NEW YORK (CBSNewYork/AP) — Thousands of flights were canceled at major airports across the country Thursday as a massive winter storm socked the Northeast.

Tracking service FlightAware.com said more than 4,000 flights were canceled within, into or out of the United States.

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Port Authority grounded flights at John F. Kennedy International Airport until 7 a.m. Friday.

A Singapore Airlines flight bound for JFK was diverted to Stewart Airport in Orange County instead, because of the weather.

Many JetBlue passengers were stuck on the tarmac for about three hours Thursday morning.

“Towards the end, it started getting impatient, but you know they were grateful that we got here safe,” traveler Luis Figueroa said.

JetBlue apologized, saying there were no gates available and it was unsafe to let passengers out on the tarmac.

Earlier in the day, the arrivals and departures board was all red at a nearly abandoned Delta Terminal at LaGuardia Airport.

“From 8 o’clock they said there was a 1, and then there was a 3 and then there was a 6. They said the 6 will be cancelled so now I have to go home,” said Maria, who was heading back home to the city.

“The flight was delayed, we sat on the tarmac for about 2 hours, then pushed back to 6 p.m., and then they just announced that the flight was canceled all together,” said a traveler named Dante. 

At Newark Liberty International Airport, more than 850 flights were cancelled, about 73 percent of normal flight activity.

“All three airports are at their highest level of staffing and at their highest level of snow alert,” Port Authority of New York & New Jersey Executive Director Rick Cotton said. “There are thousands of tons of salt and sand stockpiled at each of the airports.”

Officials are expecting a lot of people at LaGuardia on Friday and urge people to use mass transit, saying the LaGuardia Link Q70 bus will be free.

Travelers are advised to call their carrier before heading to the airport.

Most airlines have been letting customers change reservations without incurring a fee, usually up to $200 on U.S. flights.

(© Copyright 2018 CBS Broadcasting Inc. All Rights Reserved. The Associated Press contributed to this report.)