077 FXUS02 KWBC 210707 PMDEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 307 AM EDT Fri Aug 21 2026 Valid 12Z Mon Aug 24 2026 - 12Z Fri Aug 28 2026 ...Hazardous heat to continue from the Lower Mississippi Valley and Southern Plains to the Desert Southwest... ...Overview and Guidance/Predictability Assessment... A favored composite of reasonably well clustered model, ensemble mean and machine learning guidance depicts a stagnant upper pattern next week as highlighted by a strong upper-high anchored over the Southwest and east-central U.S. mean troughing. Hazardous heat across much of the U.S. southern tier may persist for the rest of August, particularly across the Southern Plains and Lower Mississippi Valley. Monsoonal moisture will stream north around the ridge to fuel scattered Intermountain West and Rockies convection as strong convection with local downpours spill across parts of the central U.S.. Shortwaves will maintain upper troughing over the east-central U.S. to favor wavy and moderating frontal surges. Wrapping rains likely for the Northeast early next week as a lead low pulls away will be followed by periodic bouts of rain/thunderstorms for portions of the East Tuesday-Friday. ...Weather/Hazards Highlights... Hot temperatures well into the 100s, with oppressive heat indices, are forecast for the south-central U.S. through next week under the strong upper high. Record-breaking warm overnight temperatures across the southern tier will further limit recovery from the heat, especially given heatwave longevity. Widespread Major to Extreme HeatRisk will persist in the Southern Plains and Lower Mississippi Valley. Several consecutive days of high heat stress can lead to heat-related illnesses. Farther west, highs near and above 110 in the Desert Southwest could also cause heat concerns next week. Areas of the Southeast may reach a few degrees above normal at times with moist dewpoints. Please see WPC's Key Messages for more on the extreme heat. Elsewhere, most areas will see near average temperatures, with the Ohio Valley and vicinity below average. A slowly exiting low will keep wrapping rain chances for the interior Northeast early next week. Through this period, monsoonal moisture is forecast to bring scattered, diurnally driven showers and thunderstorms to the Intermountain West/Rockies. Isolated flash flooding is possible especially across sensitive areas like burn scars, favored terrain like slot canyons, and urban areas. WPC Excessive Rainfall Outlook (ERO) Marginal Risk areas are in place there. Areas of rain and strong to severe storms are also possible in the central U.S. as shortwaves round the northeast side of the ridge and dive southeast. Placement of these shortwaves and thus the rainfall positions and amounts offer uncertainty over the central/east U.S., but frontal proximity and a pooling moisture/instability focus over the central Plains prompted introduction of an ERO Marginal Risk area to further monitor given cell training threat. Rain chances working to the Midwest/Great Lakes and Southeast Tuesday may periodically develop and also overspread much of the Eastern Seaboard over mid-later next week. Schichtel 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 $$