Most airlines were giving up on flying in and out of New York, Boston and other cities in the Northeast Friday as a massive storm threatened to dump snow by the foot on the region.

Airlines were generally shutting down operations in the afternoon at the three big New York-area airports as well as Boston, Providence, Portland, Maine, and other Northeastern airports. They're hoping to resume flights on Saturday.

Flight-tracking website FlightAware said airlines have canceled more than 4,200 flights on Friday and Saturday in advance of the storm.

Many travelers were steering clear of that part of the country altogether. Airlines waived the usual fees to change tickets for flights in the affected areas.

Airlines try to get ahead of big storms by canceling flights in advance rather than crossing their fingers that they can operate in bad weather. They want to avoid having crews and planes stuck in one area of the country. They also face fines for leaving passengers stuck on a plane for more than three hours under a rule that went into effect back in 2010.

Airlines began canceling Saturday flights on Friday.

"That's when the meteorologists start to have reliable predictions and the FAA holds conference calls to discuss which airports are shutting down," said Daniel Baker, CEO of FlightAware.

Airlines are also at the mercy of mass transit in each city they fly to. They'll often wrap up flying around the same time that commuter trains and subways begin shutting down, he said.

Storms like this one jam up airline call centers, so airlines are increasingly automating the process of re-booking passengers.

Delta is rolling out software it calls "VIPER" — Virtual Inconvenienced Passenger Expedited Reprotection — to find a replacement flight for passengers whose flights have been canceled.

As any frequent traveler knows, during a bad storm, the fastest route from, say, New York to Minneapolis may be through Atlanta, or Salt Lake City. Airline workers are adept at finding such routes manually. The new Delta system looks for such "creative routings" automatically and sends a message to the traveler telling them about their new flight, Delta Air Lines Inc. CEO Richard Anderson said on an employee hotline message last week.

"We need the ability to use automation to figure out for our passengers the quickest and fastest way to (re-accommodate) them on Delta or other carriers," he said. "Our goal is to get them to their destination as promptly as possible."

More than any other airline, JetBlue Airways Corp.'s route network is centered around the East Coast. Its meteorologist gives JetBlue executives a rolling seven-day forecast, and by Friday it was canceling flights. It scrubbed more than 400 flights scheduled for Friday and Saturday.

Waiting too long to cancel flights means "customers are headed for the airport, they're in their cars, they get to the airport and if your flight's canceled that's when bad things start to happen from a customer standpoint," said Rob Maruster, Jet Blue's chief operating officer.

The snow was snarling air travel in Canada, too, with 240 flights canceled on Friday at Pearson International Airport in Toronto. Environment Canada senior climatologist David Phillips said Toronto hasn't seen a snowfall exceeding 5 inches since Dec. 19, 2008. The current storm was expected to dump up to 11 inches of snow as it moves along.