A giant winter "bomb cyclone" walloped the US East Coast on Thursday with freezing cold and heavy snow, forcing thousands of flight cancellations and widespread school closures -- and even prompting the US Senate to cancel votes for the rest of the week.
Millions of Americans faced potential power outages in bitterly cold sub-freezing temperatures, with some 45,000 people in Virginia and thousands more in Georgia, South Carolina and Florida already affected.
Relief in the form of warmer temperatures was not expected until next week, according to the National Weather Service, and the snow was expected to only taper off by late Friday as the storm moves northeastward into Canada.
Roads were closed in northern Florida and southeastern Georgia, where Governor Nathan Deal declared a state of emergency in coastal areas.
Florida's Governor Rick Scott urged people there "to prepare for extreme cold weather conditions, including potential snow, sleet or ice accumulations."
With up to a foot (30.5 centimeters) of snow expected in New York, accompanied by powerful windgusts, 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.
Blowing and drifting snow reduced visibility, and downed trees and power lines were likely.
"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 in a forecast discussion.
"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 3,000 flights so far due to the storm.
Nearly 75 percent of flights out of Boston and New Jersey's Newark airports were cancelled due to the snow and other adverse weather conditions, according to flight tracker FlightAware, though only about a quarter of flights to and from New York's main John F Kennedy airport were scrapped.
The storm owes its moniker to a process known as bombogenesis, in which a weather system experiences a sharp drop in atmospheric pressure and intensifies rapidly, with blizzard-like conditions combined with hurricane-force winds.
About 12 deaths have already been blamed on the frigid temperatures this week.
Winter storm warnings were in effect from the Mid- Atlantic region northward through New England, meaning that hazardous conditions were imminent.
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, federal agencies opened two hours late and Senate Majority Leader Mitch McConnell announced the chamber would hold no further votes this week, shortening what was already a short work week 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.