(CNN) — More than 1,500 flights were canceled in the United States on Wednesday as a winter storm hit the country.

As of 6 p.m. ET Wednesday, a total of 1,548 flights in and out of the US had been canceled, according to flight tracking site FlightAware. Regional carrier SkyWest, which operates through partnerships with United, Delta, American and Alaska Airlines, had canceled more than 350 flights. Delta and Southwest had both canceled more than 250 flights.

Minneapolis−Saint Paul International Airport, Denver International Airport and Detroit Metropolitan Wayne County Airport were the most heavily affected, according to FlightAware. Toronto Pearson International Airport in Canada, Chicago O’Hare International Airport and Milwaukee Mitchell International Airport were also experiencing disruption.

Nearly 400 Thursday flights had already been canceled by early Wednesday evening.

Southwest Airlines has issued winter weather waivers for about a dozen airports.

Delta Air Lines has issued waivers for Upper Midwest winter weather and Rockies and Mountain regions winter weather.

American Airlines and United have also issued winter weather waivers for travel this week.

The flight disruption comes after winter weather alerts were issued across 29 states, including California, Minnesota and Maine, with warnings of severe icing, extreme cold and sleet as part of the three-day storm.

The Minneapolis area of Minnesota is at risk of picking up at least 15 inches of snow, with the National Weather Service in Minnesota’s Twin Cities warning the powerful storm “will bring widespread accumulating snow, with blowing and drifting snow mainly Wednesday through Thursday.”

Correction: A previous version of this story misidentified Delta Air Lines.