The temperature soared to 100 degrees here yesterday for the third time in six days and local officials grew increasingly concerned about public safety problems the heat wave could spawn.

Water pressure dropped in parts of the area, threatening the ability to fight fires; electrical demand taxed the capacity of local utilities and as many as six deaths may have been caused in part by the continuing high temperatures and humidity.

The oppressive heat was moderated somewhat last evening as a line of thunderstorms accompanied by high winds rolled through the area, dropping daytime temperatures by about 20 degrees in one hour. The respite was brief, however, and high temperatures and humidity are expected again today.

The storms caused widely scattered power failures brought on chiefly by downed power lines, and at one time more than 30,000 homes were without electricity -- 1,000 of them when a home in the 6000 block of Arlington Boulevard was struck by lightning.

Power to most areas had been restored by late last night, power company spokesman said.

The weather service said temperatures today would soar into the high 90s, but moderate by the weekend. A return of hot, muggy weather is expected by early next week.

Dr. Brian D. Blackbourne, deputy medical examiner for the District, said 15 bodies were taken to the morgue, chiefly from hospitals, between midnight and midafternoon yesterday, about double the normal number. (Approximately one-third of all dead persons are taken to the morgue.)

Although Blackbourne said the precise cause of the deaths won't be known until tests are completed, he said, "There is no question some of them, perhaps five or six, were precipitated by the heat."

Dr. Edward Zimney, another deputy medical examiner, said, "It's hard to tell" whether heat was a factor, because many of the victims were elderly, with heart ailments. But the heat, he said, "might have tipped them over."

A record demand for electricity forced the Potomac Electric Power Co. to reduce voltage by 5 percent between 1:06 p.m. and 3:50 p.m. yesterday. Even with the cutback, Pepco customers used 4,135 megawatts (a megawatt is one million watts between 4 p.m. and 5 p.m., nearly matching the 4,142 megawatts that were used last Wednesday, when voltage was not cut back.

A spokeswoman for Pepco, which serves the District, the Maryland suburbs and Rosslyn, said the voltage reduction was ordered throughout the Maryland, New Jersey and Pennsylvania power pool, permitting participating power companies "to meet the extraordinary demand without asking customers to voluntarily limit consumption." She said the voltage reduction "means no inconvenience to customers, and causes no damage to motors." It's only noticeable affect, she said, might be in reducing the size of television pictures. It should not make lights flicker, or cause other problems associated with "brownouts."

The Virginia Electric Power Co. did not reduce its voltage levels, according to a spokesman.

Metro bus and train passengers also suffered from the heat. Metro spokesman Al Long said about 100 of the 1,578 buses normally were sidelined with overheated engines. Many more buses were operating without air conditioning, but Long said, "we figured it's better to get people home to cool houses and drinks than make them wait" at bus stops for air-conditioned vehicles.

On Metro rail, Red Line trains operated with two fewer cars than usual and Blue Line trains with one less car because of air conditioning malfunctions. Also, some rail passengers complained that water from the roof-top air conditioners spilled into the cars, forming puddles on seats and surprising riders. One woman who got on a train at the Foggy Bottom station "came up screaming" after sitting in the cold water, according to passenger John Cable.

Youngsters seeking relief from the heat have created problems for firemen in the Far Northeast and Southeast sections of the District because they have opened 48 fire hydrants over the past few days.

Sam Jordan, acting deputy director of the city's emergency prepardeness office, pleaded for parents to urge their children not to open the hydrants. Firemen reported water pressure was so low in some areas that "if we get a fire, there's not enough water to contain it," Jordan said. At some higher elevations, he said, houses were without tap water because of the lowered pressure.

Because not even darkness has brought the temperatures down (the overnight low yesterday was 82), Jordan said he will ask the District Recreation Department to extend the hours that city pools operate, or at least make them open to children at all hours (some pools reserve night hours for adults. Hours had been reduced this summer to help cut costs in the city's budget crisis.

Meanwhile, workmen were busy installing new caps on opened hydrants, and employes of the city's summer youth employment program were assigned to guard hydrants in affected neighborhoods.

The Air Quality Index hit 145 at midafternoon, but the Council of Governments' monitoring team did not issue a health advisory, as it usually does when the index hits 100 at two locations, because of expected relief from wind and rain.