Government officials prepared for evacuations of low-lying sections of the water-logged Washington region last night as record-setting rainfall continued and commuters, homeowners and federal agencies struggled to cope.

Several large federal offices remained closed. Local governments opened emergency command centers and sheltered more residents reeling from rains of historic magnitude. The silt-laden Potomac River neared flood stage, and other waterways verged menacingly on overflowing.

Fearing that Lake Needwood, north of Rockville, was breaching its leaking dam, Montgomery County officials early this morning began evacuating people living along Rock Creek below the dam, county spokeswoman Donna Bigler said.

As another day of downpours came to an end, the driving rain continued in spots, with one inch falling in 15 minutes in Annapolis shortly after 10 p.m., according to National Weather Service forecaster Andy Woodcock.

By yesterday afternoon, the sluggish system that had brought the record rainfall showed signs of heading out of the area last night.

But forecasters at the time pointed to a tropical system to the southeast that they said could help bring an additional three to five more inches of rain before all the storms dissipated. However, at 11 last night, the latest forecast indicated that no more than one to three additional inches remained in store, principally for northern and central Maryland.

Flash flooding was reported late last night in Rockville and in Harford County, Md. But radars showed areas of clearing on the western edges of the region.

From Friday morning through yesterday morning, 12.11 inches of rain fell at Reagan National Airport, according to unofficial tabulations.

"This amount of rain in four days should occur once every 200 years, and we just lived through it," said Jim Lee, meteorologist in charge at the National Weather Service. Forecasters warned that an additional couple of inches of rain would lead to even more severe flooding, debris and damage.

The impact of the storm was seen everywhere: Struck from behind on a rain-slick street, a taxicab swerved and crashed into McFadden's Restaurant and Saloon, at 24th Street and Pennsylvania Avenue NW. The 11 p.m. incident sent four people to a hospital.

In Frederick and Carroll counties, Maryland State Police searched until late into the night for two missing boys, 14 and 15, whose families feared they had been washed away by rising waters after venturing to a stream about a mile from their homes in Keymar, Md.

In Montgomery County, firefighters used ropes and flotation gear to rescue a man and woman from a car that stalled in standing water on Brighton Dam Road in Brookeville.

With an eye on Maryland's rain-swollen Patuxent River, Anne Arundel County Executive Janet S. Owens (D) asked last night that residents of low-lying communities in the Laurel area head for shelter.

Also last night, the District, prompted in part by the predictions of more rain, declared a state of emergency to aid in the mobilization of resources.

As heavy rain fell in the evening, washed-out streets and downed signal lights on several major commuter routes left drivers stuck across the city. Traffic lights turned green on other major thoroughfares, such as K Street NW, but drivers went nowhere. Cars jammed intersections, and the honk of horns filled the humid evening air.

D.C. Transportation Department spokesman Erik Linden said the backups were primarily caused by downed signals on Constitution and Independence avenues. "If you can avoid driving, it really very much helps us out," he said.

Bus passengers were stuck in the same backups. Metro's Orange Line trains were so crowded that some riders at downtown stations had to wait for several trains to pass before they could get on. They said their trips home were delayed by as much as an hour.

Metro workers added sump pumps and positioned 12 tons of sandbags near vent shafts and grates at Federal Triangle, Archives-Navy Memorial-Penn Quarter and other vulnerable stations to prevent a repeat of the flooding that hobbled the transit system Monday.

In a preemptive move, the District placed a team of arborists on patrol last night to make quick assessments of weakening tree trunks and damaged limbs in hopes of keeping problems from worsening. Significant damage has affected about 250 trees since the storms started, one of the arborists said.

The Old Post Office building and the Ariel Rios Federal Building in downtown Washington remained closed yesterday, and federal officials announced that the headquarters of the Justice Department, Internal Revenue Service and National Archives would stay shut for the rest of the week. The National Zoo opened for most of the day, but only for visitors who arrived on foot.

