FOUS30 KWBC 181558 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1158 AM EDT Tue Aug 18 2026 Day 1 Valid 16Z Tue Aug 18 2026 - 12Z Wed Aug 19 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE MISSOURI VALLEY... ...16Z Update... The main change this update was to reduce the coverage of the inherited Slight Risk to focus on the Missouri Valley while maintaining a Marginal Risk into the Midwest/Great Lakes region. The latest 12Z deterministic and probabilistic guidance from the CAMs is less aggressive with rainfall amounts into the Midwest/Great Lakes with storms firing along the cold front later today. There is also less agreement in a favored corridor of heavier rainfall to support a Slight Risk, with some of the CAMs shifting further north over southern Wisconsin and portions of Michigan compared to the inherited Slight Risk over northern Illinois, and depicting a relative minimum in precipitation to the west-southwest over Iowa. Meanwhile, the latest guidance is well aligned favoring a corridor of heavy rainfall through the Missouri Valley from northeastern Kansas/southeastern Nebraska southeast through central Missouri. Expect storm coverage to ramp up into the overnight hours as an LLJ develops with clustering/potentially backbuilding storms progressing southeastward along a similarly oriented frontal boundary. Both the HREF and RRFS show moderate probabilities (40-60%) for rainfall exceeding 2 inches with deterministic guidance supportive of locally heavy amounts in the 3-4" range. The Marginal Risk was also extended a bit further to the west into southwestern Nebraska/northeastern Colorado where the latest guidance shows the potential for some heavier downpours with afternoon convection that could bring an isolated flash flood threat. Putnam ...Previous Discussion... ...Midwest to Missouri Valley... Current WV satellite imagery depicts the arrival of our first shortwave migrating southeast around the broad trough to the north and over the northern fringes of the ridge dominating the Southern CONUS. Pathway of least resistance will maintain a consistent approach of northwest to southeast trajectory of the next series of shortwaves wandering through the mean flow between the two key upper features over the next 72 hrs. The first disturbance will be the precursor to a more potent shortwave ejecting out of the Northern Rockies the second half of the period. Anticipating the current shortwave analyzed over the Wyoming Front Range through the Sandhills to propagate east-southeast with the main energy pivoting through the Corn Belt and Upper Mississippi Valley before kicking east towards the southern portion of the Great Lakes by later this evening and continuing its progression eastward. This disturbance will be the headline feature for areas of the Midwest between MN/WI/IL with the latest CAMs maintaining some variability in how they want to handle the convective scheme during the afternoon and evening period today. Current prospects are generally 50/50 on whether or not we see any significant thunderstorm genesis across the aforementioned area as relevant positive vorticity advection and the introduction of a cold front should manifest at least some scattered convective activity across the Upper Midwest towards the western Great Lake areas spanning I-94/41 between Chicago and Milwaukee. Deterministic outputs around the Great Lakes maintain at least some moderate totals with a few more aggressive CAMs noting more substantial rainfall prospects as the setup yields a better organized convective cluster, thus tipping the scales to a more enhanced rainfall prospect by the late-afternoon and early evening time frame prior to the arrival of the cold front from the northwest. Latest 00z HREF neighborhood probs for >2" is hovering between 20-35% across northeastern IL into southern WI with the primary focus lying between I-94 and I-80 in the above region. Considering the antecedent soil moisture anomalies remaining elevated, especially the top layer soils which have taken a beating the past week from incessant convective complexes and the like, as well as elevated stream flows lingering from said impacts, the prospect for localized flash flooding is a bit higher than normal. As such, with a signal still present in some of the CAMs and the local Chicago WFO messaging the threat for locally enhanced flood concerns with the disturbance, there was no reason to deviate from the previous SLGT risk in place as the environmental conditions and synoptic pattern do favorable the potential for later today before the cold front finally ushers in some much needed drier air in its wake. Further to the west and southwest, the threat is a bit less variable in its presentation within the latest CAMs suite as the second, more prolific shortwave to eject out of the Northern Rockies is all but certain to provide to a more enhanced convective episode across areas of eastern NE down through the quad-state area of NE/MO/KS/IA as the shortwave amplifies and digs southeast out of the Sandhills. Fairly textbook setup of regional diffluence ahead of the trough as it migrates through the High Plains into