The impending winter storm is set to begin across the south-central U.S. on Friday.

This index, from NOAA's National Weather Service, attempts to scale the impacts from winter weather, taking into account factors such as snowfall, accumulated snow on rooftops, ice accumulation, the potential for a flash freeze, and blowing snow. Not all factors are in play in a given winter storm.

Track the winter storm as it's forecasted to bring heavy snow and extremely cold weather to the U.S., reaching millions from the southern U.S. to the Northeast.