Extended Forecast Discussion NWS Weather Prediction Center College Park MD 431 PM EDT Wed Sep 02 2020 Valid 12Z Sat Sep 05 2020 - 12Z Wed Sep 09 2020 ...High heat for parts of the Western/N-Central U.S. this week turns much colder early next week... ...Weather/Hazard Highlights and Guidance/Predictability Assessment... As per an amplifying upper level pattern expected to evolve over the course of the medium range period over the CONUS, temperature anomalies remain one of the main sensible weather headlines. The amplified pattern will open up the northern portions of the country for periodic intrusions to Canadian high pressure, the first of which is expected to arrive late in the weekend or early next week. Model guidance shows good agreement through Sunday, but for Monday-Wednesday of next week the 00 UTC ECMWF/Canadian ensembles, the 00 UTC Canadian, and especially the 00 UTC ECMWF digs the cooling/unsettling upper trough much further west into the West/Rockies while the GFS/GEFS are quite a bit more progressive and dug over the central Plains. I still believe that the forecast track of current west-Pacific Typhoon Maysak northward into Korea/NE Asia will act to enhance downstream flow amplification over the Pacific/Alaska then CONUS. Accordingly, the WPC medium range product suite was primarily derived from the 00 UTC ECMWF ensemble mean. This solution represents a composite of the solutions that dig more energy into the West/Rockies. It is not as robust as the closed trough solutions of the 00 UTC ECMWF and especially now with the 12 UTC ECMWF. It's not all apples-apples, but the newer 12 UTC GFS/Canadian/UKMET and GEFS have overall trended westward. In this pattern, heat will remain in place over the Southwest U.S. for much of the period and may strengthen into the weekend and surge northward into the Pacific Northwest. High temperatures well into the 90s are forecast for the hottest locations over the interior Northwest, before moderating early next week. Meanwhile expect a rush of much below normal post-frontal temperatures, especially through the Rockies/N-central states by next week. Enhanced snows will also be possible across the central/northern Rockies next week as per the latest winter weather outlook. The best chance for organized heavy rainfall during this period may develop from the south-central Plains toward the upper Midwest as the cold front associated with the first surge of cool air and moisture return from the Gulf lifts north and interacts with the slowing boundary, but there remains quite a bit uncertainty with axis specifics. Schichtel 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 Southern Plains, the Middle Mississippi Valley, the Upper Mississippi Valley, and the Great Lakes, Mon-Tue, Sep 7-Sep 8. - Heavy rain across portions of the Middle Mississippi Valley, the Great Lakes, and the Upper Mississippi Valley, Sat, Sep 5. - Flooding possible across portions of the Lower Mississippi Valley and the Southern Plains. - Flooding occurring or imminent across portions of the Mid-Atlantic, the Lower Mississippi Valley, and the Southern Plains. - Excessive heat across portions of the Central Great Basin, California, and the Southwest, Sat-Mon, Sep 5-Sep 7. - Excessive heat across portions of California, Sat-Tue, Sep 5-Sep 8. - Much above normal temperatures across portions of California, the Pacific Northwest, and the Northern Great Basin, Sat-Wed, Sep 5-Sep 9. - Much above normal temperatures across portions of California, the Central Great Basin, the Pacific Northwest, the Northern Rockies, and the Northern Great Basin, Sat-Mon, Sep 5-Sep 7. - Much below normal temperatures across portions of the Central Plains, the Central Great Basin, the Northern Plains, the Northern Rockies, the Central Rockies, and the Northern Great Basin, Tue-Wed, Sep 8-Sep 9. - Much below normal temperatures across portions of the Central Plains, the Central Rockies, the Northern Plains, the Southern Rockies, the Upper Mississippi Valley, and the Southern Plains, Wed, Sep 9. - Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Sun-Tue, Sep 6-Sep 8. - Heavy rain across portions of mainland Alaska, Sun-Mon, Sep 6-Sep 7. - High winds across portions of mainland Alaska, Sun-Mon, Sep 6-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