At grassy parks, by the sandy shore or in sometimess hectic transit, thousands of Washingtonians marked the first full day of the first long weekend of the sunshine season yesterday under appropriately sunny skies with no sign of rain in sight.

The temperatures reached a high of 84 degrees here yesterday, three degrees below the year's high of 87, recorded Friday and expected to be matched today. Tomorrow, on Memorial Day, the mercury is expected to reach above the 90 degree mark for the first time in 1981.

Traffic leading out of town, described as heavy Friday night and heavy yesterday morning, had subsided substantially by yesterday afternoon. Between 7 a.m. and 8 a.m., 2,542 vehicles crossed the Chesapeake Bay bridge, headed east. Two hours later, the figure dipped below the 2,000 mark, where it remained for the rest of the day.

A two-engine plane being used by a Baltimore televison station lost power in one engine on takeoff from Ocean City airport and fell into Sinepuxent Bay. The three persons aboard escaped without serious injury.

In Washington the 900 parking spaces at the National Zoo were filled, and the Tiday Basin pedal boat concession was "going solid all day" with 509 individual rentals, an employe said.