Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1230 AM EDT Wed Jun 17 2020 Valid 12Z Sat Jun 20 2020 - 12Z Wed Jun 24 2020 ...Overview and Weather/Hazard Highlights... A slow moving upper-level low centered over the central Appalachians Sat is forecast to gradually weaken as it moves eastward through the weekend, eventually becoming absorbed in stronger westerlies ahead of an amplifying northern stream trough. Showers and thunderstorms across the Appalachians and Mid-Atlantic associated with this system should gradually diminish in coverage over the weekend. Upstream, another upper low in the northern stream is forecast to move east relatively slowly across the Canadian plains 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 over the weekend. The trailing end of this frontal boundary is expected to stall across the Central/Southern Plains and linger through early next week with some reinforcement, as the main low pressure system moves into eastern Ontario and Quebec. The front will focus showers and thunderstorms Sat from the Plains to the Midwest, with locally heavy rains possible. From Sunday onward, 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 then the Appalachians. The Southern Plains may see the greatest potential for locally heavy rain under an upper-level weakness. Out west, upper-level ridging should gradually strengthen over the Southwest through early next week, but Pacific shortwave energy and associated cold fronts will still affect the Northwest, with one system moving late Sat into Sun, and another one off the coast by Tue-Wed. Given that the Sun-Mon system will be heading into broad anticyclonic flow, and will be weakening as it moves ashore, precipitation across the Northwest should be relatively light, and primarily across the Olympics and northern Cascades. 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-Wed across much of the Great Basin. ...Guidance/Predictability Assessment... A blend of the 12Z ECMWF/18Z GFS was heavily used during days 3-4 (Sat-Sun). These solutions were reasonably compatible during that time frame, and a blend represented the overall consensus well. During days 5-7 (Mon-Wed), more weight was placed on the ECENS/GEFS ensemble means, along with some continued use of the ECMWF. The GFS had some significant differences from the overall consensus later in the forecast period, becoming substantially more amplified than other guidance with shortwave energy crossing the Northwest and northern Rockies/Plains Mon-Tue. Ensemble means showed general consensus that this feature should lose amplitude as it moves inland into a broad and expanding ridge. The ECMWF was perhaps a bit more similar to the ensembles here, but it has struggled a bit with run-to-run consistency by that time. Thus, the heavier/majority emphasis place on the ensemble means later in the forecast period. 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