Big changes are on the way for parts of the Western and Central states late this week and into this weekend.
Similar to the days prior to Thanksgiving, the worst weather will focus on the days prior to Christmas as millions of travelers take to the roads and airways.
Despite a brief warmup in Chicago, wintry conditions will span the weekend.
Warm air is forecast to surge into much of the eastern half of the nation by the weekend and will be accompanied by heavy rain and flooding risk in some locations.
Thunderstorms in parts of the South this weekend may become strong enough to threaten lives and property.
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Riverside RS (Yellowstone Park), WY: -59 degrees, lowest U.S. temp in December.
Arctic blast causes temperatures to plunge to 20 to 30 below zero.
Philadelphia, PA (1991)
High of 30 degrees; only 5th day in 1991 with a high below freezing.