The biggest snowstorm to hit Washington in decades blanketed much of the metropolitan area with more than 18 inches of dry, grainy flakes yesterday, paralyzing roadways and mass transit and turning the region's attention from the threat of terror to the wonder of winter and the near-impossibility of getting around.

Snow fell steadily all day at a rate of about an inch an hour, starting at 2 a.m., as two major storm systems converged on the Washington area. The National Weather Service predicted that the snow would continue all night and until midday today, with accumulations generally ranging from 16 to 24 inches, with up to 28 inches predicted in Loudoun County. That total would equal the amount that fell here in 1922.

"Historically, this does not happen," said AccuWeather meteorologist John Dlugoenski. "Something always prevents it from getting above 20 inches."

Top officials in the District, Maryland and Virginia declared states of emergency, clearing the way for them to seek federal help paying for snow removal and, if necessary, receive assistance from the National Guard and the Red Cross. Maryland Gov. Robert L. Ehrlich Jr. (R) also issued an order banning nonessential vehicles from state roads until at least 4 a.m. today, although officials said exceptions would be made for medical or other emergencies.

D.C. Mayor Anthony A. Williams (D) cut short a four-day weekend in Puerto Rico with his wife, flew to Newark International Airport and then caught a southbound train that brought him back to Washington last night.

In every jurisdiction, plows were struggling to clear major roads even as they filled again with fresh layers of snow, and officials said crews would work through the holiday weekend and into Tuesday in hopes of making streets passable when the workweek begins.

"Our overall goal is get the roads clear enough . . . so that people can get out of their subdivisions and get to work," said Joan Morris, a spokeswoman for the Virginia Department of Transportation. She said 40 percent of the nearly 1,700 crews working in Northern Virginia were already in residential neighborhoods. "We are not leaving those till last."

By midday, Washington landmarks that hadn't closed because of snow since the blizzard of 1996 were shut down: The memorials. The monuments. The entire Smithsonian Institution, including museums and the National Zoo. Most shopping malls, too, were shuttered.

Washington Archbishop Theodore E. McCarrick issued a statement saying it was not a sin for Catholics to stay home from Mass because of the weather. Some churches held services, but few were courageous enough to brave the elements and attend.

Raymond Johnson, a gardener from Southeast Washington, decided to forgo his usual trip to Mount Bethel Baptist Church in Northwest after his brother-in-law, a deacon there, said he wasn't going.

Instead, Johnson took Metrobus to Georgetown and earned a little extra money by shoveling out three longtime customers.

"I'm not saying church was closed, because there's usually someone there to open the place up," Johnson said, while riding the bus back home about 10:30 a.m. "But he wasn't going. And that was okay with me."

By 10 p.m., 23 inches of snow was on the ground in Damascus, with 17" in Great Falls, 16" in Alexandria and 12" on Capitol Hill.

The Presidents Day parade planned for Old Town Alexandria today was canceled, as were most other events related to the holiday weekend. Arlington, Fairfax and Prince William county school officials canceled their plans to have classes on the federal holiday, and officials said there was a good chance that most public school systems would be closed tomorrow. Virginia Railway Express and MARC commuter rail announced that they would not run train service today.

One of the few events to go on as scheduled yesterday was a Disney on Ice show at MCI Center. Organizers at first said tickets would not be refunded for those who could not make it downtown. After numerous complaints, they said customers unable to attend could exchange their tickets for passes to the Barnum & Bailey circus in April or get their money back.

But the Washington Wizards canceled this afternoon's game against the Toronto Raptors at MCI Center, at which the crowd was supposed to help superstar Michael Jordan celebrate his 40th birthday.

Good Samaritans with sport-utility vehicles ferried hospital workers to and from their jobs. Others who needed to get to work found themselves relying on Metrobus and Metrorail -- with mixed results. Some city buses made it across town in record time, but others broke down mid-route. Trains suffered long delays while crews cleared snow-covered tracks at outlying stations. By 11 a.m., Metro had gone to a schedule in which each station would have service in each direction only once an hour. By evening, the schedule was reduced to one train every two hours.

"It's hard for me to understand why the trains that are running underground are delayed," said Jeff Duffy, who waited 45 minutes at Gallery Place to ride two stops on the Red Line to Farragut North.

Reagan National and Baltimore-Washington International airports shut down completely. Although one runway stayed open at Dulles, most flights were delayed or canceled, and Amtrak and VRE halted southbound service from Washington, stranding thousands.

