Lets start with the basics. In the cool season the North Pole is pointed away from the sun. As the polar regions lose solar heating and the days shorten, the atmosphere cools significantly, and we see a huge semi-permanent polar air mass covering much of Canada and northern Russia. Parts of it break off regularly as frontal systems and polar highs. The large expansion of the cold air mass boots the jet stream out of Canada, pushing it down into the United States.