Walter Scott stood on a muddy slab of concrete grinning sheepishly at his predicament while his hands played the line he had just thrown - thrown, not cast - into the Potomac.

"I sold my fishing rod to my buddy," he explained . "I didn't think I would feel like fishing."

But there he was yesterday, drawn cut for the first time this year, throwing a line into the Potomac, just above Key Bridge, trying to coax a fish or two out of the water. And not having much luck.

"They doing more eating than we doing catching," he said to his friend, William Lynch.

Neither seemed to care that nothing bigger than a six-inch perch was bitting. It was that kind of a day yesterday. The sun was sparkling on the water, the sky was a robin's egg blue with big cumulus clouds floating by and not a drop of rain in sight.

Yesterday was the kind of day that inspires people to run errands that don't need running and to find reasons why they don't have to go back to work after lunch.

It sounds a little silly to be saying it at this late date - midway into May - but summer seems finally to be on its way. After colder-than-normal temperatures in the first 18 days of May and an inch more rain already than the area normally receives for the entire month, official predictions now are for temperatures to stay in the upper 70s and 80s with slightly more than normal rainfall.

Downtown parks yesterday were full of women in summer dresses and businessmen with ties loosened and coats off, catching the first clear sight of the sun in days. Sidewalk cafes were filled with office workers eating leisurely lunches.

In Lafayette Park, across from the White House about 200 masked Iranian students were demonstrating against the shah and Savak, the Iranian secret police. Across Pennsylvania Avenue, helmeted pard police kept a wary eye on the demonstration. And behind the White House, sprawled under an oak tree, four park policemen from the mounted section were lounging, shooting the breeze and passing the time - keeping one ear on their walkie-talkies for a call to mount up and ride out to control the demonstrators.

A fifth park policeman dozed in the front seat of one of the two green vans - each containing four horses - parked nearby. When the radio squawked, one officer stopped in mid-sentence for a moment and then said, "That's the Eye-ranian demonstration," and resumed talking.

There was no rush about anything yesterday. Debbie Henderson, sunning herself in McPherson Square, brought a timer along "in case I doze off." She described the day succinctly: "It's lovely isn't it?" she said, looking up at the sun. For the weekend, she said, she had no plans "except to get as many rays as I can."

But others did have plans. With temperatures predicted in the low 80s today (after a high of 80 yesterday), Ocean City police said they were expecting the biggest crowd of the year. Maryland state police said they were expecting heavy traffic heading toward the Eastern Shore and that radar patrols would be out in force to cut down on speeding.

The Northern Virginia Regional Park Authority said it will open its swimming pools at Bull Run and Pohick Bay regional parks at 10 a.m. today - a week early - because of the anticipated warm weather. Pools in the District and Maryland still are not scheduled to be opened until next weekend.

The chance of rain today, according to the Weather Service is near zero, with temperatures in the low 80s and slightly lower near the shore. The water temperature, however, will be cold - upper 50s and lower 60s.

Sunday, the bubble may burst. A storm front, coming actoss the Mid-west, is expected to bring showers or thundershowers with a high in the low 80s.