According to the National Weather Service, a polar vortex will impact most of the country this weekend, here is what you need to know.

A blast of Arctic air is set to cover much of the United States with temperatures below freezing starting on Friday and into next week, impacting millions of Americans in nearly all of the contiguous states.

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.

Extreme cold is set to grip much of the country starting this weekend with the return of the polar vortex. Some regions are expected to see temperatures in the negatives early next week. TODAY’s Al Roker tracks the frigid forecast.

People living "basically anywhere from the Rockies eastward" will see extremely cold temperatures over the next several days, a meteorologist says.

The polar vortex is always around. It is a channel of strong winds high up in the atmosphere that circles the poles. One over the North Pole and one over the South Pole. When the vortex is at it's strongest, the vortex is well defined and has a single circular shape to it.

Intense snowy wintry weather in the US has been fuelled by activity of the polar vortex, slipping down to engulf large parts of the country. Early January's winter storm prompted seven states to declare states of emergency,

Americans were bracing Thursday for frigid temperatures that forecasters said could produce life-threatening conditions, with Donald Trump's inauguration expected to be the coldest in 40 years.