A Negative Arctic Oscillation leaves the Plains & Eastern US more vulnerable to polar/arctic plunges. Guess what's now negative? While this isn't the end all be all, tell-tale sign of colder days time come, it does strongly suggest a major air mass change for the contiguous US by the end of the week. Temperatures are already -10 to -20 degrees below zero near the Canaidan-Alaskan border. So, the cold is already there, it just needs that shove south!