The polar vortex is always present in the Arctic Circle. It is a broad area of low pressure usually contained by the Polar Jet Stream.
A strong polar jet stream keeps the cold Arctic air bottled north. When the Polar Jet Stream weakens, the polar vortex can split and move south.
Bitterly cold times experienced in Chicago are when the Polar Vortex moves south toward the Great Lakes. This can produce many days of extreme cold for Chicago.
The last major Polar Vortex was in late January 2019.
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