The Brooklyn Bridge in the early hours of the major winter storm. (Photo by Spencer Platt/Getty Images)

As a blizzard bears down on the Northeast, thousands of flights have been canceled and transit agencies have begun limiting or halting service.

There were more than 4,400 flight cancellations on Friday and Saturday, according to FlightAware.

More than 1,400 of the flights canceled for Friday were bound for just five destinations: New York’s three primary airports (Newark Liberty International, John F. Kennedy International and LaGuardia), Logan Airport in Boston and Philadelphia International Airport. And more than 1,000 flights scheduled to depart from those five airports on Friday were also canceled.

Airlines had begun to cancel flights and issue waivers earlier this week. The major airlines posted lists of cities where travelers could reschedule flights without any extra charge or, for some airlines, re-book without paying for the fare difference.

Officials at the Washington region’s three airports urged anyone with travel plans to check with their airline on the latest cancellations or delays.

Amtrak also announced plans to reduce service in the Northeast as the storm loomed. The rail agency is halting all service between New York and Boston on Friday afternoon. The last southbound train from Boston was scheduled to depart at 1:40 p.m., while the last northbound train leaving New York’s Penn Station was called for 1:03 p.m.

But Northeast Corridor service south of New York was not expected to be affected, according to Amtrak.

[Update on Friday at 4:22 p.m.: Amtrak announced on Friday afternoon that no Acela trains would run between New York and Boston on Saturday. Northeast Regional trains would run limited service between the two cities, with the first southbound train leaving Boston at 11:40 a.m. and the first northbound train departing New York’s Penn Station at 10 a.m. Again, Northeast Corridor service south of New York is not expected to be affected.]

Cities projected to see the heaviest snowfall were also preparing for limited or halted transit. The Massachusetts Bay Transportation Authority announced that it would suspend all service on Friday at 3:30 p.m., shutting down subways, commuter rail lines and buses in Boston and the surrounding area.

In New York City, the Metropolitan Transportation Authority warned travelers that subway and bus service could be limited or suspended because of the weather. Planned weekend work on the system was canceled, the MTA announced.

Similarly, the Southeastern Pennsylvania Transportation Authority decided to cancel construction that would have interrupted train service on one of its rail lines in and out of Philadelphia.

The Greater Hartford Transit District warned its riders that service could be shut down by Friday at 6 p.m., while the Rhode Island Public Transit Authority announced plans to suspend service on Friday at noon.

Visit the Capital Weather Gang for the latest news on this historic blizzard as well as local forecasts.

[This post has been updated. Most recent update: 4:22 p.m.]