BOSTON — A storm that forecasters warned could be a blizzard for the history books, with a potential for up to 3 feet of snow, clobbered the New York-to-Boston corridor on Friday, grounding flights, sending office workers home early and knocking out power to half a million customers across the Northeast.

By Friday night, more than 18.5 inches of snow had fallen in parts of central Connecticut, and more than 16 inches covered parts of Mansfield, Mass., a half-hour drive southwest of Boston. Throughout the Northeast, more than 500,000 homes and businesses lost electricity as wet, heavy snow, freezing rain and howling winds caused havoc.

Earlier, as meteorologists warned of the impending blizzard conditions, shoppers from New Jersey to Maine crowded into supermarkets and hardware stores to buy food, snow shovels, flashlights and generators, something that became a precious commodity after Superstorm Sandy in October. Others gassed up their cars, another lesson learned all too well after Sandy. Across much of New England, schools closed well ahead of the first snowflakes.

"This is a storm of major proportions," Boston Mayor Thomas Menino said. "Stay off the roads. Stay home."

The wind-whipped snowstorm mercifully arrived at the start of a weekend, which meant fewer cars on the road and extra time for sanitation crews to clear the mess before commuters in the New York-to-Boston region of roughly 25 million people have to go back to work. But it could also mean a weekend cooped up indoors.

Rainy Neves, a mother of two in Cambridge, just west of Boston, did some last-minute shopping at a grocery store, filling her cart to the brim.

"Honestly, a lot of junk — a lot of quick things you can make just in case lights go out, a lot of snacks to keep the kids busy while they'd be inside during the storm, things to sip with my friends, things for movies," she said. "Just a whole bunch of things to keep us entertained."

In heavily Catholic Boston, the archdiocese urged parishioners to be prudent about attending Sunday Mass and reminded them that, under church law, the obligation "does not apply when there is grave difficulty in fulfilling this obligation."

Halfway through what had been a mild winter across the Northeast, blizzard warnings were posted from parts of New Jersey to Maine. The National Weather Service said Boston could get close to 3 feet of snow by Saturday evening, while most of Rhode Island could receive more than 2 feet, most of it falling overnight Friday into Saturday. Connecticut was bracing for 2 feet, and New York City was expecting as much as 14 inches. East of New York City, nearly a foot of snow had fallen before midnight Friday.

Early snowfall was blamed for a 19-car pileup in Cumberland, Maine, that caused minor injuries. Most of the region's power outages were in Massachusetts and Rhode Island.

Forecasters said wind gusts up to 75 mph could cause more widespread power outages and whip the snow into fearsome drifts. Flooding was expected along coastal areas still recovering from Superstorm Sandy, which hit New York and New Jersey the hardest and is considered Jersey's worst natural disaster.

Meteorologist Jeff Masters, of Weather Underground, said the winter storm was a collision of two storms and may end up among the Boston area's Top 5 most intense ever.

"When you add two respectable storms together, you're going to get a knockout punch with this one," he said.

It could break Boston's all-time snowstorm record of 27.6 inches, set in 2003, forecasters said. The storm also comes almost 35 years to the day after the Blizzard of '78, a ferocious storm that dropped 27 inches of snow, packed hurricane-force winds and claimed dozens of lives.

Masters said the region could get a break from warmer air trailing behind that is expected to push temperature up to the 40s by Monday.

"It's going to be not that difficult to dig out, compared to maybe some other nor'easters in the past, where it stayed cold after the storm went through," he said.

Drivers were urged to stay off the streets lest their cars get stuck, preventing snowplows and emergency vehicles from getting through. New York City ran extra commuter trains to help people get home before the brunt of the storm hit.

Amtrak stopped running trains in cities around the Northeast on Friday afternoon. Airlines canceled more than 4,300 flights through Saturday, and New York City's three major airports and Boston's Logan Airport shut down.

Interstate 95 was closed to all but essential traffic in Rhode Island, where the governor said power outages remained the biggest threat.

"With tree branches laden with heavy, wet snow, the winds picking up and the temperatures plunging all at the same time, it's a bad combination," Gov. Lincoln Chafee said.

In Massachusetts, Gov. Deval Patrick enacted a statewide driving ban for the first time since the Blizzard of '78. Hours before the ban went into effect at 4 p.m., long lines formed at gas stations, some of which were almost out of fuel.

James Stone said he was saving the remaining regular gas at his station in Abington, south of Boston, for snowplow drivers.

"It hasn't snowed like this in two years," Stone said. "Most people are caught way off-guard."

In New York, Fashion Week, a series of designer showings with some activities held under tents, went on mostly as scheduled, though organizers put on additional crews to deal with the snow and ice, turned up the heat and fortified the tents. The snow did require some wardrobe changes: Designer Michael Kors was forced to arrive at the Project Runway show in Uggs.

For Joe DeMartino, of Fairfield, Conn., being overprepared was impossible: His wife was expecting their first baby Sunday. He stocked up on gas and food, got firewood ready and was installing a baby seat in the car. The couple also packed for the hospital.

"They say that things should clear up by Sunday. We're hoping that they're right," he said.

Said his wife, Michelle: "It adds an element of excitement."