What is the polar vortex and why is it so cold?

The cold front was forecast to hit the Midwest especially hard, with wind chill expected to plummet to minus 50F (minus 46C) in Chicago by Tuesday evening and two feet (60cm) of snow predicted for Wisconsin, according to the National Weather Service.

from US News Headlines, Latest USA News, America Breaking News - Times of India http://bit.ly/2DHLPL1