Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1232 AM EDT Tue Jun 16 2020 Valid 12Z Fri Jun 19 2020 - 12Z Tue Jun 23 2020 ...Overview and Weather/Hazard Highlights... An upper-level low initially centered across the central Appalachians is forecast to gradually weaken as it moves eastward Fri through the weekend, eventually becoming absorbed in stronger westerlies ahead of an amplifying northern stream trough. Scattered showers and thunderstorms across the Appalachians and Mid-Atlantic on Fri associated with this system should gradually diminish in coverage by the weekend. Farther west, another upper low in the northern stream is forecast to move east relatively slowly across the Canadian plains Fri through the weekend, with associated height falls and cold front crossing the U.S. Northern Plains and Upper Midwest. Ahead of this trough/cold front, high temperatures are forecast to be 5 to 15 deg F above average from the Great Lakes to the Northeast Fri and into the weekend. The trailing end of this frontal boundary is expected to stall across the Central/Southern Plains Fri-Sat, and linger through early next week as the parent low pressure system moves into eastern Ontario and eventually Quebec. The front will focus showers and thunderstorms Fri-Sat from the Central Plains to the Upper Midwest, with locally heavy rains possible. By Sun-Mon as the trough/upper low moves farther east, shower and thunderstorm activity should shift farther south and east, from the Southern Plains and Lower Mississippi Valley to the Ohio Valley and eventually the Appalachians, with areas of heavy rainfall remaining possible. Farther west, upper-level ridging should gradually strengthen over the Southwest over the weekend and into early next week, but Pacific shortwave energy and associated cold fronts will still affect the Northwest, with one system moving inland Sun-Mon, and another one off the coast by Tue. Given that the Sun-Mon system will be heading into broad anticyclonic flow, and will be weakening as it moves ashore, precipitation impacts across the Northwest will be relatively minimal, with only light precipitation expected. Additionally, as heights rise across the West next week, warming temperatures are also expected, with highs forecast to be 5 to 15 deg above average Sun-Tue across much of the Great Basin. ...Guidance/Predictability Assessment... A blend of the 12Z ECMWF/18Z GFS was heavily used for the WPC forecast during days 3-4 (Fri-Sat). These solutions showed some differences with respect to how quickly the eastern upper low lifts out, and with the timing of the northern stream upper low crossing Canada, but a blend of the two solutions represented consensus well. As spread increased through time, weighting of ECENS/GEFS ensemble means was gradually increased, with a majority of the forecast comprised of ensemble means by days 6-7 (Mon-Tue). Overall, ensembles showed broad agreement at large scales through the extended period, with general consensus on a building ridge in the West next week, and troughing over the East. Ryan 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