FOUS30 KWBC 110104 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 904 PM EDT Mon Aug 10 2026 Day 1 Valid 01Z Tue Aug 11 2026 - 12Z Tue Aug 11 2026 ...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MIDWEST, OHIO VALLEY, & SOUTHERN ARIZONA... 01Z Update... Hi-res models performed poorly through the day across the Midwest-Ohio Valley, indicating convection would break out much earlier than what has occurred. However, convection has finally started organize, sparked by a mid-level shortwave moving from northern Illinois into Indiana that is interacting with a moist (PWs 1.75-2 in) and very unstable airmass (MUCAPEs 4000-5000 J/kg). This should continue to support storm development through the evening, with the coverage of storms producing 2+ in/hr rates increasing. While much of the 12-18Z guidance has verified poorly, the more recent of runs of the HRRR/RRFS appear to have a better handle on the current conditions and therefore, along with radar/satellite trends, were the basis for much of the updates for the central to eastern U.S. While some of the totals advertised by the daytime guidance may be well-overdone and/or spatially misplaced, still feel the Slight Risk centered over the Ohio Valley is warranted. Heavy rates, along with areas of training are expected to support a swath of heavy accumulations, with the heaviest amounts centering along a northwest-southeast axis extending from northern Indiana to West Virginia. Overnight localized totals of 3-5 inches can be expected along this axis. Refer to WPC #999 for additional details concerning the near-term heavy rainfall and flash flooding threat across this region. Across the Southwest, made some minor adjustments based on observation trends, including a small expansion of the Slight Risk centered over southeastern Arizona. Pereira Day 2 Valid 12Z Tue Aug 11 2026 - 12Z Wed Aug 12 2026 ...THERE ARE A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE OHIO VALLEY... ...Midwest and Ohio Valley... 20Z Update: Continued active setup across the Midwest to Ohio Valley will lead to widespread heavy rainfall from multiple convective complexes that will also exhibit potential for training and back-building over areas of IN/OH during the period. A growing consensus within both the CAMs and global deterministic allowed for a higher confidence forecast across much of southern Ohio back into eastern Indiana to connect with the already forecast MDT risk over portions of West Virginia. Latest HREF blended mean QPF depicted a long axis of 2-4" totals between areas just east of Indianapolis all the way towards Charleston, WV with some of the local maxima actually closer to 6-8" when assessing individual deterministic. This setup is conducive for these totals with a classic summer convective scenario of multiple MCS propagating through a persistent northwest to southeast aligned mean flow thanks to a squeeze player between the northern periphery of a broad ridge across the Southern U.S. and a deep cyclonic gyre spinning up over east-central Canada. The combined effort between the two distinct upper features will create a mid to upper level "highway" for several shortwave impulses to migrate out of the High Plains and traverse the central Midwest into the Ohio Valley. The coupling of favored jet dynamics and increased large scale forcing from the prevalent mid-level vorticity maxima will interact with a suitable thermodynamic environment positioned over the Ohio and Tennessee Valley's as noted via some prolific theta_E advection occurring during Tuesday afternoon. Area PWAT anomalies on the order of 1-2 standard deviations above normal will be present across much of the Ohio Valley back into the central and southern Midwest providing a sufficient deep, moist profile that will be conducive for efficient rainfall processes necessary for more formidable heavy rain accumulations. Increasingly backed flow in the lower boundary layer will occur over portions of IN/OH thanks to the disturbance advecting into the area by later Tuesday leading to a period of training and/or back- building convection after initiation, carrying through at least the first half of the evening hours. Remnant cold pool from today's convection will be a focal point for the training potential and the recent CAMs have been locking in on the area extending from eastern IN into southwestern WV as the most likely orientation and location of the surface feature in question. This correlated to some significant rainfall forecasts from the 12z CAMs suite within that zone leading to enhanced neighborhood probabilities for both the >3" (50-80%) and >5" (30-50%) thresholds during the period. 