(Adds snowfall totals, wind gusts, falling temperatures and power outages)
By Jonathan Allen and Scott Malone
NEW YORK/BOSTON, March 14 (Reuters) - A late-season snowstorm swept the mid-Atlantic and northeastern United States on Tuesday, closing public school systems from Washington, D.C., to Boston, grounding thousands of airline flights and knocking out electricity to 200,000 customers.
Tens of millions of residents from Maryland to Maine faced a "rapidly intensifying Nor'easter" that was rare for its arrival in mid-March, just a week before the official end of winter, according to the National Weather Service (NWS).
The storm also capped an unusually mild winter that saw otherwise below-normal snowfalls for much of the Atlantic Coast.
Many residents heeded official advice to stay home, as temperatures plunged 10 to 25 degrees below average across most of the eastern third of the country. Snow fell from the lower Great Lakes and central Appalachians to the eastern seaboard as far south as North Carolina.
The heaviest snow, with accumulations of a foot (30 cm) or more, was reported across New England, upstate New York and parts of Pennsylvania. Gale-force wind gusts also buffeted much of the region, creating blizzard conditions.
By comparison, the nation's capital received just a few inches of snow by late afternoon, enough to delay opening of federal government offices for three hours. The storm's greater impact for Washington was perhaps that the city's celebrated cherry blossoms, a tourist attraction and an early harbinger of spring, were encased in ice.
Another rite of March, the national college basketball championship tournament, was disrupted as Tuesday's scheduled game between Syracuse University and the University of North Carolina at Greensboro was postponed for one night.
Hundreds of thousands of public school students got a day off, as classes were canceled for the day in New York City, Philadelphia, northern New Jersey and Boston. Schools were to be closed again in Boston on Wednesday.
Still, the weather service dialed back forecasts for some urban areas, notably New York City, where residents had been warned to steel themselves for potentially record-breaking snow.
Only 4 inches (10 cm) fell in Manhattan's Central Park - less than forecast. By afternoon, as snow turned to sleet, city officials were anticipating Wednesday's morning rush hour would be largely back to normal and that schools would reopen.
After being canceled earlier in the day, above-ground parts of New York City's subway service and some Metro-North commuter trains to the suburbs resumed in the evening.
Train service to Boston and Albany, New York, was halted. Connecticut officials said roads there would reopen to general traffic on Tuesday evening.
Governors in New York, New Jersey, Pennsylvania and Virginia declared states of emergency at the outset of the storm.
"Mother Nature is an unpredictable lady sometimes," the state's governor, Andrew Cuomo, said at a news conference. "She was unpredictable today."
While children and dogs took to the streets to play in the snow, many New Yorkers welcomed the storm as a respite from the usual bustle of daily life.
"It's a ghost town," Ali Naji, 33, said as he sat listening to Mexican pop music amid the emptiness of his usually busy convenience store in Brooklyn's Fort Greene neighborhood.
More than 6,000 commercial airline flights across the United States were canceled for the day, according to tracking service FlightAware.com, including all of American Airlines flights into New York's three major airports - Newark, LaGuardia and John F. Kennedy International Airport.
JetBlue Airways reported extensive cancellations and Delta Air Lines canceled 800 flights for New York, Boston and other northeastern airports. United Airlines said it was halting all operations at Newark or LaGuardia.
Utility companies likewise reported widespread power outages, affecting more than 220,000 homes and businesses at the peak of the storm.
The young at heart seemed to take the weather in stride. At the open-sided Thomas Jefferson Memorial in Washington, the ice-slick marble floor served as a skating rink for some of the 71 eighth graders visiting from St. Mary's Academy in Englewood, Colorado.
Math teacher Michael Pattison, 65, rattled off a list of all the monuments and museums the students would see that day.
"This weather is not going to stop us," he said, clapping his gloved hands.
"No, it's not," a couple of students shouted back.
(Additional reporting by Laila Kearney and Gina Cherulus in New York, Ian Simpson in Washington and Valerie Vande Panne in Boston; Writing by Steve Gorman and Daniel Wallis; Editing by Bill Trott and Cynthia Osterman)