Extended Forecast Discussion NWS Weather Prediction Center College Park MD 426 PM EDT Tue Sep 01 2020 Valid 12Z Fri Sep 04 2020 - 12Z Tue Sep 08 2020 ...High heat rebuilds over the West late this week while much cooler air arrives for the north-central U.S. early next week... ...Weather/Hazard Highlights and Guidance/Predictability Assessment... An amplifying upper-level pattern is forecast for the CONUS. This will open up the northern portion of the country for periodic intrusions of cool air from Canada through early next week. The first surge of cool air is forecast to reach the northern Plains on Sunday behind a cold front and a digging upper-level trough. Both the GFS and CMC support an earlier development of a low pressure system across southern Canada and near the northern U.S. border on Sunday in contrast with a slower development shown by the ECMWF. This has led to a weaker surge of cool air into the Plains on Monday according to the ECMWF. But all three global models agree that there will be another cool air surge into the northern High Plains by next Tuesday, when high temperatures of 15 to more than 20 degrees below normal across the northern Plains will plunge toward the central Plains. In contrast, the warm Southwest U.S. ridge appears to strengthen late this week into the weekend before expand toward the northwest into the Pacific Northwest and off the coast of British Columbia. This will result in an general intensification and expansion of the heat across the western U.S. High temperatures soaring above the century mark will be increasingly common for the interior Valleys of California by this weekend, which will challenge their daily records at many locations. High temperatures well into the 90s to near 100 are also forecast for the hottest locations over the interior Pacific Northwest through early next week. By Tuesday, the heat may begin to ease a bit as the cool air mass over the Rockies edges closer toward the region. Excessive precipitation is not expected across the U.S. although a lingering front will keep a good chance of thunderstorms for Florida through the medium range period. By next Tuesday, models are indicating the possibility of heavy rain across the central Plains toward the upper Midwest as the cold front associated with the first surge of cool air slows down. The WPC forecast package was composed of a blend the 00Z ECMWF/CMC and the 06Z GFS with more weights of their ensemble means but less weight from the ECMWF toward 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 Hazards: - Heavy rain across portions of the Central Plains, the Lower Mississippi Valley, the Great Lakes, the Middle Mississippi Valley, the Upper Mississippi Valley, the Southern Plains, and the Ohio Valley, Mon-Tue, Sep 7-Sep 8. - Heavy rain across portions of the Southern Plains, Fri, Sep 4. - Heavy rain across portions of the Great Lakes, Sun, Sep 6. - Flooding possible across portions of the Lower Mississippi Valley and the Southern Plains. - Excessive heat across portions of California and the Southwest, Fri-Mon, Sep 4-Sep 7. - Excessive heat across portions of California, Sat-Tue, Sep 5-Sep 8. - Much above normal temperatures across portions of California, the Central Great Basin, the Pacific Northwest, the Northern Rockies, and the Northern Great Basin, Fri-Mon, Sep 4-Sep 7. - Much above normal temperatures across portions of California, the Pacific Northwest, and the Northern Great Basin, Fri-Tue, Sep 4-Sep 8. - Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Sat-Mon, Sep 5-Sep 7. 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