More than 20,000 customers remained powerless this afternoon following the thunderstorms that blew through and lit up the region yesterday, injuring four people in Loudoun County who were struck by lightning.

The storms were whipped up suddenly, albeit predictably, when a cold front that moved into the region confronted 95 degree heat and the waterlogged air mass that had oppressed Washington for the past few days. Temperatures plummeted in minutes by 20 degrees.

Today's high is expected to be in the 80s.

At the height of the power outages, more than 150,000 homes were without electricity.

"We're saying 8 p.m. Friday for [restoration of power to] the very last customer," said Pepco spokeswoman Mary-Beth Hutchinson last night. "Most people, I feel certain, will be back well before that."

Today, most of the remaining outages were in Maryland, Pepco reported on its Web site, with about 16,000 customers out in Montgomery County and 3,500 in Prince George's County. Some outages were also reported in Anne Arundel and Howard counties.

About 800 people were without power in the District, Pepco said.

In Northern Virginia, Dominion Virginia reported that just under 500 customers were without electricity after 4 p.m.

In the morning, when outages were still more prevalent, traffic signals were inoperative in some areas, particularly in Silver Spring. Authorities asked motorists to treat darkened intersections as four-way stops.

Motorists were also cautioned to be alert for downed branches and debris in the streets.

In the most severe episode from the storm, Mary Maguire, a spokeswoman for the Loudoun fire and rescue department, said one person suffered cardiac arrest when struck by lightning and was revived by rescue workers. The four people injured in Loudoun, who were together when struck, were transported to area hospitals and their conditions were not available late last night.

The incident occurred about 5:40 p.m. in the 14600 block of New Valley Church Road in the Lucketts area, north of Leesburg. A dog that was with them died after being struck by lightning.

The storms snapped trees and produced strong, gusting winds and hail in many areas. The foul weather had largely moved out of the region by late last night.

Before the searing heat abated, about 300 people at the opening events of the Boy Scout National Jamboree at Fort A.P. Hill in Caroline County were treated for conditions due to excessive heat, Scout officials said late last night.

Emergency medical technicians from surrounding towns and counties responded, and most of the 300 were treated at the on-post medical clinic. Scout officials reported today that two people were hospitalized but all were in stable condition.

The clashing weather systems triggered spectacular lightning displays, but also ignited house fires and blasted power boxes. Utility crews scrambled to identify short-circuited wires and blown fuses.

Along the outer loop of the Capital Beltway, drivers heading home witnessed vivid vistas of forked lightning, as they dodged tree branches that blew before slow-moving traffic. Downed tree limbs and other debris blocked roadways in the District's Rock Creek Parkway, along Maryland's Route 29 corridor and the major arteries of Prince George's County.

Brian Mooney, 29, of Germantown said that he usually loves thunderstorms and that he had looked forward to last night's boomers. "This was supposed to be our relief from the heat," Mooney said by telephone, before lightning burned a hole in the roof of his neighbors' condominium building.

Mooney said the lightning crash next door knocked him off his feet. The resulting fire in the 12900 block of Church Hill Ridge Circle reportedly left several families without housing and caused about $200,000 worth of damage.

"It was heavy-duty. There's no joke -- the lightning put a car-sized hole in the roof of that building," Mooney said.

The weather front was a part of long band of showers and thunderstorms that stretched from Pennsylvania to eastern Tennessee, reportedly causing similar wind damage in those areas, the Weather Service said. The storms produced golf ball-size hail in areas of Montgomery and dumped about a half-inch of precipitation in most areas, as gusts of up to 54 mph kept the storm clouds from lingering.

"The storms were moving at a rather brisk pace," the Weather Service's Meadows said.

Meadows said the storms are expected to bring daily high temperatures into the mid-80s for several days, bringing "relief from the hot muggy conditions."

Prince George's fire officials reported two electrical fires caused either by lightning or power surges. They that storm hit the county hardest in the Hyattsville and Landover Hills areas.

No injuries were reported in any of the fires, officials said.

Staff writers Allan Lengel, Nick Anderson and Hamil R. Harris contributed to this report.