A line of powerful thunderstorms, fueled by four days of 90-degree heat, ripped through the Washington area yesterday afternoon, sending winds gusting over 50 miles an hour, blowing down trees and limbs and knocking out power to more than 70,000 customers.

Most of the power failures were reported in Northern Virginia, where about 60,000 Virginia Power customers -- about 15,000 of them in the Fredericksburg area -- lost power for part of the afternoon, a utility company spokesman said. By 8 p.m. power had been restored to all but 5,500, he said.

Accompanied by thunder and repeated flashes of lightning, the swiftly moving storms tore across the area about 3 p.m., capsizing sailboats on the Potomac River, strewing hailstones in many places, and creating what appeared on radar to be a tornado in Charles County, Md.

The line of storms, which formed about 11 a.m. -- uncharacteristically early -- in the West Virginia Panhandle and churned southeastward, was "probably one of the more severe that have hit Washington in the past couple of years," said National Weather Service forecaster Erik Pytlak. Two dozen people taking shelter under a metal roof during an archery event in Preston County, W.Va., were struck by lightning about 1 p.m., and four were reported critically injured, authorities said.

Yesterday's mix of air currents and weather patterns, which included a cold front advancing from the northwest and a disturbance in the upper atmosphere as well as low temperatures aloft and steamy temperatures at ground level, provided the ingredients for a meteorological explosion.

A Prince George's County fire department spokesman said officials received many calls in the Fort Washington area for downed electrical wires and malfunctioning alarm systems, but added that no major fires or other incidents were reported.

The Weather Service received reports of hailstones -- golf-ball size in Stafford County, Va., three-quarters of an inch at the Marine Corps base in Quantico, Va., and marble- to pea-sized in many parts of the District.

Rain fell in wind-whipped bursts, with an inch reported in an hour in Fauquier County, Va. Planes were held on the ground as far away as Jacksonville, Fla., and Miami, until the storm cleared.

"Lots of capsizes," Steve LeBel, manager of marinas for Guest Services Inc., said after the storm, referring to numerous sailboats that experienced trouble in the Potomac. He described the wind as "tremendous."

Although Virginia Power bore the brunt of the storm, Potomac Electric Power Co. reported that 11,000 customers lost power in the District and Maryland.

After the four 90-degree days that led to yesterday's violent clearing of the air, today is expected to be fair and milder, Pytlak said.

"A very refreshing day." he said.