Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1136 AM EDT Mon Jun 18 2018 Valid 12Z Thu Jun 21 2018 - 12Z Mon Jun 25 2018 ...Guidance/Predictability Assessment... Models show broad agreement on the large scale flow pattern across the CONUS during the medium range, with a modestly progressive pattern, but with at least some continued tendency for at least occasionally blocked flow and amplification. An upper ridge across central Canada persisting through the forecast period will support trough amplification across the northeastern U.S. (initially on Thu and then again by early next week), as well as across the Pacific Northwest. Meanwhile, energy crossing from the Rockies into the central U.S. will see potential for deepening and/or cutting off, moving through a region of relatively weak upper flow beneath the Canadian ridge. The WPC forecast during days 3-5 (Thu-Sat) was based on a majority deterministic blend including the 00Z ECMWF/06Z GFS. These solutions represented consensus will with respect to the trough lifting out of New England and the upper low moving from the central plains toward the Mississippi Valley before transitioning to an open wave across the Great Lakes. This blend also sufficiently represents additional shortwave energy crossing the northern Rockies Thu-Fri. Model differences begin to grow after 12Z Sat as energy crosses the Rockies and moves toward the plains. The GFS has been relatively consistent in trying to close the energy off either as it crosses the Rockies or as it nears the High Plains Sun/Sun night. The ECMWF on the other hand has generally kept a more progressive solution, but with more significant run-to-run variability. Given the amplified nature of the flow across the higher latitudes from the North Pacific across Canada, the pattern seems, at least conceptually, to also favor greater potential for shortwave energy to close off across the CONUS. The ensemble means also seem to support this idea, with a significant number of NAEFS/GEFS/ECENS members showing a solution at least somewhat closer to the GFS (the 00Z ECENS mean was slower and more amplified than the deterministic ECMWF). Farther downstream, additional uncertainty enters the forecast with respect to trough amplification across eastern North America by Sun-Mon. The 00Z/06Z GFS runs were highly amplified by Mon, with significant height falls getting into New England. The ECMWF was similar but a bit less amplified than the GFS. Ensemble means showed good consensus at least with respect to the general timing of any trough amplification, however, with the main differences pertaining to the amplitude. Given the increased uncertainty, weighting of ensemble means (00Z ECENS/NAEFS) was boosted to a slight majority of the forecast weighting during days 6-7. Among the deterministic components, the 06Z GFS was weighted a bit more relative to the 00Z ECMWF, to support at least somewhat more amplification of features. ...Weather Highlights/Threats... A frontal boundary with multiple waves of low pressure will persist from the central U.S. into the Ohio Valley and Mid-Atlantic through much of the forecast period, promoting scattered to numerous showers and thunderstorms, with areas of heavy rainfall potential. Meanwhile, above average temperatures will continue for much of the western U.S. as well as the Southeast, with high temperatures 5 to 10 deg F above average possible across these areas through at least a portion of the medium range. The largest high temperature anomalies are expected across portions of California and Oregon Fri-Sun, where highs may reach 10 to 15 deg F above average. Ryan WPC medium range forecasts of 500 mb heights, surface systems, weather grids, quantitative precipitation, and winter weather outlook probabilities can be found at: http://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif http://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif http://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html http://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml http://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4