For one glorious December Saturday, Washington was suspended between a Bermuda High and a Mississippi Valley Low.

By 4 p.m. yesterday, the mercury had hit 75 degrees, equaling a record for December set in 1948. The National Weather Service said the unseasonable warmth will continue today, with highs in the mid-70s. Scattered showers and thunderstorms are expected tonight, with more warm temperatures and more showers Monday night. By midweek, the mercury is expected to drop into the seasonal 50s.

But yesterday, the Washington area reveled in seasonal masquerade. It was a day when a man on a beat-up Honda motorcycle could let his sleeves fly and feel the sun on his forearms, when heads were thrown back toward a low-powered sun, when sweat gathered above lips and dogs could be walked slowly, out of joy instead of hustled necessity.

Abdul the incense man in Adams-Morgan had his shoes off, toes wiggling in the 75-degree air.

At Hains Point, James Henry Adams had his shirt off and the trunk of his Monte Carlo open, installing two more speakers to his equalized, synthesized, AM/FM/Dolby cassette system with the LED readout.

At Fire Station No. 33 on Lee Highway in Fairfax City, firefighters were fighting dirt rather than flames, giving their big red pumper a wash and a shine, and on 19th Street in Hillcrest Heights, little green sprouts of confused tulips had nudged through the dirt and into the day.

No less confused were three cherry trees on the south side of the Capitol. Their blossoms defiantly open, pink and white, they too, felt the faint call of spring.

All along the East Coast, temperatures reached record heights for the day. Mount Washington, N.H., the highest point on the East Coast, where the high temperature at this time of year is usually 20, reported 45. In Boston, which was buried under a blizzard Dec. 6, 1981, the temperature hit 70 degrees, breaking the 63-degree record for the date set in 1950. Baltimore's 74-degree weather broke a 109-year-old record for the date of 73 degrees. New York recorded 72, Richmond, 77, and Orlando was the nation's hottest spot with a high of 85.

In the rest of the nation, flooding continued in central Missouri, Louisiana, Oklahoma and the Tennessee Valley. Travelers' advisories for snow and gusty winds were posted for the Oregon Siskiyou and Cascade Mountains, and snowy conditions were bad enough yesterday morning to stop traffic near Casper, Wyo.

Washington's fine weather was due to a high-pressure system in the Atlantic Ocean near Bermuda that provided a southerly flow of air, holding off a storm system in the Mississippi Valley.

All of which, said Abdul the incense man, led to "a right-on kinda feel-good day."

Or as Jeffrey Moscowitz, 19, visiting the Mall from Merrick, Long Island, said, "A day when you want to find a good outdoor cafe."

It was a day when the U.S. government lived up to its word. The National Weather Service had said last week that winter this year would be warmer than normal. And though it's not quite winter, it was quite warm. "Warm enough to make you want to smile," said Caryn Moscowitz, Jeffrey's twin sister.

It was a day of contrasts: a group of George Washington University students in shorts walking past the flashing lights and tinsel of a drugstore Christmas display on 19th Street. Sort of like spending Christmas in Jamaica and watching Santa plow into the beach on water skis.

A day of oddities too. Azaleas and other rhododendron flowered along with the cherry blossoms. Harold Green, retired horticulturist for the national Park Service, said the early appearance of flowers was caused by natural confusion. "When you get a warm base like this . . . the flowers get confused and think it's spring. But we should still have cherry blossoms in the spring."

Jos Roozen, owner of Roozen's Nursery and Landscaping in Oxon Hill did say, however, that the earlier flowering of the blossoms will mean a bit less grandure for the Cherry Blossom Parade. "We'll only have 80 or 90 percent blooming in the spring," he said.

The only real problem with the warm weather, horticulturally speaking, is business, said Helmut Jaehnigen, of Behnke Nurseries Co. in Beltsville.

"With this kind of weather," he said, "nobody is thinking about buying a Christmas tree."