FOUS30 KWBC 101556 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1156 AM EDT Mon Aug 10 2026 Day 1 Valid 16Z Mon Aug 10 2026 - 12Z Tue Aug 11 2026 ...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MIDWEST, OHIO VALLEY, & SOUTHERN ARIZONA... ...Midwest and Ohio Valley... 16Z Update: Only minor changes necessary to the SLGT risk positioned over the central Midwest near Chicago, across the Ohio River Valley with the greatest threat likely over Central Ohio into West Virginia. 12z CAMs indicate a pretty solid threat for >3" for areas between I-80 to US50 in OH as the HREF neighborhood probs come in between 40-60% for the 3 inch threshold within the aforementioned zone. The Columbus area is the most notable urban center in the middle of the higher prob fields, however, the more complex terrain and drainage areas of eastern OH into WV will be noteworthy for more localized significant flash flood prospects beginning this evening, carrying through the overnight hours. Environmental conditions are ripe for heavy rainfall rates between 2-3"/hr in the strongest cells with intra-hour rates likely to push upwards of 4-5"/hr in a few cells. This is in part to a very buoyant environment and modest shear located across the region allowing for maturing mesocyclone initiation and maintenance to occur during the peak of the convective regime. Already seeing some alignment in this mornings convection across OH orient more northwest to southeast in part to the cold pool fanning from the storms themselves as the the upwind shear vectors align within the general mean flow on the southern periphery of the upper low situated over Canada. This will be a theme for the period, and in the coming days for this general convective forward motion and potential for repeating as the prevailing steering pattern will see very little deviation in what is evolving currently. A high-end SLGT is forecast over much of OH and the western half of WV with the western extent back into eastern IN. Expect another round of convection to form across northern IL later this afternoon with guidance initiating convection after 19z across areas north of I-80. The Chicago suburbs and potentially the city of Chicago will see another round of heavy rainfall that will add to an already saturated environment in and around the metro area thanks to last nights convective impacts. This round of heavier precip does look to be a bit south of the previous episode, so that could offset the worst case of overlap, however the environment is more than capable for rates between 1-2"/hr during the time frame of impact, so scattered flash flood prospects fall within the general SLGT risk threshold, leading to only a minor adjustment of the SLGT to encompass the I-94 stretch between Milwaukee and Chicago to account for some guidance promoting the potential overlap of those harder hit areas yesterday/last night. Upstream over the Dakotas, the first in several shortwaves will eject out of the Front Range into the Northern Plains around the northern periphery of the ridge situated across the Southern U.S. Convective initiation will occur this evening as the nocturnal LLJ promotes deeper layer shear and a greater advection of moisture within the lower boundary layer. Addition of the large scale forcing and maturing low-level environment will help ignite the next round of thunderstorm activity across SD with the highest potential for heavy rainfall likely to occur east of the Missouri River into the I-29 corridor on the eastern side of SD. FFG's in the areas are higher than areas further east which should limit a more substantial flash flood scenario, however rates will likely be prolific enough to engage in isolated to widely scattered flash flood prospects, especially as you move closer to the interstate over the eastern side of the state. A MRGL risk remains in place, however the risk is likely on the higher side and bordering the necessity for a higher risk, so will have to monitor closely. Kleebauer ..Previous Discussion.. A deep layer cyclone across Hudson Bay rules the roost, keeping the equatorward edge of the Westerlies from the Northern Plains into portions of the Ohio Valley. This is near where a front and/or resultant outflow boundaries from convection will align and where an instability gradient is most likely to set up. Confluent low- level westerly flow is forecast near this boundary. Two to three rounds of convection are expected within an area of diffluent 1000-500 hPa thickness, implying organized convection. The ingredients available support hourly rain amounts to 3" where cells merge/train or where mesocyclones form. While the guidance is showing an emergent signal for heavy rain across portions of SD and IA, it is neither high enough nor agreeable enough for a Slight Risk but do believe the threat is on the higher end of a Marginal Risk. Farther to the east across portions of the Midwest/eastern Corn Belt south of the Great Lakes into the Upper Ohio Valley, a broad swath of rainfall is anticipated. Flash flood guidance is modest to low here, so even though the mesoscale signal for 3"+ in 24 hours was modest and spotty, so saw no issues with the high end Slight Risk inherited from continuity. Local totals to 6" are possible on an isolated basis. Roth ...Southeast... 