Airlines have canceled more than 5,000 flights for Thursday and preemptively nixed more than 3,200 for Friday as extreme winter weather sweeps the nation right before Christmas, according to tracking website FlightAware, spelling trouble for people trying to rush home for the holidays. Airports in Chicago and Denver have been hit hardest, with around 25 percent of the two hubs’ flights canceled as of Thursday. Temperatures across the Midwest are expected to plummet below zero as more than a foot of snow could hit in Chicago and its surrounding areas. Several airlines have issued weather waivers, giving travelers some flexibility as they navigate canceled and delayed flights going into the holiday weekend.