WeatherWhys for December 13,2013 from accuweather.com

The prerequisites for a brutal blast of cold air across the United States are for a large dome of high pressure to build and strengthen in the vicinity of British Columbia as a polar vortex forms over eastern Canada. A polar vortex occurs above the core of the coldest polar air. The jet stream between the high and the polar vortex ushers the frigid air into the United States.