Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1200 PM EDT Tue Jun 26 2018 Valid 12Z Fri Jun 29 2018 - 12Z Tue Jul 03 2018 ...Overview and guidance evaluation/preferences... Model guidance this morning continues to forecast for a heat wave to spread eastward from the central Plains into the East Coast by this weekend while general troughing will keep conditions cool over the western U.S. Model agreement remains good regarding a broad upper-level trough and the associated cold front moving out of the northern Rockies into the northern Plains through Day 5 Sunday. Thereafter, models begin to show differences on how an upper-level closed low over the Gulf of Alaska evolves as it drops towards the west coast of Canada early next week. The ECMWF has been the fastest on bringing cyclonic vorticity inland across British Columbia toward northern Idaho on Day 7 Tuesday while the GFS keeps the cyclone center farther back along the coast of British Columbia. The ECMWF has settled into the faster solution for the last couple of runs while the GFS continues to show inconsistency between its deterministic solutions and the ensemble means (i.e. the GEFS). The GEFS has been comparable to the 00Z EC ensemble mean with a weaker surface low center around Alberta Canada on Day 7. Therefore, the WPC morning package was based on a general compromise between the 06Z GFS/06Z GEFS and the 00Z ECMWF/00Z EC mean through Day 5. More weights were then given to the 00Z EC mean for Day 6 and 7, with 70% assigned to the 00Z EC/EC mean on Day 7. ...Weather threats/highlights... Low pressure tracking northeast from the Plains into Canada from Fri onward along with its associated fronts and upper shortwave support will likely produce convective episodes capable of producing areas of heavy rainfall. Best signal for highest totals is across the northern Plains/Midwest/upper MS Valley/upper Great Lakes. Southward progression of the cold front may bring convection into the central Plains. Some activity may be strong to severe as well. Check latest SPC outlooks as details become more clear in coming days. Expect showers/thunderstorms to extend farther eastward later in the period but likely with lesser amounts as best upper dynamics lift into Canada. Meanwhile there should be diurnally favored convection over portions of the South/Florida through the period with some locally heavy amounts. It remains a question mark to what extent upper energy/surface feature(s) flowing around the deep layer high to the north may enhance this convection. A heat wave is expected to spread into the eastern U.S. this weekend and only dampened a bit early next week with a better chance of showers and thunderstorms. Within the large area of hot and humid weather over the central/eastern U.S., highest min/max temperature anomalies of plus 10-20F should extend from the Midwest through the Great Lakes/Ohio Valley and Northeast. The heat/humidity will gradually erode from the west over the course of the period as a Plains cold front moves into the Great Lakes. There are many locations that may see one or more days of min temps at least close to daily record warm values. Daily record highs should be less numerous and most likely confined to the Great Lakes/Ohio Valley/Northeast during Sat-Mon. The humidity will contribute to high heat index values. Below normal temperatures will be most common over the West and High Plains, with the northern Rockies/High Plains likely to see one or more days of highs 10F or more below normal Fri-Sun. Ejection of the initial western trough aloft will help readings trend to moderately above normal levels over the central/southern parts of the West by early next week. Kong/Rausch WPC medium range forecasts of 500 mb heights, surface systems, weather grids, quantitative precipitation, and winter weather outlook probabilities 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