A man walks in the snow after a winter storm arrived in Washington January 22, 2016. [Photo/Agencies]

The storm developed along the Gulf Coast, dropping snow over Arkansas, Tennessee and Kentucky on Friday. On the East Coast, warm, moist air from the Atlantic Ocean collided with cold air to form the massive winter system, Pereira said.

"This has life-and-death implications and all the residents of the District of Columbia should treat it that way," Washington Mayor Muriel Bowser said.

The storm was forecast to move offshore in southern New England early next week. Philadelphia and New York were expected to get 12-18 inches of snow before the storm abated.

Low-lying areas of New York and New Jersey - some still rebuilding after Superstorm Sandy in 2012 - might see flooding during high tides on Saturday and Sunday, officials said.

More than 7,200 flights were canceled within, into or out of the United States on Friday and Saturday, according to flight tracking website FlightAware.com. Amtrak modified service on train routes along its busy Northeast Corridor.

The Washington Metropolitan Area Transit Authority, which includes the country's second-busiest subway system, took the rare step of suspending operations from late Friday through Sunday.

New Jersey Transit shut all bus, rail and light rail service early on Saturday and said service would be restored as "conditions permit."