Extended Forecast Discussion NWS Weather Prediction Center College Park MD 201 AM EDT Sat Aug 15 2020 Valid 12Z Tue Aug 18 2020 - 12Z Sat Aug 22 2020 ...Dangerous heat for much of the West through next week... ...Overview... A strong upper high will meander over the Southwest next week. Troughing will settle into the Tennessee and lower Mississippi Valleys by midweek as Atlantic ridging noses in from the east into Florida. Well above normal temperatures will approach and exceed records for many locations from Oregon/Idaho southward through the Colorado River Valley and across the Desert Southwest. Rainfall will be focused along a stalled front along the Gulf Coast through the Carolinas with lighter amounts over the Upper Midwest in the vicinity of another wavy frontal boundary. ...Guidance/Predictability Assessment... With good ensemble agreement overall, the deterministic models showed acceptable clustering Tue/Wed so that a blended approach was used as a starting point. Thereafter, favored the 18Z GFS and 12Z ECMWF with an increasing contribution of their ensemble means as uncertainty in shortwave timing/track increased. Over the Gulf of Alaska, ensembles were spread NW-SE with the location of the upper low as the GFS/ECMWF/GEFS showed lower heights into British Columbia and Washington vs the Canadian and ECMWF ensembles. Split the difference for now given the lead time and lack of trend. ...Weather/Hazard Highlights... Record heat is expected for many areas west of the Rockies into midweek while some showers/storms may fire on the east side of the Rockies over Colorado and New Mexico with limited moisture. Record high max and min temperatures will persist over the Great Basin, Desert Southwest and inland areas of California with some places only dropping into the 80s to around 90 overnight. Daytime temperatures in the 100s will be common across the Great Basin while 110s to low 120s are expected in the lower deserts into Death Valley. Little to no rainfall is expected under the upper high. With troughing over the East, showers/storms will be favored over the Southeast especially in the afternoon. Moisture may surge into the Mid-Atlantic midweek as the front wanders along the coast. 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. 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