More than 1,200 U.S.-based flights were canceled on Thursday as an arctic blast with cold, heavy snow and wind swept toward the Midwest, according to FlightAware.

Millions of people in the Upper Midwest and the Plains have been placed under blizzard and winter weather warnings as the weather system moves into the continental U.S. before heading east just days ahead of Christmas.

Most major airlines have announced waivers to allow passengers traveling through affected cities to rebook their flights for an upcoming date once the storm passes.

“We had a great Thanksgiving week with minimal disruptions,” Transportation Secretary Pete Buttigieg said on MSNBC’s “Morning Joe” on Wednesday. 

“Unfortunately, it’s not going to be that way heading into Christmas,” Buttigieg continued. “We have not just a large portion of the country affected by these winter storms, but that includes some of the key hubs for many of our airlines.”

As of 8:15 a.m., at Chicago Midway International Airport, 25 percent of Thursday flights were canceled, the highest percentage of any U.S. airport, according to FlightAware.