlantic coast outside of more sensitive urban areas (like the Norfolk vicinity) and thus a more limited flood risk. Second, the location of the front and thus corridor of heavier rainfall remains uncertain, with guidance such as the ECMWF focused on the Tidewater through DelMarVA and southern New Jersey, and the GFS further offshore with the heavier rain limited to the Tidewater and southward into northeastern North Carolina. The CAM guidance may provide more confidence in higher rainfall amounts and their location that would lead to a greater risk for flooding despite the relatively high FFGs. Another scenario for an upgrade would be if the front/corridor of rainfall were also to set up a bit further northwest and begin to reach portions of the I-95 corridor which wouldn't require as much confidence in higher amounts given the increased coverage of more sensitive urban areas. Putnam ...Previous Discussion... ...Ohio Valley to Mid Atlantic... Troughing for areas east of the Mississippi will continue with modest amplification occurring as shortwaves dive down the backside of the mean trough out of Canada and allow for further height falls across the Ohio Valley to Mid Atlantic. Cold front from the past few periods will continue making progress to the east coast with the frontal positioning a critical part in the convective posture over much of the above areas. The consensus for frontal placement and eventual convective development puts the best opportunity for heavy rainfall across the Tennessee Valley and the northern reaches of AL/MS up through the central Mid Atlantic with the core of heaviest rains focused along the Delmarva and VA Tidewater. Latest global deterministic and ensemble means note a more robust low- level convergence pattern initiating across the coastal areas of the Mid Atlantic as the timing of the front coupled with diurnal instability maxima and low-level convergence along the western periphery of a ridge centered in the Atlantic will allow for a heavier axis of QPF to materialize on Thursday afternoon and evening. Proximity of the front will act as a focal point for the setup with the projected low-level flow to run parallel to the front over areas of eastern VA up through the Delmarva Peninsula with the northern fringes lining through the southern half of NJ. Ensemble mean QPF already shows a solid 1-1.5" areal QPF average with some of the deterministic forecasting between 2-3" over the time frame, a testament to a relatively steady signal with a relatively pronounced range of 1-3" potential. NBM 90th percentile is also solidly within that 2-3" range, albeit a much larger footprint of higher totals with the heavy precipitation output even down into northern NC. NBM probs for >2" are between 20-35% across all of the Delmarva, extending south into the northern reaches of the VA Tidewater. This signal has been consistent the past 4 cycles of the NBM lending credence to the threat as the mean hasn't really wavered much the past few days. In this case, the MRGL risk inherited was maintained and will hold off on any upgrade until we get closer to CAMs range heavier QPF signals may present themselves if the synoptic evolution remains fixed to the area. ...Southwest U.S. into the Southern Rockies... Relatively textbook monsoonal pattern across the west will lead to scattered thunderstorm development across much of the Desert Southwest extending north into parts of the Great Basin and through the Southern Rockies. As of now, the signal is mainly seasonal with regards to the PWAT anomalies and general expectation as the most notable areas will likely be in those terrain focused zones across AZ (Mogollon Rim), NM (Sangre de Cristos and Sacramento Mtns.), and UT (Zion National Park). Low probs for >1" on the latest ECENS/GEFS suites and 01z NBM allow for a standard MRGL risk issuance for the area as the setup is genuinely favored for pulse convection and slow cell motions tied to the terrain as a broad 597dm ridge centers over the Four Corners and makes little progression in the D3 window. The previous MRGL was extended into southern UT to account for the increasing signal in the area of Zion National Park where slot canyons provide an inherent danger to any slow moving thunderstorm activity. Kleebauer Day 4 and Day 5 Valid 12Z Fri Aug 21 2026 - 12Z Sun Aug 23 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF VIRGINIA AND THE CAROLINAS ON FRIDAY... A deepening negatively tilted upper level trough extending southeastward from over the Great Lakes towards the Mid-Atlantic will be a primary driver of heavy rain over the region on both Friday and Saturday. As the trough digs, a stalled out surface front will support large scale lift over Virginia and the Carolinas both Friday and Saturday afternoons. Plentiful tropical moisture has been drawing north up the Eastern Seaboard ahead of the front. PWATs on Friday will still be around 2 inches. Unidirectional southwesterly flow through all levels of the atmosphere will support backbuilding and training storms. The front will uplift that moisture-rich and sufficiently unstable air to allow for numerous rounds of showers and thunderstorms. The front may be stalled along the Appalachians, which will support those storms forming over areas where the terrain will enhance the severity and quicken the formation of any downstream flash flooding. While total rainfall amounts into western NC and VA will likely be lower than further east along the coastal plain, the terrain effects also bring that area into the Slight Risk category. The heaviest rains will be along the coastal plain, where sea breeze convergence, cold pools from prior storms, and neighboring storms themselves will all provide additional localized uplift, supporting new storm formation. Thus, the higher confidence at Slight Risk level impacts on Friday is from the Hampton Roads area of Virginia south and west, roughly following I-95, to northeastern South Carolina. Elsewhere, the monsoon will continue to support isolated to widely scattered showers and thunderstorms over the Four Corners region. Storms forming along the lake breezes and the terrain of the eastern U.P. may also allow for some isolated instances of flash flooding on Friday, where a new Marginal Risk has been issued, as well as further south and east into portions of OH, PA, and western NY on Saturday. Terrain impacts and low FFGs also warranted a Marginal Risk issuance for those areas. Wegman 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 $$