Extended Forecast Discussion NWS Weather Prediction Center College Park MD 335 AM EDT Tue Aug 21 2018 Valid 12Z Fri Aug 24 2018 - 12Z Tue Aug 28 2018 ...Pattern Overview and Guidance Evaluation/Preferences... Model guidance continues to show strong support for a strengthening upper ridge across the northeast Pacific during the medium range, which supports a downstream mean trough across the northwestern U.S., and an expanding ridge from the Lower/Mid-Mississippi Valley to the Mid-Atlantic. Deterministic solutions showed relatively modest timing/amplitude differences with several shortwaves traversing the northern tier Fri-Sun, and a multi-model determinsitic blend (including the ECMWF/GFS/UKMET/CMC) served as a basis for the forecast for days 3-5. By days 6-7 (Mon-Tue), models continue to show some spread as to the degree of western U.S. trough amplification that will occur, and also with the downstream ridge amplification across the Mississippi Valley. Given the degree of flow amplification expected across the North Pacific, as well as the extratropical transition of Typhoon Soulik across eastern Asia, would expect a greater degree of flow amplification to occur across the CONUS as well. Given the relative increase in model spread by days 6-7, weighting of ECENS/GEFS means was boosted to a majority of the forecast during that time period. ...Weather Highlights and Hazards... A lingering stationary frontal boundary along the Southeast U.S. coast through most of the medium range period will result in scattered to numerous showers and thunderstorms with locally heavy rainfall possible. Areas of potentially heavy convection will also be possible across the mid/upper Mississippi Valley and Upper Midwest through much of the period as a couple waves of low pressure traverse a frontal boundary across the region. Convection across the central U.S. may be confined more to the Midwest and Great Lakes by the weekend as upper-level heights begin to rise across the central U.S., increasing the likelihood of a capped environment farther south. Scattered afternoon/evening showers and storms are also expected to persist through the week across the Four Corners region, with monsoonal moisture remaining in place. Areas from the Mid-Mississippi Valley to the Mid-Atlantic will begin the medium range period with high temperatures 4 to 8 deg F below average, but as the upper ridge expands overhead, a gradual warming trend will follow as high temperatures 5 to 10 deg F above average are forecast to expand eastward from the central U.S. toward the eastern U.S. during the weekend. Finally, height falls across the Northwest will lead to a cooling trend with temperatures falling to more than 10 deg below average in the interior by Sunday into Monday. 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