Polar vortex eases grip on frozen US

The United States is warming up from the polar vortex.

As it continued its northward retreat, the arctic blast still held sway over the upper Midwest, where it bore down with below-zero lows early Friday.

This should be its last stand in the United States, as temperatures bump up day by day, the National Weather Service said.

However, the bone-chilling cold, snow and ice that gripped much of the country -- affecting about 200 million people -- brought about the biggest economic disruption delivered by the weather since superstorm Sandy in 2012.

The crippling cold has left at least 21 people dead, including seven in Illinois and six in Indiana.

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A woman bundles up against the cold in New York City

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A commuter ferry navigates through the ice flow in the Hudson River between New Jersey and lower Manhattan in New York

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The frozen US side of the Niagara Falls is pictured in Ontario

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Kids climb into a car to be driven to school in sub-zero temperatures in Minneapolis. Schools in Minneapolis reopened after being closed for two days due to dangerously low temperatures and wind chills

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A snow covered scooter is seen in Chicago, Illinois

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Aldo, a pet dog, leaps through the snow as he runs after his owner, Cyril River, who skates on a Lake of the Isles in Minneapolis

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Ice flows jam the Delaware River in Trenton, New Jersey

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A local police car blocks an intersection where water and ice form from a water main break in Silver Spring, Maryland

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