Extended Forecast Discussion NWS Weather Prediction Center College Park MD 314 PM EDT Thu Sep 16 2021 Valid 12Z Sun Sep 19 2021 - 12Z Thu Sep 23 2021 ...Overview and Guidance Evaluation/Preferences... Elsewhere across the CONUS, high pressure is forecast to be the dominant feature for the East Coast through the middle of next week. Ridging will remain in place over much of the East and Southeast during the extended period while an amplified trough tracks from the Pacific Northwest to the north-central states. In general, the large scale pattern and its evolution have been in fairly good agreeance the first couple of days, but the spread in timing and location increases beyond day 5 particularly for the trough/surface front near the Northern Plains and Great Lakes region. This trend has been persistent that past 4 or 5 days. A majority of the guidance show a flattening trough, therefore a more progressive system to move trough the Pacific Northwest and Intermountain West into the Plains. Previous runs of the ECMWF advertised a slower trough that eventually cuts off over the Plains; and now, the GFS and CMC solutions are hinting at this posibility but not for the same time periods. The preferred blend for this forecast consisted of nearly equal weights for day 3-5 of the ECWMF, GFS and CMC with increasing percentages of the ensemble means for day 6 and 7 to account for some of the uncertainty in the northern Plains and Great Lakes. ...Weather/Hazard Highlights... Warm, moist air will surge northward from the Gulf of Mexico prior to the trough and associated deepening low pressure system that will cross the Intermountain West and enter into the Northern Plains by early next week. As such, the daily temperatures will be warmer than average and with the mositure surge there will also be scattered to numerous showers and thunderstorms. Parts of the Gulf region may have periods of heavier rainfall. The greatest temperature departures from normal will be for the northern tier and Upper Midwest where daily highs 10F to 20F above normal will be possible. A significant drop in temperatures can be expected across the Northwest and northern tier states as a "season-changing" cold front passes through. Scattered to widespread rain will shift from the Northwest coastal areas inland toward the Intermountain West. Some of the highest peaks may have snow as the heights and temperatures fall. Maximum temperatures of 10F to as much as 20F below normal will be possible Sunday and Monday across the Pacific Northwest and Intermountain West. In contrast, the daily lows across the CONUS are largely forecast to be above normal, with some significant departures from normal likely Sunday into early next week across the northern tier and Great Lakes. Lows 15F to 20F above normal are possible. Campbell Hazards: - Flooding occurring or imminent across portions of the Lower Mississippi Valley. - Heavy rain across portions of the Northern Rockies and the Northern Great Basin, Sun, Sep 19. - Heavy rain across portions of the Mid-Atlantic, the Southern Appalachians, and the Tennessee Valley, Tue, Sep 21. - Heavy rain across portions of the Upper Mississippi Valley, Tue-Wed, Sep 21-Sep 22. - High winds across portions of the Northern/Central Great Basin and the Northern/Central Rockies, Sun, Sep 19. - Much below normal temperatures across portions of California and the Pacific Northwest, Mon-Tue, Sep 20-Sep 21. - Much below normal temperatures across portions of the Northern/Central Great Basin and the Northern/Central Rockies, Tue-Wed, Sep 21-Sep 22. Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook 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