A giant winter "bomb cyclone" walloped the US East Coast on Thursday with heavy snow and freezing cold that made for treacherous travel conditions and bone-chilling misery.
Four people were reported killed in the southeastern states of North and South Carolina, where icy roads sent vehicles skittering.
A cold wave gripping a large section of the United States had already been blamed for a dozen earlier deaths.
Thousands of flights were cancelled and schools were closed in many localities as snow piled up and blizzard conditions began taking hold in the northeast.
Temperatures were so low in northern New York that Niagara Falls -- the giant waterfalls straddling the US-Canadian border -- froze.
Relief in the form of warmer temperatures was not expected until next week, according to the National Weather Service, and the snow was not expected to taper off until late Friday as the storm moves northeastward into Canada.
Weather forecasters dubbed the event a "bomb cyclone" -- their nickname for a phenomenon known as "bombogenesis," in which a weather system experiences a sharp drop in atmospheric pressure and intensifies rapidly, unleashing hurricane-force winds.
Americans along the East Coast faced potential power outages in bitterly cold sub-freezing temperatures -- so far, some 24,000 customers in Virginia, 9,000 in Massachusetts, and thousands more in Georgia, South Carolina and Florida already affected.
The southeast was the first to feel the storm's icy lash -- Florida saw its first snow in nearly three decades.
North Carolina Governor Roy Cooper lamented the deaths of three people in his state, including two killed when their pickup truck slid off a bridge and landed on its roof in a creek bed.
Roads were closed in northern Florida and southeastern Georgia, where Governor Nathan Deal declared a state of emergency in coastal areas.
With up to a foot (30.5 centimeters) of snow expected in parts of New York, accompanied by powerful wind gusts, schools were closed. More than a foot of the white stuff was expected in Boston.
Mayor Bill de Blasio urged New Yorkers to stay off the roads and take the storm "very seriously."
The storm could bring "very dangerous conditions," he warned Wednesday.
Wind gusts of up to 55 miles (88.5 kilometers) per hour were expected through Friday on Long Island and southeastern Connecticut, with wind chills as low as minus 20 degrees Fahrenheit (minus 29 Celsius), increasing the risk of frostbite and hypothermia from prolonged exposure.
"Very anomalous and potent winter storm system currently developing as it moves northward towards New England bringing multiple hazards: moderate to heavy snow, low visibility, strong to damaging winds, coastal flooding, and hazardous seas," the National Weather Service said.
"Anticipate travel and economic impacts today and tonight with accumulating snow and white-out conditions. Expect downed trees and power lines resulting in scattered to widespread outages."
Airlines have scrapped more than 3,700 flights into and out of the United States so far due to the storm, and delayed 1,400 others, according to flight tracker FlightAware.
Nearly 75 percent of flights out of Boston and New Jersey's Newark airports were cancelled due to the adverse weather conditions, though only about a quarter of flights to and from New York's main John F. Kennedy airport were scrapped.
Air France announced it was cancelling all flights Thursday and Friday from Paris to New York and Boston.
Winter storm warnings were in effect from the Mid-Atlantic region northward through New England.
Virginia Governor Terry McAuliffe declared a state of emergency.
"Virginians living in the impacted areas should stay off the roadways to allow local and state road crews and first responders to do their jobs safely and efficiently," he said.
"With continued frigid temperatures expected to last for several more days, road conditions will remain treacherous beyond the expected end of the snowfall today," he added.
National rail line Amtrak cancelled service between Washington and Newport News, Virginia.
In Washington, despite only a dusting of snow, federal agencies opened two hours late, many schools were closed and Senate Majority Leader Mitch McConnell announced the chamber would hold no further votes this week, shortening what was already a short workweek due to the New Year's holiday.
Senators had only returned to the upper chamber Wednesday, and they face a series of critical votes in the coming weeks, including on funding to prevent a government shutdown and spending cuts.