WASHINGTON (AFP) -- An “extremely dangerous” blizzard expected to dump record amounts of snow pounded the eastern United States Saturday, closing down the U.S. capital and threatening to trap millions indoors for days.

The National Weather Service (NWS) put the Washington-Baltimore metropolitan area under a rare 24-hour blizzard warning starting at 10:00 pm Friday.
Forecasters issued a blunt warning: help area emergency workers “by staying off the roads.”
The NWS forecast up to 30 inches (76 centimeters) of snow in the capital region, which would shatter Washington's 88-year-old record snowfall of 28 inches, in the “Great Knickerbocker Storm.”
That blizzard, which slammed the region in January 1922, got its name from the collapse of the Knickerbocker Theatre in Washington, killing nearly 100 people.
The blizzard warning means “severe winter weather conditions,” which includes “falling and blowing snow with strong winds... that will lead to whiteout conditions,” the NWS said.
President Barack Obama, who lived in snowy Chicago for years, mocked Washington-area schools last year for closing due to a few inches of snow and ice.
“Folks in Washington don't seem to be able to handle things when it comes to the weather,” the newly-sworn in president said at the time.
On Friday he had changed his tune.
“Even a transplanted Hawaiian to Chicago has sufficient respect for a forecast of up to two feet of snow,” said White House press secretary Robert Gibbs.
Washington's two main airports, Reagan National and Dulles International, had limited flights Friday. All flights were canceled for Saturday at Reagan National, while some international flights were still to operate at Dulles, the Metropolitan Washington Airports Authority said.
It urged passengers against traveling to either airport and to re-book cancelled flights.
Late Friday, the national capital was left without bus service as the metropolitan transit authority deemed the roadways “impassable.”
The metro shut down 40 above ground stations, with Metro Acting Deputy General Manager Dave Kubicek saying the service was “fast-reaching the point where we risk trains becoming stranded on snow-covered tracks.”
The measures effectively meant that Washington's outlying suburbs have been left without public transportation.
The governors of Virginia, Maryland and Delaware declared states of emergency, a move that puts the National Guard on alert.
In nearby Delaware, Governor Jack Markell ordered all vehicles off the roads starting at 10:00 pm.
“Until further notice, the emergency declaration allows only emergency vehicles and essential personnel on the roads,” a statement from Markell's office said.
Courts, schools, government offices, even military bases closed early to beat the storm which started to dump snow on the capital and its suburbs by mid-day.
Jane Bate, 41, who emigrated to the United States from the Philippines in March, headed out to a local supermarket early Friday after neighbors warned her to prepare for the worst.