Nearly 6,000 flights canceled as major blizzard hits Northeast

— -- Thousands of flights were preemptively canceled on Tuesday in anticipation of what could be the biggest blizzard of the year.

The storm is forecast to bring heavy snow and strong winds that could reduce visibility to a quarter mile or less for several hours, making travel “nearly impossible in many areas,” the National Weather Service warned in a statement late Monday.

As of early Tuesday, 5,401 flights into or out of the U.S. were cancelled, while an additional 87 were delayed, according to the Houston, TX-based aviation site FlightAware.

The three New York City area airports accounted for nearly half of the cancellations -- more than 2,600 in total for John F. Kennedy, LaGuardia, and Newark Liberty airports. Boston, Baltimore Philadelphia and Washington, D.C. airports also reported significant numbers of cancellations.

In New York City, above-ground MTA trains are scheduled to stop at 4 a.m. local time, while underground subways will continue to operate at a reduced pace.

New York City Mayor Bill de Blasio declared a state of emergency and warned people to stay off the roads.

The city plans to deploy 1,600 snow plows from the sanitation department, plus another 80 snow plows from Department of Transportation.

"Do not expect to see blacktop for quite a while," de Blasio said at a news conference late Monday.

The National Weather Service issued a blizzard warning through Tuesday evening for several areas in the Northeast -- including the New York City metro area -- from the New Jersey Shore to Connecticut.

A blizzard warning refers to sustained or frequent winds of 35 mph or higher.

Major cities spanning from Philadelphia to the Boston could see between 10 and 20 inches of snow.

ABC News' Lloyd Whitney, Melissa Griffin and Darren Reynolds contributed to this report.