The fiercest snowstorm of the winter slammed the northeastern United States on Thursday, leaving a foot (30 cm) of snow in places, cancelling thousands of flights and shutting down schools. At least two deaths were blamed on the storm.

The storm, which came a day after temperatures had been a spring-like 50 to 60 degrees (10 to 16C), had wind gusts up to 50 miles per hour (80 kph) and left roads and sidewalks dangerously slick in densely populated cities such as New York, Boston and Hartford, Connecticut.

The storm's winds reached as far south as Virginia, where a truck driver died after his tractor-trailer was blown off the Chesapeake Bay Bridge-Tunnel, Tom Anderson, the facility's deputy director, said in a phone interview.

A New York City doorman died while shovelling snow as he slipped and fell down a flight of stairs, crashing into a window that cut his neck, police reported.

Some areas experienced "thunder snow," violent bursts of weather featuring both snow and lightning.

Nearly two-thirds of the flights into or out of the three major New York-area airports were cancelled, as were 69 percent of those at Boston Logan International Airport, according to Flightaware.com.

Nationwide, about 4,000 flights were cancelled and 5,700 delayed.

"The roads are dangerous," New York Governor Andrew Cuomo told reporters. "I don't care if you have a four-wheel-drive car and you think you're a super hero ... if you don't have to be out, don't be out."

David Hassan, 50, attested to the ugliness of the weather as he packed up his mobile coffee cart in New York's Times Square.

"I don't like coming out in this weather but I have three kids going to school and I have to work," Hassan said as he prepared for the two-hour trip back to his home in Parsippany, New Jersey.

New York received about a foot of snow, while Boston was braced for up to 20 inches.

Many schools systems were closed in the area, and Boston schools would remain closed on Friday, Mayor Marty Walsh said.

Many government offices also were shuttered with Massachusetts and Connecticut ordering non-emergency workers to stay home.

Blizzard warnings were in effect for the New York's eastern Long Island suburbs, southern Connecticut and Rhode Island, as well as the Massachusetts coast.

Temperatures were expected to fall to single-digit Fahrenheit levels overnight in the Boston area.