16Z Update: Little change necessary from the previous issuance as the forecast remains on track. Most notable change was the removal of the MRGL risk across eastern FL as the greatest potential looks to lie across the western half of the state with the most prominent threat across the western coast in urban corridor between Tampa to Fort Meyers along the sea breeze. Modest probs for >3" exist in that zone, as well as over the interior Southeast and the TX coast, but thankfully high FFG's should preclude a greater threat. Urban centers will be the most at risk for flash flooding considering the antecedent environment with the soils. Kleebauer ..Previous Discussion.. Disorganized convection with brief, locally high rain rates are expected during daytime heating. The more concentrated area of heavy rainfall lies along portions of the Middle and Upper TX Coasts during Monday morning as onshore flow brings a slug of 2"+ precipitable water values and instability off the Gulf of Mexico. The Marginal Risk from continuity appears to cover the situation sufficiently. Roth ...Southern Arizona... 16Z Update: No changes to the previous forecast as the current setup remains favorable for an elevated flash flood threat over southern AZ with the highlight remaining the I-19 corridor and adjacent desert over southeastern Pima county. HREF probs for >1" continue to reside within the 60-90% range with >2" probs more modest (25-45%), but still noteworthy for that particular area of the CONUS. A high-end SLGT is forecast over the above area for the threat. Kleebauer ..Previous Discussion.. Monsoonal moisture increases across southern Arizona on Monday. Storms are expected to initiate along the Mogollon Rim, and the numerous ranges of southeastern Arizona and terrain in NM during the afternoon hours. Unlike prior days, enough easterly flow, especially in the upper levels, will successfully push those showers and storms off the mountains and towards the lower elevations and valleys of southwestern Arizona and could lead to some level of organization. Hourly rain amounts to 2" are possible where cells sit, train, or backbuild. The inherited Slight Risk is little changed -- southeast AZ is forecast to see the longest period of heavy rainfall. Roth Day 2 Valid 12Z Tue Aug 11 2026 - 12Z Wed Aug 12 2026 ...THERE ARE A MODERATE RISK OF EXCESSIVE RAINFALL IN AND NEAR PORTIONS OF OHIO AND WEST VIRGINIA... ...Midwest... The general flow pattern is similar to Monday. The equatorward edge of the Westerlies is forecast to lie from the Northern Plains into portions of the Ohio Valley where there is also low-level confluence, difluent 1000-500 hPa thickness, and an instability gradient to focus organized convection. This led to similar risk area placement. Hourly rain amounts to 3" and local totals to 6" remain possible given the available ingredients. There is a decent likelihood that prior days' rains over these same areas will have saturated the soils further by Tuesday, and when combined with the general very flashy nature of the rivers/terrain of West Virginia and coordination with RLX/the Charleston WV forecast office, a Moderate Risk area was introduced for portions of southern OH and western WV. ...Southern Arizona... Southeast AZ remains most at risk for more persistent heavy rainfall, so left the Slight Risk intact. Across the Great Basin, the Marginal Risk was broadened as moisture subtly increases from Monday to Tuesday as an upper level trough settles in to the CA Coast and ridging near the Four Corners weakens, which should lead to less mid-level capping and broader convective coverage. Roth Day 3 Valid 12Z Wed Aug 12 2026 - 12Z Thu Aug 13 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE OHIO VALLEY, MIDWEST, & CENTRAL HIGH PLAINS... ...Portions of the Plains/Ohio Valley/Midwest/Corn Belt... Early to mid next week, broad cyclonic flow expected to persist across the northern tier of the Lower 48 from the Northern Plains/Upper MS Valley, eastward through the Midwest/Great Lakes and into the Northeast, to the south of a closed mid-level low that has shifted into Quebec. A frontal boundary will lie along the southern periphery of this broad trough from portions of the Northern and Central Plains into portions of the Ohio Valley and Appalachians. Embedded shortwaves in this broad cyclonic flow will accentuate lift in the vicinity of this front where PW values are expected to rise to ~2" ahead of each set of shortwaves and low- level confluence is expected. The guidance is showing a growing signal for heavy rainfall that at the moment is most centered over the NE Sand Hills. Due to the hydrology there, thought it prudent to maintain the inherited Marginal Risk. When looking at the ingredients, organized convection looks most likely near portions of the Ohio Valley and the Corn Belt Wednesday into Thursday morning, where the better 