the Missouri River Valley, entering an environment that will yield much more favorable low to mid-level shear and elevated PWAT anomalies as flow becomes backed for a time with the troughs approach. Convective initiation across NE will ensue late this afternoon, carrying through the evening as the mean flow will take any activity and press it southeast with sights on northeastern KS and northern MO as we move into the evening. From there, a modest nocturnal LLJ will nose up through KS allowing for a localized convergence signature and a narrow corridor of low-level shear enhancement to maintain better mesocyclone structures that will wander southeast through northeast KS into MO. Heavy rainfall will maintain a southeastward heading with a pretty solid consensus of 1-2", locally higher amounts of rainfall extending from the KS/NE border down through central MO. This is highlighted by modest >2" probs off the latest HREF with the axis aligned perfectly with the expected frontal positioning by the end of the forecast. These are the more likely locations to see flash flooding given the more favorable forcing, moisture presence, and potential for repeating leading to a maintenance of the SLGT risk from the previous forecast(s). Kleebauer ...Eastern Florida Peninsula... Broad trough expanding across the east will dig slightly into the Southeast Tuesday, pushing a cold front and pre-frontal trough towards the Gulf Coast. This pre-frontal trough is progged to push down the Florida Peninsula in response to a push of a modest shortwave impulse embedded within the flow. As this trough/shortwave dive southward, the overlapping ascent will merge with the pinned sea breeze into robust thermodynamics characterized by PWs of 2-2.25 inches and MUCAPE above 3000 J/kg. This will support scattered to widespread thunderstorm development, with rainfall rates eclipsing 2"/hr. While this setup is favorable for locally enhanced rainfall and totals between 3-5" in the hardest hit locations, antecedent soil moisture remains low and probs for FFG exceedance maintain a meager signal as FFG's remain upwards of 3.5-4"/hr across all of northeastern FL into the FL Panhandle. Considering the nature of the precip being scattered in nature and the signals within the latest CAMs a little less robust than the previous forecast, decided to remove the previous MRGL risk, but maintain a posture for a non-zero flash flood threat with the greatest potential likely between Jacksonville to Daytona Beach. Kleebauer/Weiss 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... ...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... ...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 ARE MARGINAL RISKS OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHWEST AND MID-ATLANTIC... ...East... Heading into Friday a mid to upper level trough will deepen over the eastern U.S. resulting in a surface frontal boundary stalling somewhere over the Mid-Atlantic. This setup remains favorable for areas of heavy rainfall, as high PWs and strong instability persists over portions of the area. However, model spread introduces uncertainty regarding the strength and timing of shortwaves traversing the trough, as well as the positioning of the surface front. The 00z GFS, AIGFS and CMC are notably more aggressive in pushing the boundary southward, advecting drier air into the northern Mid- Atlantic and displacing the heavy rainfall risk further south across the Carolinas and far southern VA. Conversely, the ECMWF and UKMET feature a stronger shortwave over the OH valley, keeping the front further north and maintaining a heavy rainfall risk northward into the Delmarva. Given these lingering spatial discrepancies, uncertainty remains regarding where the optimal overlap of forcing, instability and deep moisture will materialize. While the setup needs to be monitored closely for potential isolated to scattered flash flooding, a Marginal risk is maintained given the low spatial confidence. Only a minor southward adjustment was made to the risk area with this update...with a larger shift withheld given we're still 4 day out and the ECMWF and UKMET remain north. By Saturday (day 5) the trough deepens further across the East. The northern stream energy appears detached from the deep moisture plume at this time. Thus, while rain is expected to stretch from portions of the OH valley into the Northeast, it remains unclear whether rainfall rates will be high enough to trigger flash flooding. The coastal Carolinas currently represent the most favorable zone for overlapping instability and high PWs, however frontal timing and positioning are uncertain at this 5 day lead time, and regional FFG thresholds are generally higher along these coastal areas. As a result, no ERO risk area is delineated at this time, although a Marginal risk may need to be introduced in subsequent updates as guidance trends clarify. ...Southwest... A Marginal risk remains in place across portions of the Southwest for both Friday and Saturday, with the risk expanding slightly northwestward on Saturday relative to Friday. 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 $$