One man was killed and another seriously injured early last night when they were struck by lightning at the East Potomac Park Golf Course during a severe thunderstorm. The storm also caused numerous power failures and knocked down trees as it rolled through the Washington area.

D.C. homicide detectives identified the dead man as Walter Hayman, 48, of 3501 28th Parkway, Hillcrest Heights, Md. Hayman's golfing partner, identified as Erma J. Morgan, 56, of 3405 Massachusetts Ave. SE, was in serious condition at George Washington University Hospital.

Detectives said Morgan apparently was pulling his golf cart behind him as the two men hurried to seek shelter from the storm under a tree. A lightning bolt struck Hayman just as they reached the tree, police said, knocking both men to the ground.

Bob Doak, night manager of the golf course, said he broadcast a warning about 7:45 p.m. to golfers still on the course to take shelter.

Doak said that about 27 persons were still on the course then, but he wasn't certain that golfers at the far end of the course, near Hains Point, could hear his message.

Doak said that during the height of the storm a man in a passing van stopped along the east side of the course to aid Morgan, who said he had been hit by lightning. The unidentified man in the van then called an ambulance and summoned U.S. Park Police, Doak said.

Morgan apparently had stumbled from the third green of the "B" course, which is several hundred yards from the clubhouse. The dead man was found lying face down under a tree near the third green, golf course employes said.

Area police, fire and power officials reported numerous power outages that affected about 1,200 homes and businesses during the storm, which followed an uncomfortably humid day during which the temperature reached 95 degrees, a high for the year.

The official temperature was only 6 degrees short of the record 101 degrees which incurred in 1930. A National Weather Service spokesman said there were scattered reports of temperatures that reached 100 degrees and that the relative humidity soared over the 90 percent mark.

Temperatures plummeted rapidly as the storm swept southeasterly across the area beginning shortly before 8 p.m. At National Airport, the official measuring station, the temperature fell from 87 to 78 within minutes of the storm's passing. About .4 inch of rain was recorded there.

Pepco officials said widely scattered areas of lower Montgomery County and northeast Washington were left without electricity for periods up to an hour or more as power lines snapped under the force of 40-mile-an-hour winds, and falling trees and branches.

Police and power company officials said two separate automobile accidents during the storm caused power outages at Duke Street and Quaker Lane and in the 5000 block of Seminary Road in Alexandria when the automobiles struck power poles. There were no reports of serious injury.