To longtime resident Gary Yoder, it's "the attack of the Volvos," the city people's winter weekend invasion of rural Garrett County some three hours and many mountains removed from Washington.

They converge especially on New Germany State Park, which offers nearly nine miles of color-coded, easy to very difficult cross-country ski trails, forests of snow-covered trees and mountain laurel, and the opportunity to bump skis with others who want to get away from it all.

Instead of bumper-to-bumper, said Sharon Smith of Annandale, "We call it tip-to-tail skiing, but it's beautiful enough that it's well worth it."

But in their quest for cross-country solitude, the legions of urban people have created what locals jokingly refer to as "the turnpike." Unlike in downhill skiing, there are no lift lines in cross-country, just lines.

"Some D.C. people we know call it Connecticut Avenue because it's a busy place where people hang out all the time," said Sandi Gregory, who, with partner Lisa Baker, operates the food concession inside New Germany's large log "warming hut."

On Saturday, the hut was filled almost to its capacity of 160 persons, and Gregory was out of nearly everything by 4 p.m. By day's end, according to gatekeeper Bill Martin, 593 skiers had checked into the park.

"It's been like this all day," said Martin as cars backed up at the entrance. "We have a few locals, but mostly it's people from the Baltimore-Washington area."

"Normally, we're a majority," said skier David Ramsey of nearby McHenry, "but we've been invaded. Weekdays, there's hardly anyone here. But this is a nice spot to come and watch all the people."

"Watch all the yuppies," added his wife Sara.

Locals, who usually ski on their own land and not in state parks, eschew designer ski wear for wool pants or knickers, a sweater and a vest, said Gregory. "People from D.C., you see more ski suits, one-piece, tight-fitting suits down to the ankles."

"They come in mass with picnic baskets of quiche, Dom Perignon, brie and bagels," said Yoder, 46, who divides his time between here and Annapolis, where he works for the state. His family moved to Cumberland in neighboring Allegany County during the Depression, making him almost a native.

The skiers must be off the state trails by 4:30 p.m., and then they jam the local restaurants and motels. The Grantsville Holiday Inn, close to New Germany, was filled this weekend by metropolitans. The inn's 12 copies of the Sunday Washington Post were gone before 9 a.m.

It amounts to a population explosion in Maryland's largest county, where there are only 39 people per square mile, compared to, say, 1,145 in Montgomery County, according to the 1980 census.

The parking lots in New Germany State Park, along with the trails, were filled this weekend, although a Saturday survey of Parking Lot 5 revealed only three Volvos and one Mercedes out of 30 cars. One car bumper sticker said, "We support Reston Soccer."

In the warming hut, some drank imported beer, others munched on granola bars or sipped hot soup made and sold by Gregory. The trash was neatly separated: Styrofoam cups in one receptacle, aluminum containers in another, for recycling purposes.

If the main green-coded trail seems like Connecticut Avenue to some, then the intersection of paths where the trail starts is Connecticut and K Street.

Among the people coming and going there the other day were four from Adams-Morgan, all young professionals but not, one insisted, yuppies. Andrea Levere, who works for the National Development Council, said, "I'm not into exercise. I'm into recreation."

Debbie Berkowitz, one of her companions, said she does Jane Fonda workouts and rides a bicycle machine back home.

In another season, many who come here would be biking or jogging in Rock Creek Park or along the C&O Canal towpath in Georgetown. But it is winter and there is snow by the foot in Garrett County, so they have come in large numbers, for the exercise, for the beauty, for the crowds.

On the trail, the skiers generally look forward, their faces showing a grim determination to get ahead. First-time skiers who pause to ponder Poplar Lick, the creek that parallels the most popular trail, soon discover the rules of the road and step aside to let others pass.

Skiers who cross paths occasionally greet each other, but the camaraderie is mostly among groups who come and go together. The mood inside the warming hut is a bit more congenial. There, longtime friends and neighbors sometimes meet unexpectedly 200 miles from home.

Bonnie Sidranksy of Bethesda, for instance, ran into Nancy Estill of Cabin John. They had worked together as nurses at George Washington University Hospital. At the other end of the hut, Gerry and Ingrid Sophar of Silver Spring encountered their friends, the Mielkes, of Chevy Chase.

"The easy trail is very crowded," said Ingrid Sophar. "It's not the kind of fun I'm used to when you can go on lonely trails . . . . "

Said Jim Mielke, who has skied in arctic isolation, "Especially if you're going down a slope, you have to be very careful not to run into people climbing up. There seems to be an awful lot of people on the trails."

To Mercer Cross of Bethesda, "The best thing about cross-country skiing is the solitude of the woods, if you can find it." The Cross clan was headed next for West Virginia's Canaan Valley, which they hoped would offer such solitude.

"The reason Lisa and I started this concession," said Sandi Gregory, "is we figured the weekend was so crowded, we'd never ski here then. Sometimes we ski here during the week. It's great. It's quiet.

"The first year out here, I went skiing at Herrington Manor," another Garrett County state park. "A young deer ran right in front of me. Stuff like that wouldn't happen here on a weekend because it's so crowded."