(CNN) — About a thousand flights were canceled in the United States on Wednesday morning as a winter storm hit the country.

As of about 8:40 a.m. ET Wednesday, a total of 1,004 flights in and out of the US had been canceled. Southwest Airlines has canceled more than 230 flights so far. Delta Air Lines and SkyWest have both canceled about 200 flights, according to FlightAware.

Minneapolis−Saint Paul International Airport and Denver International Airport were the most heavily affected, according to early data from flight tracking site FlightAware. Chicago O’Hare International Airport, Detroit Metropolitan Wayne County Airport and Toronto Pearson International Airport were also experiencing disruption.

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

Delta Air Lines has issued waiver 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.”

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