Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Mon Aug 31 2020 Valid 12Z Thu Sep 03 2020 - 12Z Mon Sep 07 2020 ...High heat rebuilds over the West late this week into early next week as waves of cool air from Canada intrude the central and eastern U.S.... ...Weather/Hazard Highlights... An amplifying upper-level pattern is forecast for the medium-range period across North America. The warm anticyclone that has been anchoring over the Desert Southwest will likely expand toward the Pacific Northwest this weekend into early next week. This will sustain the hot and dry conditions over the Desert Southwest (with daily highs above 110) while heat is expected to build toward the Pacific Northwest. In fact, high temperatures soaring above the century mark will be increasingly common for the interior Valleys of California by this weekend; and by next Monday, record high temperatures may be established at many locations. Triple-digit high temperatures are also forecast for the hottest locations over the interior Pacific Northwest. Slightly cooler air from western Canada could filter into Washington by Sunday into Monday behind a cold front. Meanwhile, more signs of autumn are in store for the central and eastern portions of the U.S. Two strong surges of cool air from Canada will likely bring large temperature swings across the northern Plains while below normal temperatures are expected to stretch from the central Plains to the Northwest. Much of the Southeast into the Deep South should remain warmer than normal with a good chance of showers and thunderstorms as the cold front associated with the first surge of cool air stalls across the region by this weekend. Farther north, the cold front associated with the second surge of cool air should trigger a around of showers and thunderstorms across the upper Midwest on Sunday, followed by the Great Lakes on Monday. Finally, areas of showers and thunderstorms are expected across Texas from Thursday through much of the weekend. ...Guidance/Predictability Assessment... Increasing signal and forecast confidence for an amplifying flow pattern over the CONUS during the medium range period, featuring upper ridging over the western US with a deep longwave trough centered over the central US. The latest model guidance shows very good agreement with this idea and overall, the deterministic models had tight clustering through day 5/6 which bolstered the forecast confidence this cycle. The WPC blend was composed of a blend the 00Z ECMWF/EC mean with the 06Z GFS/GEFS suite with increasing weights toward the ensemble means as the forecast hour increases. On Day 7, the 00Z ECMWF indicated that the front moving into the Northeast was too far ahead of its ensemble mean and the GFS/GEFS solutions. Therefore, only a very small portion of the deterministic ECMWF was used for Day 7. Kong 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