Flight cancellations are continuing to mount today as the U.S. is being hit with dangerous winter weather from a storm moving across the country.

Airlines have canceled more than 9,900 U.S. flights for Sunday, according to the flight-tracking site FlightAware. That’s the most for a single day since the start of the COVID-19 pandemic and more the double the number of U.S. flights that were canceled for Saturday. 

The National Weather Service says the storm is bringing widespread heavy snow, sleet and freezing rain from the Southern Rockies to New England through Monday. 

“Extremely cold air will follow, prolonging dangerous travel and infrastructure impacts into next week,” it said on Sunday. “Severe thunderstorms may produce damaging gusts and tornadoes across the eastern Gulf Coast states Sunday morning and afternoon.”

More than 1,800 U.S. flights have already been canceled for Monday. 

Ronald Reagan Washington National Airport advised travelers that snow began at midnight and said no flights can come or go while its team clears snow from the airfield. It warned of widespread cancellations Sunday. 

Even before the winter storm arrived in North Texas, flight cancellations at both major airports there piled up. Dallas Fort Worth international Airport said airlines were expected to operate a reduced number of flights Sunday and encouraged passengers to check their flight status with their airline as schedules could change. 

“DFW’s teams are actively treating roads, bridges and airfield surfaces to maintain safe operations,” it said in a statement.

LaGuardia Airport in New York said on its website that “significant travel impacts” from the storm were expected. 

Airports in Atlanta, Charlotte and Philadelphia were also expecting disruptions. 

Flight tracking service Flightradar24 said American was the most impacted airline this weekend, followed by United and Delta. 

Major U.S. airlines were issuing travel waivers, allowing customers to change their plans without penalty.