(CNN) -- This is the definition of rare air.

The National Weather Service warned Sunday that much of the United States will see this week the coldest temperatures in almost 20 years as an arctic cold front descends on 140 million people

"A piece of a polar vortex, the coldest air in the Northern Hemisphere ... is coming," CNN Meteorologist Alexandra Steele said.

And in addition to the cold, there will be heavy snow in the eastern Plains and Great Lakes, as much as a foot Sunday.

Temperatures will be 30 to 50 degrees below average Sunday from the Plains to the Upper Mississippi Valley, forecasters said.

By Wednesday, nearly half the nation will shudder in temperatures of zero or lower, forecasters said. Even the Deep South will endure single-digit or sub-zero temperatures.

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