A rash of sudden thunderstorms that took swimmers and sunbathers by surprise has left a deadly legacy at area beaches in the past few days: three people killed by lightning and another three injured.

Although frequently discounted by people, lightning is a powerful force that can leave little time for escape, especially on beaches, experts warned. That seemed to be the case in the most recent injuries.

"The lightning caught everybody by surprise," said Joe Wilson, deputy director for Virginia Beach's emergency medical services department, when describing the storm that injured three people on Saturday. "There were just some little clouds in the sky. Not much of a forewarning."

The latest casualty occurred Saturday night in Delaware when Robert Randall, 29, of Golden, Colo., was struck and killed by a bolt of lightning while sitting on Dewey Beach with a friend, police said. His death followed an incident Thursday in nearby Rehoboth Beach that left a Bethesda man and his son-in-law dead, according to police.

The three people injured in Virginia Beach were sitting on the sand under a beach umbrella when the storm came up. One of them, a 19-year-old unidentified Navy enlisted man, was in critical condition at Virginia Beach Hospital yesterday, a spokeswoman said. His heart stopped after the lightning strike, but rescue workers were able to resuscitate him. The two other people were treated at the hospital and released.

According to weather forecasters, the hot, humid days of July and August can produce stampedes of turbulent afternoon and late-evening thunderstorms. There is a 30 percent chance of such summer storms today, with rain expected to be heavy in some areas, according to the National Weather Service.

"The very warm air in the lower levels and the cold air in the upper levels of the atmosphere can create some very unstable conditions," meteorologist Edward Schoenberg said.

About 100 people are killed nationwide each year by lightning, said weather forecaster Edwin Danaher. Swimmers and boaters are the most vulnerable in thunderstorms. "Lightning is more dangerous near water and flat areas," Danaher said. "It tends to strike the highest object, so people are more exposed."

Golfers, hikers and other outdoor enthusiasts also face heightened risks from lightning.

Lifeguards and beach patrols at ocean resorts order swimmers out of the water when a storm is threatening, officials said. But sunbathers have the option of whether to leave the beach, said Jay Hancock, spokesman for the Ocean City, Md., police department.

"The city does not direct people out of the rain," Hancock said. "We urge them to use common sense. If someone wants to stay on the beach in a storm, we don't send someone to drag them off."

The last lightning fatalities in Ocean City occurred in August 1986, when four Northern Virginia people were killed while huddled under a beach umbrella, Hancock said.

Officials advise some basic precautions to reduce the danger of being hit by lightning. If you are on a beach, get to shelter quickly or lie down in a ditch or other low-lying area. Stay away from trees. Avoid contact with metal, such as beach umbrellas and golf clubs. And, according to one scientist, skin wet from rain or perspiration may aid in surviving a lightning strike.

Sometimes, the suddenness of the storm can leave little time for escape, officials said. "People should keep an eye on the sky," Danaher said. "When you start seeing puffy clouds with dark bases, you should head indoors."

Thursday, as the thunderclaps moved closer, Bethesda resident Milton Harper and his family rushed to pack their picnic gear on Rehoboth Beach, police said. But Harper, 62, and his son-in-law, Barry Brennan, 36, of New Milford, Conn., were struck by lightning before they reached safety, police said. Harper was pronounced dead the same day; Brennan died Friday.