Thematic of the 12Z GFS is maintain warmth in the West days 6-10 beneath a titanic upper-level high pressure ridge which extends north and northeastward in the 11-15-day period causing the Canadian polar vortex to dive southeastward toward the Northeast U.S. The 12Z GFS is loaded with arctic sir and the shifting polar vortex forces the big chill into the Northeast Corridor in the 11-15-day period.