Extended Forecast Discussion NWS Weather Prediction Center College Park MD 225 AM 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... Through the 12Z/18Z guidance, the latest deterministic runs offered good enough clustering such that a blend of their solutions served as a reasonable starting point. Greatest uncertainty lied over northern Canada with shortwaves sliding into the backside of the upper low over Hudson Bay as well as over the Pacific Northwest downstream of the Gulf of Alaska upper low. The 12Z GFS was likely too fast in the east by next weekend and was too strong (along with the Canadian) with another upper low over Canada compared to the ensembles. Trended toward a mixture of the 18Z GFS/GEFS mean and 12Z ECMWF/ECMWF ensemble mean to round out the forecast. ...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 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