Airlines canceled more than 1,200 U.S. flights on Friday ahead of a major winter storm that will put carriers to the test during one of the busiest travel periods of the year.

A winter storm warning is in effect starting Friday afternoon in New York City, New Jersey and Long Island, with snowfall totals potentially reaching 9 inches, most of it falling overnight, the National Weather Service said.

Over 350 flights, or more than a quarter of the day’s schedule, were canceled as of 1 p.m. Friday to and from New York’s John F. Kennedy International Airport, according to flight-tracking site FlightAware. More than 200 were also scrubbed at Newark Liberty International Airport in New Jersey, and more than 100 were canceled at Philadelphia International Airport.

American Airlines, Delta Air Lines, United Airlines, Southwest Airlines, JetBlue Airways and other carriers waived change fees for restrictive basic economy tickets and said they won’t charge a difference in fare for any other customers flying in and out of a host of airports in the Northeast U.S.

Customers must travel by the end of the year if they change their flights, the airlines said. Flying as early as possible is likely the best bet with few seats available during the busy Christmas week.

Airlines for America, the industry lobbying group, expects carriers to fly a record 52.6 million people between Dec. 19 and Jan. 5, with this Friday and Sunday among the busiest days.

Airlines generally cancel flights ahead of time for major weather events in the forecast, like blizzards or hurricanes, to avoid planes, connecting travelers and crews from getting stranded and worsening disruptions.