24hr 10yr-ARI exceedance probs across the same areas are fairly robust with a bullseye of 40-60% located between Muncie, IN to Wilmington, OH with the same area already seeing 20-35% probs for even the 100yr-ARI exceedance threshold, a testament to the anticipated magnitude of heavy rainfall for the coming setup. In coordination with the OHRFC and local WFO's from IN/OH/WV, the previous MDT risk was expanded across portions of eastern Indiana, the southern-third of Ohio, into western side of WV, mainly along and west of the I-79 corridor. Significant flash flooding is likely in these areas with widespread flash flood warnings and rapid riverine responses likely to cause issues lingering through the rest of the week as the overall setup extends beyond the D2 period. Kleebauer ...Southern Arizona... 20Z Update: Latest CAMs and associated HREF continue to pin another round of heavy rainfall across the southern and southeastern portion of AZ thanks to elevated PWATs between 2-3 deviations above normal and a very buoyant environment conducive for heavy rainfall development. Prob fields for >1" remain very high (70-90%) over the area extending from Cochise county over into the eastern half of Pima, extending north towards the I-10 corridor in Tuscon. I-19 remains the focal point for heavier rainfall, however the ridge positioning would invite more of southeast AZ into the picture for locally heavy rains, including the Huachuca Mountains that will likely be another focus for development as we move into Tuesday afternoon. The previous SLGT risk forecast was maintained as the signal remains consistent with little run-to-run variability. Kleebauer ..Previous Discussion.. 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... 20Z Update: The previous SLGT risk with high-end SLGT risk potential remains across portions of the Ohio Valley as the signal for heavy rainfall continues over the region, despite continued inconsistencies within the global deterministic. The setup on exactly where the heaviest rainfall will focus is still on the tricky side as the location of the remnant cold pool from Tuesday/Wednesday AM convection will be a focal point for where the next round of convection will transpire. There is a growing likelihood that the area extending from northern KY through southern OH will be the primary area of interest when you couple with the expected heavy rains the prior periods leading to priming, or even already flooded areas before the next round occurs. The global deterministic is all over the place with where the heavier rains will occur, but one consistency is the areal expanse of heavy convection is broader than the previous periods leading to a much larger SLGT risk situated from the Midwest through much of the Ohio Valley. The prospects for a risk upgrade where any overlap of heavy rainfall is high, so the expectation is for an addition of a higher risk probable at this point in time. That said, we need a bit more consensus from ensembles and deterministic, alike to prompt an upgrade over a particular area. The most notable area for a consideration to an upgrade is likely the southwestern quadrant of WV into southeast OH where complex terrain and funneling within the valleys west of I-79 and within the bounds of I-64 have the greatest potential just based off the National Blend and modest probs for >1" and >2" thresholds. For now, felt it was prudent to provide another round or two of model runs before inserting any upgrades, but will make mention that the opportunity in the future is higher than normal for continued significant flash flooding and an expected upgrade. Kleebauer ..Previous Discussion.. 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/00z ECMWF the lowest and the 00z Canadian Regional/RDPS the highest. Due to the uncertainty in the heavy rain signal, held off on a Moderate Risk for the time being. Roth ..The West/Central & Northern Plains... 20Z Update: The previous SLGT risk forecast was generally unchanged from the overnight issuance as models remains steadfast in their presentation of heavy rainfall situated across southeastern AZ. This area will have seen multiple days of heavy rains leading into the period, so the prospects for scattered flash flood concerns are elevated relative to normal. Totals of 1-2", locally higher are forecast across the area with multiple areas across the Interior West and the Northern Plains also likely to see similar precip outputs thanks to a favorable setup across the western half of the CONUS. FFG's are much higher outside the southern deserts, so the threat for more significant impacts are lower, so a broad MRGL exists elsewhere to account for the threat. Kleebauer ..Previous Discussion.. 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... 20Z Update... The 12Z guidance did not present any reason to make significant changes to the previous outlook. Therefore, made only minor adjustments. Pereira Previous Discussion... 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 $$