1000-500 hPa thickness difluence is forecast to remain. Hourly rain amounts to 3" and local totals to 6" remain possible. A high end Slight Risk lies across portions of the region due to previous days of rains further saturating soils. There is a bit of spread of the heavy rain signal seen in the guidance, with the 00z UKMET/old 12z ECMWF the lowest and the 00z Canadian Regional/RDPM the highest. Due to the uncertainty in the heavy rain signal, held off on a Moderate Risk for the time being. ..The West/Central & Northern Plains... A broadening upper level trough is forecast across portions of the West and Great Basin Wednesday into Thursday morning. Broad Marginal Risk areas exist near and ahead of this upper level trough for portions of the Southwest and Great Basin. Increased upper level troughing opens the door to strengthening low-level level flow and broader convective development/coverage across the West and Plains when compared to previous days as the mid-level cap further erodes. Above average precipitable water values slowly shift northward through the region. While there continues to be low confidence with the details, locally heavy rains with the potential of 2.5" an hour amounts could be problematic. Urban areas, arroyos/dry washes, box/slot canyons, and burn scars would be most at risk for flash flooding. A Slight Risk was introduced across southeast AZ. Roth Day 4 and Day 5 Valid 12Z Thu Aug 13 2026 - 12Z Sat Aug 15 2026 ...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL FROM THE MID- MISSOURI VALLEY THROUGH THE OHIO VALLEY, PORTIONS OF THE HIGH PLAINS, AND SOUTHEASTERN ARIZONA TO WESTERN NEW MEXICO ON THURSDAY... Out west, the monsoon will have one final day where moisture across Arizona and New Mexico will be well above normal. Flattening upper level troughing that lifts northeastward away from Arizona and southern New Mexico should allow the showers and storms that form in that region to reduce the total atmospheric moisture quite a bit, with no replenishment overnight Thursday night. The Slight risk in this region follows where numerous prior days heavy rains impacted portions of southeastern Arizona and western New Mexico. Low FFGs still support widely scattered instances of flash flooding due to highly favorable hydrology (hydrophobic soils) in the flood prone areas. A large Slight Risk was introduced for much of the Front Ranges and High Plains from the Sangre de Cristos of northern New Mexico north through southeastern Montana. The aforementioned lifting upper level trough will have the opposite effect as further south, namely greatly increasing convective coverage and intensity in this generally flood prone region. A potent upper level shortwave riding northeastward along the eastern periphery of the positively tilted longwave trough will locally greatly increase convective coverage across the Plains of Colorado in particular. With a potent easterly low level jet advecting deep tropical moisture that has been in place across the eastern half of the country westward, the storms will run into an increasingly moisture rich and unstable environment. This will support backbuilding and training storms as the fuel for the storms is continually replenished. Further upgrades to the risk outlook are possible in the coming days, especially should urban areas such as Denver remain in the forecast for multiple inches of rain Thursday afternoon and evening. Finally, the Slight across portions of the Midwest and lower Ohio Valley was expanded east through much of West Virginia. With continued rain for a third consecutive day expected into West Virginia, despite somewhat lower total expected rainfall, the flood prone nature of the terrain there supports a higher risk for flooding. Downstream along the Ohio River, they will also be enduring the latest of a multi-day rainfall event on Thursday, also suggesting that FFGs by Thursday will be much lower across much of the Ohio Valley and supporting the Slight Risk upgrade. A plume of deep tropical moisture will bank itself along a stationary front suck just south of the southern ends of Lakes Michigan and Erie. This will support backbuilding and training of convection from Iowa straight through to West Virginia in the form of numerous rounds of storms that each follow a similar track, resulting in repeating storms. Some areas in this zone too may require risk upgrades as the event draws closer and moves into CAMS range. By Day 5/Friday, the signals for heavy rain become much more diffuse and spread out, with a few areas still highlighted, including northeastern Colorado, much of Iowa, and southern West Virginia. All 3 of these areas may have been hit hard with heavy rains on Day 4/Thursday, but since it's highly likely there will be large changes in both the placement and intensity of the heavy rain signals in each of these areas, have opted to keep all 3 in a Marginal risk for now, albeit a higher end one. Slight Risks will definitely be considered should these signals remain in future forecasts. 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 $$