The National Park Service made it official yesterday: With the flowering of the Tidal Basin's cherry trees, spring is a capital notion.

Under a blanket of sunshine, temperatures soared to 70 degrees. Although only a lucky few were free to enjoy the gift of midweek sunshine that sneered at last weekend's deluge, many Washingtonians took time yesterday to enjoy it.

Those who were working settled for eating their fast-food lunches on the outdoor tables from Rockville Pike to McPherson Square, or clipping forbidden branches of forsythia in Rock Creek Park, or greeting passers-by from the balcony of a Gaithersburg apartment complex.

"It's a great day to be outdoors," agreed Montgomery County employe John Peay, leaning on his spiked litter stick alongside Montrose Road and wiping his forehead on his rolled-up cuff. "But not if 'outdoors' means working."

And a Beefsteak Charlie's kitchen worker on his day off spent a busman's holiday at Dietle's Tavern near White Flint Mall. "It's nice," he shrugged, "but nowhere to go. Everybody else is working."

Today, however, the National Weather Service says a midwestern cool front will stub its toes on the Blue Ridge Mountains, bringing back clouds and a 30 percent chance of rain. The temperatures will stay high, however, perhaps hitting 75 degrees. National Weather Service forecasters say temperatures will remain high until the weekend, when they mutter of cooler weather.

Still, as far as the Washington area goes, spring is always a fling. So there were flashes of spring delirium yesterday in the way the rear windows of a Montgomery County school bus were filled with upraised bare feet; in the puckish grin of a motorcycle messenger whose unbuttoned shirt whipped into wings behind him; and in the Huck-ish back view of a dusty pickup with a brace of fishing reels hung on the gun rack.

But spring threatens a jolt to the more sedentary types, who emerge from their winter clothing to discover new and unwelcome additions to their anatomy just when bathing suits and bareback Hawaiian print dresses are filling the display windows of Hecht's.

And so there was a particular poignance to the bent, upended handlebars of the exercycle that someone had tossed onto the conveyor belt at the Shady Grove solid waste transfer station.

It may have been intended to announce a rebirth by someone whose old-fashioned bike will take him out to see the world. But the shards of mirror alongside it suggested the early demise of a leftover New Year's resolution.

And that is truly the end of winter.