475 FXUS02 KWBC 081919 PMDEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 319 PM EDT Sat Aug 8 2026 Valid 12Z Tue Aug 11 2026 - 12Z Sat Aug 15 2026 ...Multi-day heavy rainfall threat extending from parts of the Mid- Mississippi Valley to Ohio Valley next week... ...Hazardous heat builds across much of the South-Central and Southeast U.S. through the end of next week... ...Overview... Upper ridge over the Four Corners shifts east midweek and remains stretched out across much of the Southeast and south-central U.S. through Friday. Meanwhile, shortwave disturbances and troughing across the Northern Tier (particularly Northeast and Pacific Northwest) brings shower and thunderstorm chances. Multiple days of heavy rainfall may lead to flooding concerns across parts of the Mid-Mississippi and Ohio valleys. Troughing extending along the West Coast and Great Basin by midweek will also increase southerly flow and monsoonal moisture into the region, increasing the coverage of shower and thunderstorm chances throughout Southwest, Intermountain West, and Interior/Southern California. ...Guidance/Predictability Assessment... Latest deterministic and ensemble guidance generally agrees on the large-scale evolution, although uncertainty remains regarding the amplitude and timing of individual shortwave impulses embedded within the northern-stream flow. Model spread increases towards the latter portion of the period, especially with the latitudinal placement of the central Plains frontal boundary. This will impact the confidence in the location for the greatest heavy rainfall threat between the High Plains and Midwest/Ohio Valley. The 06Z GFS was a southern outlier with respect to this feature, but trended favorably towards consensus with the 12Z run. The broader signal for precipitation potential across parts of the Upper Midwest into the Great Lakes and Northeast is reasonably well supported. General model weights were used in the forecast through day 6 before trending away from the 06Z GFS and towards ensemble means. ...Weather/Hazards Highlights... The increasing troughing over northern/eastern U.S. will bring thunderstorm and showers development, especially over areas where deep moisture, instability, and slow- moving boundaries overlap. Additionally, the shortwave crossing the northern Rockies/High Plains will move towards the Great Lakes/Upper Midwest on Tuesday. A surface low and trailing frontal boundary will accompany this feature as it interacts with moist and unstable airmass, leading to thunderstorm development along the front within west-northwest flow aloft. With the slow- moving, organized convection, locally heavy rainfall will be possible across the Great Lakes/Midwest into the Ohio Valley and portions of the Mid- Atlantic. The 00Z run of the ECMWF EFI highlights parts of the Upper Ohio Valley for potentially extreme rainfall through Wednesday. By the middle to later part of next week across portions of the High Plains and central U.S., a decent moisture connection from the eastern subtropical Pacific in connection to troughing over the West will allow for moisture to overrun a frontal boundary draped across the central U.S., this will lead to a stripe of additional heavy rain and flash flooding potential extending from the central High Plains to the Ohio Valley, with daily thunderstorm chances occurring along this axis through Saturday. The Day 4 and 5 Excessive Rainfall Outlooks maintain broad Marginal Risks with embedded Slight Risks centered around the Upper Ohio Valley. Over into southwestern U.S., the persistent ridging will start to move eastward, but monsoonal moisture will continue to generate thunderstorms over the Southwest and increase in coverage throughout the week. Multiple days of showers and thunderstorms may lead to locally heavy rainfall and flash flooding across the Southwest, central Great Basin, Rockies, and parts of Interior/Southern CA, particularly over burn scars and steep terrain. The ridge will support hazardous heat over the southern Plains, Southeast and Gulf Coast throughout much of the week. Widespread Major HeatRisk is forecast to extend into parts of the central Plains and Lower Ohio Valley on Tuesday before the area shrinks southward with the approaching frontal boundary and shower/thunderstorm chances. Longest duration of above average and potentially dangerous levels of heat are expected through Saturday across parts of the Southern Plains to Southeast. This is depicted on the WPC Day 3-7 Hazards Chart. Highs are forecast in the mid- upper 90s (triple digits across parts of OK and TX), with lows only dropping into upper 70s. Heat- related impacts could become significant where the combination of high temperatures and humidity persist for several consecutive days. Oudit/Snell Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages are at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw $$