Extended Forecast Discussion NWS Weather Prediction Center College Park MD 215 AM EDT Thu Sep 16 2021 Valid 12Z Sun Sep 19 2021 - 12Z Thu Sep 23 2021 ...Overview and Guidance Evaluation/Preferences... Upper level ridging is forecast to remain in place across the Southeast and East Coast during the medium range period while a strong/amplified trough progresses through the Pacific Northwest toward northern Plains. This scenario is well agreed on by the latest deterministic and ensemble guidance through about day 5. As the trough pushes into the Plains, low pressure is forecast to develop and lift toward the Great Lakes region. Aside from the 12Z ECMWF, most of the models show a flattening trough and therefore a more progressive system. Meanwhile the ECMWF advertised a slower trough that eventually cuts off over the Plains. For this cycle, the blend consisted of near equal weights for day 3-5 then a higher use of the ensemble means for day 6 and 7 to account for some of the uncertainty in the northern Plains and Great Lakes. Elsewhere across the CONUS, high pressure is forecast to be the dominant feature for the East Coast through the middle of next week. ...Weather/Hazard Highlights... Ahead of the approaching trough and associated deepening low pressure system for the Intermountain West and northern Plains by early next week, warm and moist air will surge north from the Gulf. This will lead to above average high temperatures and daily chances of scattered to numerous showers and thunderstorms, some of which could be locally heavy along the Gulf Coast region where the airmass will be very tropical in nature. 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. As the upper level trough digs and advances over the northwest and northern tier, expect much below normal high temperatures in its wake. Maximum temperatures of 10F to as much as 20F below normal will be possible Sunday and Monday across the Pacific Northwest and Intermountain West. Meanwhile, 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. Taylor 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