LAS VEGAS (AP) — A massive winter storm set the stage for a brutal travel day Sunday, with airlines warning of widespread cancellations and delays at some of the nation’s busiest airports.

Widespread snow, sleet and freezing rain threatened nearly 180 million people — more than half the U.S. population — in a path stretching from the southern Rocky Mountains to New England, the National Weather Service said Saturday night. After sweeping through the South, forecasters said the storm was expected to move into the Northeast, dumping about 1 to 2 feet (30 to 60 centimeters) of snow from Washington through New York and Boston.

More than 13,500 flights have been canceled across the U.S. since Saturday, according to flight-tracking site FlightAware. About 9,600 of those were scheduled for Sunday. Aviation analytics company Cirium says its data shows that Sunday will be the highest cancellation event since the pandemic, with over 29% of all U.S. departing flights axed.

Ronald Reagan Washington National Airport warned travelers on its website of widespread flight cancellations. Nearly all of its departing flights scheduled for the day — 414 flights, or 97% — have been canceled.

Significant disruptions were also expected at major airport hubs in Dallas-Fort Worth, Charlotte, Philadelphia and Atlanta, home to the nation’s busiest airport, as well as New York’s John F. Kennedy International Airport and LaGuardia Airport.

American Airlines had canceled over 1,400 flights for Sunday, according to FlightAware. Delta Air Lines and Southwest Airlines each reported about 1,000 cancellations for the day, while United Airlines had more than 800. JetBlue had more than 560 canceled flights, accounting for roughly 70% of its schedule for the day.

My flight was canceled, now what?

If you’re already at the airport, get in line to speak to a customer service representative. If you’re still at home or at your hotel, call or go online to connect to your airline’s reservations staff. Either way, it helps to also research alternate flights while you wait to talk to an agent.

Most airlines will rebook you on a later flight for no additional charge, but it depends on the availability of open seats.

Can I get booked on another airline?

You can, but airlines aren’t required to put you on another carrier’s flight. Some airlines, including most of the biggest carriers, say they can put you on a partner airline, but even then, it can be a hit or miss.

Am I owed a refund?

If your flight was canceled and you no longer want to take the trip, or you’ve found another way to get to your destination, the airline is legally required to refund your money — even if you bought a non-refundable ticket. It doesn’t matter why the flight was canceled.