Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1201 PM EDT Sat Aug 11 2018 Valid 12Z Tue Aug 14 2018 - 12Z Sat Aug 18 2018 ...Pattern Overview and Guidance/Predictability Assessment... Several slow moving cut-off upper lows will initially situate over the mid-Atlantic and Southern/Central Plains, respectively. With the exiting closed low over the mid-Atlantic...weak upstream height falls should eventually nudge the system offshore in the following days entering the Canadian Maritimes by late Thursday. Shifting focus to the longwave trough sitting over the Great Lakes and points southward, there is plenty of ensemble spread with regards to placement and magnitude. Additionally, run-to-run continuity is lacking suggesting future shifts in the newer guidance. Farther upstream, models remain uncertain on the poleward extent of ridging across western North America late in the period. It does appear a 594-dm mid-level ridge will re-establish a semi-permanent summer home over the Desert Southwest. Accordingly, started the WPC forecast process with an evenly weighted blend of the well clustered 00 UTC GFS/ECMWF and 00 UTC GEFS/ECMWF ensembles day3/Tuesday. With uncertainty building across many key features on the map, increasingly weighted ensembles over time through medium range time scales at the expense of deterministic models. By early next weekend, this blended approach still supports a weak negative height anomaly across the Great Lakes and its vicinity while a mean ridge sets up across a large chunk of the western U.S. ...Weather Highlights and Hazards... Wet weather in store for the Southern/Central Plains, and models show some healthy local downpours. Otherwise, there will be a daily chance for thunderstorms almost anywhere east of the Mississippi River given the slow moving longwave trough combined with a robust warm sector. Across the Desert Southwest, monsoonal convection should break out each afternoon/evening. Recent GFS runs have shown very high amounts compared to most other guidance, but these appear contaminated by convective feedback. Rubin-Oster/Schichtel 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