WASHINGTON — Life in the nation’s capital ground to a halt Friday as steady snow fell, the beginning of a storm that forecasters said could be the biggest in modern history.

A record 2 1/2 feet or more was predicted for Washington, where snow was falling heavily by evening, with big amounts expected elsewhere throughout the Mid-Atlantic. Authorities already were blaming the storm for hundreds of accidents and the deaths of father-son Samaritans in Virginia.

The region’s second snowstorm in less than two months could be “extremely dangerous,” the National Weather Service said. Heavy, wet snow and strong winds threatened to knock out power, clog roads and paralyze the region’s transportation and retail.

Airlines canceled flights, schools closed and the federal government sent workers home, where they could be stuck for several days in a region ill-equipped to deal with so much snow. Some area hospitals asked people with four-wheel-drive vehicles to volunteer to pick up doctors and nurses to take them to work.

The National Zoo closed at noon and the Smithsonian museums were to close Saturday, as they did during a major storm in December. U.S. Park Police spokesman Sgt. David Schlosser said the Lincoln Memorial and other monuments in Washington would remain open as long as conditions allowed.

At Baltimore-Washington International Thurgood Marshall Airport, most flights had been canceled by early Friday afternoon. In the virtually empty Southwest terminal, someone announced Flight 677 to Denver — “our last flight till Sunday morning to the West Coast.”

Gilles Conti, scrambled in vain to find a way to get to Los Angeles from Dulles International Airport in suburban Washington, where all flights through Saturday afternoon were canceled. It was the same story at Reagan National, the city’s second airport. Conti said he hoped to get out by Sunday.

“I’m just going to wait, I mean, what can I do?” he said. “I’m going to go back to the hotel I was in and I guess I’m going to stay there.”

Flights were also scrapped elsewhere in the region, and Amtrak stopped most trains heading south from Washington.

Starting earlier in the day, shoppers jammed aisles and emptied stores of milk, bread and shovels.

There were already 20 to 30 people waiting when a Trader Joe’s in Falls Church, Va., opened at 8 a.m.

Errol Bailey, a 55-year-old tailor who works in northern Virginia, said he had stocked up on food at his home in Largo, Md., but realized he should have provisions at work too.

“I’ve got some cashews, some orange juice, some bread, cheese and I’m about to pick up a bottle of wine here now,” Bailey said. “I hope I’ll be enjoying the wine at home, but if it gets real bad maybe I’ll have to pop that open at work.”

Many residents scrambling for food and supplies found they were too late.

At a Safeway in Hanover, Md., dairy products and other perishable items were scarce. There wasn’t a single egg in the store, and a few bottles of milk remained on the shelf.

“I’ve come from two other places that are out of milk and sour cream,” said Cheryl Conner, 50, of Hanover. “This one’s out of sour cream, too, it’s crazy.”

Conner said the snow was altering her weekend plans. “I told my husband, we’re not having a Super Bowl party,” she said. “No one can get up our driveway.”

Metro, the Washington-area rail system, said ridership Friday morning was down about one-fourth from the same day last week, a sign people were heeding official warnings to stay home. Metro warned it would likely have to close all but the underground portions of the system during the storm.

In western Virginia, a tractor-trailer struck and killed a father and son who had stopped to help another driver who had wrecked in snow on Interstate 81, Virginia State Police said. William Edward Smith Jr., 25, of Mooresburg, Tenn., and 54-year-old William Edward Smith Sr. of Sylva, N.C., died at the scene, authorities said. They reported more than 200 accidents through Friday afternoon.

Across the region, transportation officials were deploying thousands of trucks and workers and had hundreds of thousands of tons of salt at the ready. Several states exhausted or expected to exhaust their snow removal budgets.

“This is not a good mix,” said Joan Morris, a spokeswoman for the Virginia Department of Transportation. “Heavy, wet snow with gusting winds is going to make it a very tough storm for us. I expect visibility will be very poor in spots, and we’ll have to deal with drifting snow.”

Virginia Gov. Bob McDonnell, who has been in office less than a month, declared his second snow emergency, authorizing state agencies to assist local governments. Then he appeared on The Weather Channel to talk about it.

Blizzard warnings were in effect for much of Delaware and southern New Jersey from Friday afternoon to Saturday night, with strong winds and blowing, drifting snow.

Philadelphia could get about a foot of snow and up to 20 inches was expected in the Pittsburgh area.

The storm comes less than two months after a Dec. 19 storm dumped more than 16 inches of snow on Washington. Snowfalls of this magnitude — let alone two in one season — are rare in the area. According to the National Weather Service, Washington has gotten more than a foot of snow only 13 times since 1870.

The heaviest on record was 28 inches in January 1922. The biggest snowfall for the Washington-Baltimore area is believed to have occurred in 1772, before official records were kept, when as much as 3 feet fell in the Washington-Baltimore area, an epic event George Washington and Thomas Jefferson mentioned in their diaries.

In Washington, tourists made the best of it Friday, spending their days in museums or venturing out to see the monuments before the snow got too heavy.

A group of 13 high school students from Cincinnati was stranded in D.C. when a student government conference they planned to attend was canceled — after they had already arrived. They tried to rebook a flight to head back home Wednesday, but there weren’t enough seats. So they spent Thursday and Friday sightseeing.

At the Smithsonian’s Natural History museum, Caitlin Lavon, 18, and Hannah Koch, 17, took pictures of each other with the jaws of a great white shark in the Ocean Hall.

“Our parents are all freaking out, sending texts to be careful,” Koch said of the snowfall predictions. “Being from Ohio, I don’t think I’ve ever seen that much snow at once.”

Beverly Hollingsworth, 46, and her husband, Roger, 61, of Chattanooga, Tenn., wore ski jackets as they visited the Vietnam Veterans Memorial. They were also in town for a conference that was canceled, and they couldn’t leave either.

Roger Hollingsworth’s best friend was killed in Vietnam, and they liked being able to find his name on the memorial without the usual crowds.

“It was nice,” Beverly Hollingsworth said. “We didn’t have to push and shove to see the name.”

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Associated Press writers Brett Zongker in Washington, Kathleen Miller in Falls Church, Va., David Dishneau in Chantilly, Va., Ben Nuckols in Hanover, Md., and Steve Szkotak in Richmond, Va., contributed to this report.