One Fairfax County teen-ager was killed and one was critically injured yesterday afternoon when they were struck by lightning during a brief but violent thunderstorm that doused the Washington area.

Northern Virginia bore the brunt of the storm that occurred on a warm muggy day that saw a high of 81 degrees, 19 degrees below the previous day's high.

The storm knocked out electricity for 18,000 Virginia Power customers, most of whom live in central Fairfax County, according to Virginia Power officials. Rescue workers also fielded dozens of calls reporting houses struck by lightning and power lines downed by falling tree limbs.

A large tree was blown down on Rte. 123 in Fairfax City, blocking traffic in the two blocks between Cedar and Warwick avenues in the late afternoon rush hour. Traffic was detoured for several hours while workers were removing the tree.

"It was a very quick storm, but very intense," said Virginia Power spokeswoman Deborah Tompkins. "For the time it was here, it did a lot of damage."

No power outages, flooding or weather-related incidents were reported in the District. Minimal outages were reported in Maryland, officials said.

In Fairfax County, the storm left tragedy in its wake. Stephen P. Black, 14, and Evelyn Laouikas, 15, were running across the front yard of Black's house trying to escape the rain when lightning struck a large tree, then hit both teen-agers in the chest, police said.

When rescue workers arrived at 2261 Richelieu Dr., both were lying under the tree in cardiac arrest, police said.

The victims were taken to Fairfax Hospital, where Black was pronounced dead about 45 minutes later. His mother, Lucy Ann Black, is an obstetrics nurse at the hospital; his father, Donald L. Black, is a retired Navy captain employed at TRW.

Laouikas, of 8527 Betterton Ct., was listed in critical condition.

"I just can't believe it," said Joyce Lin, who lives across the street from Stephen Black's house. "Such a fine boy. He mowed our lawn."

Stephen Black, who would have been a sophomore at George C. Marshall High School, was active in sports, family members said. This summer he swam the butterfly and back strokes for the Freedom Park community swim team and played outfield in a Vienna youth baseball league team. He was a member of the Vienna Presbyterian Church.

Fire and rescue workers in Fairfax were swamped with telephone calls reporting a rash of house fires, toppled trees and downed power lines. Virginia Power officials were scrambling to restore power to about 8,000 homes by yesterday evening.

"We've had numerous houses struck by lightning, from Penn-Daw {in southeastern Fairfax} to Chantilly {in western Fairfax}, which is basically the scope of the county," said Lt. Michael T. Reilly, spokesman for the Fairfax County Fire and Rescue Department.

There were reports yesterday of delays at the county's new Public Safety Communications Center. Asked about the delays, police spokesman Warren Carmichael said the emergency operations center was "deluged with calls," but that the longest delay in dispatching was five minutes.

Because as many as nine reports of house fires were received simultaneously, he said a number of nonemergency calls for problems such as downed wires were held indefinitely.

Most of the house fires were quickly extinguished, but there was $15,000 worth of damage to a house in Burke and $5,000 in damage to the roof of a house in Chantilly, authorities said.

The storm was triggered by a cold front that was expected to pass through the area last night. The highest winds were clocked at 50 mph at Andrews Air Force Base.

According to the National Weather Service, the cold front spread moist unstable air over the region, and as the air warmed, thunderstorms developed at midday.

The storm, which dropped .33 inches of rain at National Airport and .12 at Dulles International Airport, moved eastward from Loudoun County to the Dulles Airport area, from Fairfax to the District and Maryland suburbs.

This weekend's forecast by the National Weather Service calls for sunny skies, with highs climbing to the low to mid-80s, with lows in the low to mid-60s.