Just when you thought it was safe to go out again, a freak storm with 55-mile-an-hour winds and some snow yesterday intruded on the area's first sunny Sunday in weeks, sending temperatures plunging 20 degrees in two hours.

Yesterday's unexpected storm was attributed to a cold front that brought more than an inch of snow to Chicago on Saturday before moving east. The front is expected to keep temperatures unseasonably cold here through Tuesday, with the mercury expected to climb no higher than the mid 40s today.

By mid week, temperatures should approach the normal 70-degree high for this time of year, said forecaster Jeff Bowman.

The last weekend that wasn't a washout was March 12 and 13, according to weather records.

At 2 p.m., National Airport was reporting a high of 58 degrees, but by 4 p.m. the mercury had dropped to 38 degrees and light snow mixed with rain had started to fall in Fairfax, Loudoun, Montgomery and Clarke counties. The snowfall was brief, however, and there was no accumulation, according to weather officials.

The unexpectedly intense winds accompanying the storm capsized many small craft on the Potomac River, and a number of Sunday sailors had to be pulled from the water by area rescue squads and the operators of larger power boats.

But during the first half of the day, rain-weary Washingtonians squeezed in a host of spring activities. Before afternoon showers forced people indoors again, the Mall, Hains Point, the Tidal Basin, other tourist spots and area parks were mobbed.

At the Vietnam Veterans Memorial, an honorary color guard exchanged a tearful group hug after presenting the colors for the last time at noon. The volunteers from South Carolina had spent the last two weeks soaking up "rain, wind, snow . . . beautiful Washington," joked Greig Fine of Charleston. But with the sun in evidence at last, he said he planned to attend the Orioles game later in the day.

At Hains Point, Sunday drivers lagged behind bikers and a procession of more than a dozen repainted Checker Cabs, flags flying and lights on, midway on a 26.2 mile "Checker Marathon" from the Washington Sailing Marina to Carderock.

Also at the Point, Bob Zimmerman of Upper Marlboro, a collection manager for a credit union, fished for "anything that'll hit the line" although his sons were hoping for bass. "Actually I'm just trying to get some fresh air," he said. "It's called cabin fever."

Although his chicken livers didn't get a nibble, Zimmerman was undaunted: his wife and his son's girlfriend were on their way with a picnic.

Buffing the wax on his gleaming '77 Lincoln Continental, Jessie Hall of Arlington said he planned to take the finished job for a spin through Rock Creek Park. His wife, Debbie, lounging across the front seat after jogging four miles on soggy grass around the Point, said, "I stayed in all day yesterday because of the cold."

Eyeing the first dark clouds over the Ellipse, bicyclist John Hendrick, an engineer from Baltimore, adjusted his helmet and mini rear-view mirror. "I might get caught in the rain going home," he said. Hendrick said he made the trip south in three hours to watch the 15th annual National Capital Open bike race--a grueling 35 laps around the Ellipse.

Storm clouds menaced the annual Songkran Festival of the Thai Community at Wat Thai in Silver Spring. But the traditional dancing and, mainly, eating came off as planned. No storm could have put out the fire of the "nam sot," a spicy combination of pork, ginger root, peanuts, lemon juice and sizzling seasonings provided by Dang Arnold of Fairfax.