Why does it get so cold so early in the west?

I know the north-south orientation of the Rocky mountain range tends to combine with the cold surface highs dropping out of Canada to pull the highs more southward initially. The clockwise circulation around the highs causes the surface winds to blow upslope to the south of the high. This upslope flow cools the air and induces height falls, which induces the surface high to move more along the mountains. Eventually it pulls out eastward but the air becomes modified by then, and the system also tends to lift back northeastward. Later in the winter there tends to be a larger area of snowcover over the Canadian prairies which limits modification and allows the colder air to intrude further eastward into the midwestern areas. Pattern changes during the winter could also account for some of this.