Extended Forecast Discussion NWS Weather Prediction Center College Park MD 129 PM EDT Sun Aug 16 2020 Valid 12Z Wed Aug 19 2020 - 12Z Sun Aug 23 2020 ...Dangerous heat for much of the West will slowly ease... ...Overview... Strong upper ridging over the Southwest will persist into next weekend as troughing settles over the Ohio, Tennessee, and lower Mississippi River Valleys. This will result in well above normal temperatures approaching and exceeding record highs Wednesday into Thursday throughout the Great Basin southward to the Colorado River Valley/Southwest. A stalled frontal boundary from the Gulf Coast and along the eastern seaboard will be the main focus for periods of showers and thunderstorms while lighter amounts are forecast across the Upper Midwest along another frontal boundary. ...Guidance/Predictability Assessment... The latest deterministic guidance was primarily used for the WPC blend for days 3-5, anchored by the 00Z ECMWF and 06Z GFS which offered the best agreement and consistency from the previous forecast cycle. The biggest model differences are with the speed and development of various shortwaves across the northern tier over Canada as well as with the development and progression of a closed low near the Gulf of Alaska. The GFS trended slower and more amplified/blocky while the ECMWF was a bit flatter/faster. Given the greater uncertainty and forecast spread, the ECENS/GEFS means were heavily weighted in the day 6/7 blend. ...Weather/Hazard Highlights... Record temperatures in the West will slowly ease later in the week but remain well above normal (by about 5-10 deg) for late August. Overnight min temps may be comparatively milder (10-15 deg above normal). Little to no rainfall is expected under the upper high but some isolated showers/storms will dot the landscape over the Rockies each day. With troughing over the East, showers/storms will be favored over the Southeast especially in the afternoon. Moisture may surge into Virginia midweek as the front wanders along the coast but otherwise highest totals may linger near Florida. Northern stream troughing will bring in cooler than normal temperatures to portions of the Plains and mid-Mississippi Valley through the week with the best rainfall chances over the Upper Midwest near an area of low pressure. Fracasso/Taylor Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indices 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml