A blast of brutally cold air - thanks to the polar vortex - is roaring across the central and eastern USA early this week, sending temperatures to levels not seen in 20 years.
The polar vortex is a strong area of low pressure that usually wanders around the Arctic throughout the winter. Counterclockwise winds around the vortex occasionally push waves of intensely cold air into the USA.
The vortex is strongest during the winter, and usually weakens or even disappears in the summer. The vortex tends to move very slowly or even stay stationary, and its position determines what part of the USA the Arctic air will invade.
This week, the core of the vortex has actually slipped all the way down over the USA. Essentially, the bitterly cold air that's typically over the Arctic is now over the central and eastern U.S.
The vortex was allowed to slip this far south in part due to a strong "blocking" area of high pressure over eastern Canada and Greenland.
Fortunately, this visit from the Arctic is expected to be short-lived, as the vortex is forecast to return to its proper location north of the border. This will allow temperatures to return to average or even above-average levels by the end of the week and the weekend.