Strong polar vortex winds normally trap cold air in the Arctic and circle the North Pole from west to east. This pattern broke down this month, allowing cold Arctic air to spill over the central United States and create record cold temperatures for the eastern half of the United States.
This isn’t the first time that the polar vortex has been weak. During late autumn and early winter in 2005, 2008, 2009 and 2010, weak polar vortex winds were associated with an increase of cold air moving south from the Arctic. During this event in 2009, North America was 3 to 18 °F cooler than normal monthly averages, and the Arctic region was more than 7 °F warmer than average.