874 FOUS30 KWBC 100653 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 253 AM EDT Mon Aug 10 2026 Day 1 Valid 12Z 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... 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 difluent 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. ...Southeast... 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. ...Southern Arizona... 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, & ARIZONA... ...Portions of the Plains/Ohio Valley/Midwest/Corn Belt... 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 $$