Power was still a problem for some. As of last night, about 6,000 customers were without power in Maryland and the District, most in Montgomery County, Pepco said. That was down from about 8,000 a few hours earlier. In Virginia, about 3,700 customers were without power as of late yesterday, according to Dominion Virginia Power. By 9:30 p.m., the figure was less than 1,000.

The Fairfax County government was considering an evacuation of the Belle View and New Alexandria communities, which flooded during Hurricane Isabel in 2003.

Fire officials in Fairfax announced that 160 homes were damaged in the Arlington Terrace area Sunday night when Cameron Run overflowed, crashing through basement windows and destroying cars, which in some cases were immersed by the waves. Three of those homes were "red-tagged," meaning the damage was beyond repair and the houses had to be condemned.

In Bethesda, residents along the 4300 block of Sleaford Road were irate yesterday because they had been without power since Sunday morning. Those grumbling about traffic tie-ups and soppy shoes were getting little sympathy from Barry Blandford, who said he got a two-day runaround from Pepco.

"They keep telling you that it's scheduled to be restored at a certain time," he said "When you call at night, they say, it'll be fixed in the morning. When you call in the morning, they say it'll be fixed in the afternoon."

About 2 p.m. yesterday, a crew showed up. Within 21 minutes, power was restored.

"You don't realize what you miss -- all these little things like the coffeemaker," said a relieved Blandford, enjoying the cold blast coming from the air conditioner.

Two barns were destroyed in St. Mary's County yesterday evening by what people in the Choptico area said was a tornado. Authorities said they were investigating.

A resident of the area, Pat Riffle, said he saw a "white wall of water coming toward us." After the barns were destroyed, he said, debris was scattered to "every point of the compass," suggesting the rotating winds of a tornado.

"We definitely had a tornado," said Joyce Cusic of Cusic Mechanic Service on Manor Road in Choptico. She said trees went down, and "we're saying big trees."

In addition, she said, "I have tin from the neighbors' barns all over the back yard and the front yard."

She said she had been about to go out for a walk shortly before 6 p.m., but her dog refused to go. "That's what saved me," she said.

In Alleghany County in the western part of Virginia, rescuers searched for an 8-year-old girl swept away by floodwaters, the Associated Press reported. The child went missing shortly before 2 p.m., according to Ryan Muterspaugh, the county's public safety director.

Farmers on the waterlogged Eastern Shore dealt with the remains of as many as 80,000 chickens that drowned.

The Potomac will continue to rise today and probably will crest tomorrow, but forecasters were not projecting significant flooding.

Officials in Alexandria were bracing for a possible surge of water created by heavy rainfall north and west of the Washington area that could travel down the Potomac and reach the District by tonight, spokesman Brian Hannigan said. That could endanger riverfront homes and businesses in Old Town.

Hannigan said the city is concerned about a trio of forces that could combine tonight during high tide at 11:20. They feared the storm surge would hit when the tide peaks and more rain might fall.

Officials in Alexandria also warned of additional flash flooding all over the city after sweeping floodwaters in the city's West End trapped motorists, swept away cars, flooded homes and caused thousands of dollars in property damage Sunday evening.

Yesterday morning's commute went much better than Monday's crusher, but the continued rainfall washed out critical routes and choked traffic for many.

Water on Interstate 395 slowed drivers heading into the city, extending the morning rush. Drivers in downtown Washington suffered a second day of gridlock as parts of Constitution Avenue remained closed, signals were out on Independence and curb lanes of 12th Street NW were blocked as water was pumped from the Internal Revenue Service building.

On 17th Street NW, Rowena Reid described traffic as "a nightmarish mess." She and a friend spent 20 minutes going two blocks as they diverted around a closed portion of Constitution Avenue -- just like everyone else.

By mid-morning, Orange Line riders were reporting trips of 40 minutes or more and expressed frustration with Metro's lack of alerts.

Riders have complained all week that they have not received timely information, and Metro managers said yesterday that they were revamping their information systems.

But Gladys McCowin was just happy it wasn't as bad as Monday morning.

"Yesterday, I saw the commute and turned back around," said McCowin, a loan analyst at the Veterans Affairs credit union. "Today, it wasn't nearly as bad."