Extended Forecast Discussion NWS Weather Prediction Center College Park MD 148 AM EDT Sun Aug 19 2018 Valid 12Z Wed Aug 22 2018 - 12Z Sun Aug 26 2018 ...Pattern Overview and Guidance Evaluation/Preferences... Models/ensembles show good consensus that a pattern change will occur during the medium range, with a shift away from the western ridge/eastern trough flow pattern that has dominated for several weeks. An upper-level ridge is expected to amplify across the north Pacific, between Alaska and Hawaii, by the middle of the week. The downstream effects of this will result in lowering heights from western Canada into the Pacific Northwest, and gradually rising heights from the Midwest/Ohio Valley to the Eastern Seaboard as broad upper ridging expands eastward. In general, a blend of the ECMWF/GFS and their respective ensemble means served as a good starting point for the WPC forecast. Forecast confidence was initially sufficient to use a majority of deterministic solutions during days 3-4 (Wed-Thu), with a gradual shift toward heavier ensemble mean weighting during days 5-7 (Fri-Sun). These solutions show typical timing/amplitude differences the upper trough lifting out of the eastern U.S. Wed-Thu, as well as with a couple additional areas of shortwave energy traversing the northern tier, from the Pacific Northwest to the northern plains, during the forecast period (each gradually lowering heights across the Northwest). ...Weather Highlights and Hazards... A cold front that is eventually expected to become stationary by the weekend near the Southeast U.S. coastline will bring scattered showers and storms with locally heavy rainfall through much of the next week. Areas of showers and thunderstorms with locally heavy rain will also be possible Thu-Sun across the northern plains and Upper Midwest in association with the passage of a couple upper-level shortwaves and a surface frontal boundary. Across the Four Corners states, monsoonal moisture will persist with scattered showers and thunderstorms a daily occurrence. A relatively pleasant air mass with high temperatures 5 to 8 deg F below average will initially be in place from the Mid-Mississippi Valley to the Mid-Atlantic and Northeast in the wake of the cold front stalling along the Southeast U.S. coast. As upper height gradually rise by next weekend, temperatures across these areas will rise, and high temperatures of 5 to 10 deg F above average are possible by late in the week from the Plains to the Midwest and Great Lakes. Ryan WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found 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