By Scott Malone and Gina Cherelus
BOSTON/NEW YORK (Reuters) - The fiercest storm of the winter slammed the northeastern United States on Thursday, bringing snowfalls expected to be a foot (30 cm) deep, forcing cancellation of thousands of flights and closing schools.
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 Philadelphia.
Some areas saw periods of "thunder snow," a violent burst of weather featuring both snow and lightning, which could drop as much as 4 inches (10 cm) of snow per hour. Some coastal roads around Boston were closed as wind-driven waves washed over them.
All flights at New York's John F. Kennedy International Airport were halted on Thursday morning due to the storm's intensity. More than half of the flights into or out of the three major New York-area airports as well as Boston Logan International airport were canceled.
Nationwide, close to 3,200 flights were canceled, according to Flightaware.com.
"This is an unusually fast, intense storm," New York Mayor Bill de Blasio told reporters. "Don't drive your car today ... It's very tough conditions out there. Very hard to get around."
New York was due to see 10 to 14 inches (25-36 cm) of snow, with Boston braced for even more.
"Right now for the Boston metro area we're looking at 12 to 16 inches (30-41 cm) by the time everything is said and done," said Alan Dunham, a meteorologist with the National Weather Service in Taunton, Massachusetts, noting that the snowfall was expected to continue into the evening rush hour.
"Travel is going to be extremely dangerous. When it comes down at 2 to 3 inches per hour it's hard for the plows to keep up."
Many schools systems were closed in the area, including Boston, Philadelphia and New York City, the nation's largest with more than 1 million students.
Many government offices also were shuttered with the governors of Massachusetts and Connecticut ordering non-emergency workers to stay home. Maine's state legislature suspended operations for the day.
Blizzard warnings were in effect for the New York's Long Island suburbs, Cape Cod, Massachusetts and the island of Nantucket.
Temperatures were expected to fall to single-digit Fahrenheit levels (below -12.8 C) overnight in the Boston area.
(Reporting by Scott Malone; Editing by Larry King and Bill Trott)