The Henley and Torp families, friends from Denver, ended up at Union Station in search of a way to pass the time after their flight back to Colorado was canceled.

"We were going to go to the Smithsonian, but we found out they were closed," said Kris Henley. "Shops are closed. Restaurants are closed. There's not much to do but go back to our hotel."

Lines to board the Amtrak trains that were running were at least 100 people long. Outside wasn't much better; as many as 150 people stood in line waiting for taxicabs to ferry them through the snowy streets.

Even President Bush, en route back from Camp David via a black Chevy Suburban instead of the usual helicopter ride, found his motorcade following a snowplow on George Washington Memorial Parkway.

Temperatures hovered in the teens, and a driving wind made it seem colder. District homeless shelters and hypothermia centers were filled almost to capacity, and officials said they would open emergency shelters at Banneker and Cardozo high schools in Northwest Washington if necessary.

"We have people sitting in chairs or standing against the wall, but we're not going to let anyone go out and freeze to death," said James E. Burton Jr., who manages the drop-in center at the city's largest homeless shelter, the Community for Creative Non-Violence. "We have lots of blankets. We have no other choice but to let people sleep on the floor."

For many who were more fortunate, however, the heavy snowfall was cause for celebration.

Downtown and in the suburbs, adventurous types strapped on skis and dug out snowboards to navigate hilly streets that suddenly looked more like ski slopes.

"I never thought I would find myself skiing in beautiful Washington, D.C.! This is absolutely amazing," Northwest resident Andrea James wrote in an e-mail just before 2 p.m., after traversing snow-covered New Mexico Avenue. "If only there were ski lifts."

D.C. Council Chairman Linda W. Cropp (D-At Large) answered her home telephone with a cheerful, "Happy snow day." The two official appearances she had yesterday were canceled, and she said the storm was providing a welcome respite from the war-related tension that had engulfed the city.

"I think the terrorists will have trouble getting out in this," Cropp quipped.

In rural Southern Maryland, even some farmers who in years past would clear driveways for $35 stayed in.

"We'll all start digging out tomorrow," one woman said. "Might as well wait until it's all finished. Then the work will start."

The holiday weekend meant two extra days for snowplows to clear the roads -- time that city, county and state officials said was desperately needed.

Public works departments across the region went into overdrive, putting all available plows on the road and hiring private contractors as well. "As soon as they treat a problem section," said Prince George's County public works spokesman Chauncey Bowers, "it keeps on snowing."

D.C. officials said more than 350 cars had been towed from parking spots on streets labeled snow emergency routes. City Administrator John A. Koskinen urged people still parked on those streets to move their vehicles as soon as possible so that the city's 176 snow-removal trucks and an additional 200 or so contract vehicles could work efficiently. Williams said the city's snow removal crews were doing well, especially considering the circumstances, but he said patience may be required of people living on side streets where clearing may take up to 60 hours.

"We will try to get it done quicker," he said. "But we want people to have measured expectations."

In Prince George's, by contrast, public works officials said primary roads would not be passable until at least 36 hours after the storm's end. County Executive Jack B. Johnson (D) said he would seek state reimbursement for the cost of snow removal, since the county had already depleted the $1.2 billion budgeted for that purpose. Crews had responded to 13 snowstorms so far this winter, county officials said in a statement, compared with zero last winter.

Officials in every jurisdiction urged the public to stay off the roads unless absolutely necessary.

"It's treacherous out there, and it's going to remain treacherous until it stops," said Morris, of the Virginia Department of Transportation.

Maryland's highway crews had more than 2,000 snow removal trucks on the road.

Warren Cohen and Nancy Tucker Bernkopf had heeded government warnings last week and stockpiled batteries and bottled water in case of a chemical or biological weapons attack. They found themselves trapped in their house in a woodsy part of Potomac, prisoners to more than 18 inches of snow. The contractor they usually hire to plow them out hadn't shown up by late afternoon. But they weren't panicked.

"It's so beautiful," Bernkopf said. "We have food and electricity, so why worry?"

Julia Seyferth and Eric Wooliever, both of Alexandria, cross the snow-covered Mall near the Washington Monument with their dog, Brutus.Arlington County firefighters come to the aid of a driver whose car caught fire off North Quincy Street. In Arlington and across the Washington region, plows struggled to get ahead of the snowfall.In Mount Pleasant, Rene Alvarez finds that digging out his car is more than he bargained for as the snow continues its relentless assault.Alex Schultz of Bethesda isn't deterred by the snow, running in Silver Spring while training for the coming D.C. Marathon.