The storm -- with about 80 million people in its path, including more than 22 million people under a coastal flood warning -- officially "bombed out" late Friday morning when it underwent bombogenesis, signaled by an extreme drop in atmospheric pressure.
"Since 10 a.m., Thursday until now, this storm has rapidly dropped in pressure, officially passing the definition of 24 millibars in 24 hours," CNN meteorologist Judson Jones said at 11 a.m. ET Friday. "It continues to strengthen."
At least two people have died, according to authorities: a 77-year-old woman in Kingsville, Maryland, and an 11-year-old boy in Putnam County, Virginia. Both were killed by falling trees.
Massachusetts Gov. Charlie Baker said he's called in the National Guard and urged vigilance.
"Do not ride out the storm if you are told to evacuate," Baker said.
The Massachusetts Emergency Management Agency predicted the storm will affect the state through Saturday.
Utility companies said 500,000 customers were without power throughout the District of Columbia, Maryland, New Jersey, New York and Massachusetts. MEMA said more than 160,000 power outages had been reported in Massachusetts by 5 p.m. ET.
"This is one of the more extreme storms in recent memory in terms of coastal flooding and damaging wind potential along the coast, but also includes heavy wet snow and heavy rain as important factors," the agency said.
The blustery, wet barrage by midday Friday was packing wind gusts exceeding 50 miles per hour and knocking out power, flooding neighborhoods and walloping homes up and down the Atlantic coast.
"We are already experiencing downed trees and limbs," Brockton, Massachusetts Mayor Bill Carpenter said mid-morning Friday on Twitter. "Be prepared for power outages, drive with care."
More than 3,000 flights canceled
More than 3,100 US flights have been canceled, including hundreds at the busiest Northeast airports in Boston, Philadelphia and New York, the flight-tracking website FlightAware.com posted Friday.
Airlines, including Delta, United and Southwest, are offering fee waivers for flight cancellations or changes involving affected airports. Airports on the East Coast have been experiencing delays and ground stops, the Federal Aviation Administration said.
Amtrak also announced it had suspended train service between New York and Boston.
Some people living in the affected region skipped work Friday to hunker down.
In the Washington area, federal offices didn't open Friday, schools closed and trash collection was suspended. The wind even directed its wrath at the Capitol, blowing out a window on the west front over the Olmstead Terrace, an official from the Architect of the Capitol office said.
As the high tide advanced in New England, authorities warned coastal residents to be ready to evacuate.
Michelle Shaffer, 45, of Hull, Massachusetts evacuated Thursday night to higher ground, she told CNN affiliate WCVB. Shaffer lost her appliances in the last storm and expected another pounding.
"This storm is going to be worse," she told WCVB, "because it's going over three high tides."
New York Gov. Andrew Cuomo said aptly that "March is coming in like a lion."
Heavy snow and strong winds already had caused power outages upstate, Cuomo said late Friday morning. He issued a travel advisory for all areas north of New York City "so plows, utility crews and first responders are able to do their jobs."
"If you do not have to travel today," he said, "I encourage you to stay off the roads."
Here's what else to expect:
With the moon full, the tide is at its highest point of the month, and the storm surge could drive as much as 4 feet of water into coastal neighborhoods, CNN meteorologists said. Massachusetts emergency officials said tides "will be astronomically high" in the next few days.
Boston Harbor, which has only seen tides above 15 feet twice -- in 1978 and in January, during the last bomb cyclone -- is expected to see the tide at that level three times during this storm, Jones said.
"I encourage all residents to be mindful of the storm and encourage employers to take the weather into consideration, which will mostly impact the coastal areas of our city," Boston Mayor Marty Walsh said.
The storm is moving slowly, and the wind, rain and flood effects will linger for days. Parts of eastern Long Island and eastern Massachusetts could get up to 5 inches of rain through Saturday. The rest of the region could see 2 to 4 inches of rain.
Winds will be so ferocious that the Port Authority of New York and New Jersey is securing cranes at a LaGuardia Airport construction project. It's also making plans to use buses if trains at JFK or Newark airports must stop because of wind.
In Boston, high wind warnings are in effect from Friday to Saturday morning, when winds could whip up to 65 mph. New Jersey officials say high winds may affect trees and power lines.
In Washington, wind gusts exceeding 50 and 60 mph have been reported at airports. Officials in the nation's capital activated a hypothermia alert and urged homeless residents to take shelter.
"Strong gusty winds will continue with sustained winds of 20 mph to 30 mph and gusts 40 mph to 50 mph possible," Cuomo, the governor, said.
Barges carrying construction materials broke loose from their moorings near the Tappan Zee Bridge, which crosses the Hudson River, said Laura Ware, public involvement manager for Tappan Zee Constructors LLC.
"Barges associated with the project are equipped with GPS tracking devices and immediately transmitted notifications to our management team when they traveled beyond project limits," she said. "TZC tug boats were immediately dispatched to recover the barges."
Strong winds will affect states from South Carolina to Maine.
Hurricane-force wind gusts and flooding pose the biggest dangers along the coast, Jones said.
Any snow that falls will be heavy and wet, likely to bring down trees and power lines, and cause power outages.
Major cities in the Northeast, such as Philadelphia and New York, may get a wintry mix of snow, rain and sleet, with no snow accumulation forecast.
Inland New England is more likely to get snow, with a foot possible in upstate New York and western Massachusetts. Some areas around Buffalo in western New York have reported 17 inches already, and locations in upstate New York could approach 2 feet.
"Heavy, wet snow fell overnight and snow will continue into the evening hours tonight," Cuomo said in a statement, describing conditions statewide. "Snow will turn dryer and lighter and combined with strong winds, will cause dangerous conditions making driving extremely hazardous from blowing and drifting snow."