350 FXUS02 KWBC 080537 PMDEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 137 AM EDT Sat Aug 8 2026 Valid 12Z Tue Aug 11 2026 - 12Z Sat Aug 15 2026 ...Overview... Upper troughing and shortwave disturbances will bring thunderstorms and showers across north-central and eastern U.S. through late week. Multiple days of heavy rainfall may lead to localized flooding concerns. The ridge will move eastward, bringing above normal temperatures across southern Plains into Southeast. Southerly flow will continue to funnel monsoonal moisture across the Southwest/Southern CA, leading to daily thunderstorm development. ...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 frontal placements. This will impact the confidence in the location for the greatest heavy rainfall threat. However, the broader signal for precipitation potential across parts of the Upper Midwest into the Great Lakes and Northeast is reasonably well supported. CMC seemed to be a outlier in some of the smaller-scale features compared to the GFS/ECMWF. Therefore, a model blend of ECMWF/GFS weight in the forecast were heavier than the CMC, while gradually phasing in the ensemble means towards day 5. ...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. 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. 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, as well as portions of the northern/central Plains through Wednesday. Across the Southeast, abundance low-level moisture will maintain a risk of thunderstorms that can lead to localized flooding where repeated storms may develop. The frontal boundary will stall and drape across the central Plains into the Ohio Valley and Northeast by late-week, reinforcing another round of thunderstorms and showers through Friday. High rainfall rates and repeated rounds of convection can lead to localized flooding concerns along the boundary. Over into southwestern U.S., the persistent ridging will start to move eastward, but monsoonal moisture will continue to generate thunderstorms over the Southwest. Multiple days of showers and thunderstorms may lead to locally heavy rainfall and flash flooding across the Southwest and parts of Southern CA, particularly over burn scars and steep terrain. The ridge will support hazardous heat building over the southern Plains, Southeast and Gulf Coast by midweek. Heat- related impacts could become significant where the combination of high temperatures and humidity persist for several consecutive days. Oudit 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 $$