Winter Storm Gia struck the Midwest region of the U.S. Friday night, freezing travel plans and prompting airlines to waive rebooking fees. The storm is moving east and is expected to persist throughout Sunday, with flight disruptions extending into the work week.

Some 350 flights were canceled nationwide and almost 950 had been delayed as of 1:40 p.m. EST, according to flight-tracking service FlightAware.com. The hardest-hit airport was St. Louis, where more than 115 combined arrivals and departures were canceled as more than 12 inches (30 centimeters) of snow fell in the area. Airports in Indianapolis and Kansas City, Missouri, also saw above-average cancellations as the storm tracked across the nation’s midsection.

The dangerous weather conditions added to travel misery as the Transportation Security Administration said it would begin closing a handful of security checkpoints at airports around the U.S. this weekend. TSA employees called in sick in higher numbers, a day after their first missed paycheck since parts of the federal government wound down on Dec. 22.

Gia is expected to bring additional snow to the Ohio Valley and Mid-Atlantic by Sunday evening, the Weather Channel reported. Flights at Chicago O’Hare, Chicago Midway, Washington Dulles, Washington Reagan National, Indianapolis and Cincinnati could be affected.

Delta Air Lines Inc., United Continental Holdings Inc., Southwest Airlines Co. and Frontier Airlines have all have waived rebooking fees for some airports because of the storm.

Rash of Crashes

The Weather Channel reported that nearly 500 weather-related motor vehicle accidents had been reported in Missouri. Hundreds of people were stranded for hours as snow and freezing rain rendered parts of Interstate 44 in Missouri nearly impassable on Friday evening. That motorway and I-70 were reopened Saturday morning.

“Our crews remain out and are making good progress but as long as it’s snowing the roads won’t be totally clear,” the Missouri Department of Transportation said on its St. Louis area Twitter feed.

AccuWeather called the storm the largest in the St. Louis area since 2014. “For many areas, this will be a long-duration winter storm that lasts more than 12 hours and perhaps as long as 24 hours in some cases,” said AccuWeather senior meteorologist Alex Sosnowski.

Sporadic power outages also resulted, the weather service said, the result of heavy, wet snow that caused some trees to snap and bring down power lines.