These severe cold events occur when the polar jet stream – the familiar jet stream of winter that runs along the boundary between Arctic and more temperate air – dips deeply southward, bringing the cold Arctic air to regions that don’t often experience it.

Dangerous temperatures are sweeping the country, with 100 million people from the Plains to the Northeast feeling the chill. Even those in the South will see some effects. Several cities have already declared weather emergencies,

Cities across the U.S. are bracing for cold temperatures as a polar vortex arrives, bringing temperatures down to much below normal for mid-to-late January. Parts of Utah are on the "very western fringe" of a cold front that will bring in air from the Arctic.