Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1122 AM EDT Fri Jul 13 2018 Valid 12Z Mon Jul 16 2018 - 12Z Fri Jul 20 2018 ...Overview... The flow pattern across the contiguous U.S. will become a bit more amplified during the medium range period with an upper-level ridge expanding across the Great Basin/Four Corners region and a series of upper waves gradually carving out a trough across the Great Lakes/Northeast. Heights are expected to fall across the Pacific Northwest by late next week as an upper low digs south from the Gulf of Alaska toward the region. ...Guidance Evaluation and Preferences... With shortwave energy passing south of Hudson Bay and skirting the Upper Midwest/Great Lakes days 3-5, models showed a range of solutions with respect to both timing and amplitude. The GFS and ECMWF were generally well centered within the spread, while the 00Z Canadian/UKMET were displaced from the consensus by about next Wednesday over western Canada (UKMET) and southeastern Canada (Canadian). Based on the gradually increasing flow amplification through the period, the preference was to go near or slightly slower/deeper than the ensemble mean consensus and this was reflected in the WPC forecast. Uncertainty increases by next Thu/Fri with the amplitude and evolution of troughing into the Ohio Valley (GFS most amplified) but the upstream/downstream ridging over the southern Rockies/western Atlantic, respectively, would support a slower/deeper trough or possibly a closed low along 85-90W by the end of the period into next weekend. However, models can have a tendency to overamplify the longer range and trend weaker as lead time decreases. Farther west, with a shortwave/upper low diving southeast across the Gulf of Alaska mid to late next week and lowering heights across the Pacific Northwest, there is uncertainty as to how deep the feature will be and whether it will dig farther south or move more quickly eastward. Ensemble means were in general agreement at 12Z next Fri on showing a mean trough axis across British Columbia while the deterministic models (and manual forecast) mostly showed a closed low. The WPC forecast initially based on a multi-model deterministic blend during days 3-5 (Mon-Wed), with most weight placed toward the ECMWF and GFS. By days 6-7 (Thu-Fri), weighting of ensemble means (ECMWF and GEFS) was gradually increased to comprise a majority of the weighting. ...Weather Highlights/Threats... Showers and storms with locally heavy rain will accompany a cold front across the Ohio/Tennessee Valleys to the Southeast/Mid-Atlantic/Northeast Mon-Wed. Monsoonal moisture will also produce scattered afternoon/evening thunderstorms across from the southern Great Basin/Southwest to the southern/central Rockies through the next week, with locally heavy rain possible especially early in the week (the building upper-level ridge may somewhat reduce the coverage of convection by later next week). Temperatures will initially be 5 to 15 deg F above average across the Northeast ahead of the cold front early next week, with a decrease closer to seasonal averages by mid to late next week (which are near/at the maximum for the year). In the wake of the cold front, high temperatures are expected to be 5 to 10 deg below average across the central/northern plains and portions of the Midwest through much of next week. Finally, hot temperatures across the Pacific Northwest (highs 5 to 15 deg above average) should moderate some by later next week as a Pacific cold front moves inland. Fracasso/Ryan WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are 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 http://www.wpc.ncep.noaa.gov/heat_index.shtml