Extended Forecast Discussion NWS Weather Prediction Center College Park MD 108 PM EDT Wed Aug 12 2020 Valid 12Z Sat Aug 15 2020 - 12Z Wed Aug 19 2020 ...Record high temperatures in The West as heat wave expands north this weekend... ...Guidance/Predictability Assessment... The ridge centered over the Southwest builds north up the northern Rockies and Canadian Rockies this weekend with the northern end tilting east onto the Canadian Prairies early next week. This shifts a low currently over the Canadian Rockies east to Hudson Bay through this weekend a negatively tilted trough reaching the upper Great Lakes Saturday and a rather amplified pattern of a ridge over the Intermountain West and a trough over the east-central CONUS. The 00Z ECMWF is more amplified with the western ridge by Saturday morning than recent GFS runs. The flatter GFS is more progressive with the negatively tilted trough into the Great Lakes along with the associated cold front that spreads across the eastern CONUS which will displace the cold air damming along the Eastern Seaboard. The 06Z GFS was stronger with this leading wave over the Great Lakes Saturday, so some 00Z GFS was included through Day 5. The 12Z GFS retains this leading feature, but is not as strong with it as the 06Z. The 00Z UKMET is less amplified with this ridge than the GFS and was included in the blend through Day 5. Another feature of note is a shortwave rounding a Gulf of Alaska low that crosses just off the OR/WA coast Sunday night. Recent GFS runs (including the 12Z) are strong with this wave that allows more amplification of the trough off the West Coast and keeps the ridge more intact in the GFS than the ECMWF with has a more active patter over western Canada next week. The 00Z ECMWF outpaces the 00Z ECENS mean with this ridge axis, so heavier reliance on ensemble means is used for Days 6/7. ...Weather/Hazard Highlights... High pressure dammed down from the Northeast will be slow to recede, likely lingering until the cold front crosses the area in the Monday night time frame. Continued heavy rain over the Mid-Atlantic, central Appalachians, and upper OH Valley are expected with the damming scenario that will feature a surface low shifting northeast from the Carolinas Sunday into Monday. Convection will also be enhanced as the leading cold front digs from the Upper Midwest/central Plains Saturday, spanning the entire Eastern Seaboard and Gulf Coast by Tuesday. Ample troughing building over the east behind a reinforcing cold front makes for temperatures 5 to 10 degrees below normal spread across the eastern CONUS from the Northern Plains starting Sunday. The ridge axis position over Arizona will continue to inhibit monsoonal flow. Much above normal temperatures will spread north with the ridge over the Intermountain West and across the Pacific Northwest this weekend into Monday. Temperatures will generally be 10 to 15 degrees above normal (15 to 20 degrees above normal in the Pacific Northwest Sunday/Monday). Record high max and min temperatures are expected to persist over many areas of the Desert Southwest through the medium range period with max temperatures exceeding 115 degrees. Additional record highest spreads over the rest of the West in tandem with upper trough amplification off the West Coast. This is a continued dry pattern with frontal relief expected only for the Pacific Northwest in the middle of next week. Jackson 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