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."

As the Earth rotates, it wobbles on its axis like a spinning top. And like a top as it slows down, the planet develops a host of different wobbles, ranging in period from a few minutes to billions of years.

Some of the major wobbles are well studied, such as the 433-day Chandler wobble and the annual wobble, which together can tilt the Earth's axis up to 30 feet from its nominal center. One long-term change alters which point of light deserves to be called the North Star every few millennia.

Earth's axis of rotation is tilted about 23.5 degrees compared to the plane in which the planet orbits the Sun each year. The daily rotation of the planet creates a bulge at the equator, and the gravity of the Sun and Moon tends to pull this bulge back toward the orbital plane.

But Earth resists this pull. The result is that the axis moves in a cone-shaped pattern, called a precession, with the celestial North Pole describing a full circle every 26,000 years or so. Right now, the north celestial pole points towards Polaris, the North Star, but it used to point to Vega, and in 14,000 years it aim at Vega again.

Smaller variations, lasting a week or so, have proved difficult to study, partly because they're masked by the more prominent wobbles.

But from November 2005 to February 2006, the Chandler and annual wobbles essentially cancelled each other out. This allowed Sebastien Lambert of the Royal Observatory of Belgium and colleagues to study the minor variations and determine why they occur when they do.

Using newly-available GPS data that establishes the exact location of the poles, the team determined that weather patterns in the Northern Hemisphere play a significant role small wobbles.

The location of high- or low-pressure centers and the relation of these systems to each other played a measurable role in generating small, short-term wobbles, the scientists report. Moving weather systems caused the pole positions to swing in small loops ranging from the size of a cell phone to a sheet of paper.

The motion of the ocean also affects short-term wobbles; the study showed that oceanic pressure variations also coincided with the polar loops. This is the first study to demonstrate that day-to-day changes in atmospheric pressure produce a measurable effect on Earth's rotation.

The study is detailed in the July 1 issue of the journal Geophysical Research Letters.

In the June issue of the respected journal Nature Physics, he and J. David Neelin, UCLA professor of atmospheric and oceanic sciences, report that the onset of intense tropical rain and magnetism share the same underlying physics.

Peters and Neelin analyzed statistical properties of the relationship between water vapor in the atmosphere in the tropics and rainfall, using remote sensing from a satellite over the tropical oceans.

"We studied properties of that relationship that are also observed in equivalent quantities for systems with 'continuous-phase transitions' like magnets," said Peters, a research scientist with UCLA's Institute of Geophysics and Planetary Physics and a visiting scientist at the Santa Fe Institute.

"The atmosphere has a tendency to move to a critical point in water vapor where the likelihood of rain dramatically increases. The system reaches a point where it's just about to rain; it's highly susceptible. Any additional water vapor can produce a large response."

Finding the simple predictions of this statement confirmed in a complex meteorological system is unexpected, and may lead to more accurate climate models, Peters and Neelin said.

Neelin is working to incorporate these findings into models for atmospheric dynamics. Predicting the effects of global warming on precipitation is currently difficult to assess, he said.

"Global climate computer simulations make assumptions about how rainfall depends on moisture and temperature that are imperfect approximations," said Neelin, a member of UCLA's Institute of Geophysics and Planetary Physics.

"This research may lead to improved ways of treating rainfall in these models, which could help scientists improve rain prediction in daily weather or how it might change under global warming."

"Our study showed that absolutely everything we dreamed of finding was actually there," Peters said. "The predictions from critical phenomena showed up in the data. This is a huge step forward in self-organized criticality and critical phenomena. There really is a critical point. We observed the system in a whole range of different water vapors. This is the strongest evidence for any physical self-organized critical system to really have a critical point."

How does a critical threshold point work?

Consider a pile of rice, Peters said. You can add a single grain of rice and measure its effect on the pile. After slowly adding rice grains, at some point you eventually trigger an avalanche; the release is very fast.

A similar principle is behind the coin machines you can find in casinos, where it looks as if dropping in one or two quarters will create an avalanche of coins that will come crashing down for you. In fact, it is much more likely that it only looks like the system is at a critical point; you are more likely to lose your quarter.

Imagine that you add one raindrop into a cloud. Like the pile of rice, where adding a single grain can produce an avalanche or nothing at all, or like the coin machine, the one additional raindrop could trigger a huge downpour, but most of the time produces nothing. You can heat a magnet to a point where it loses its magnetization; it no longer has a north and south direction.

"When a magnet is near the critical temperature, a slight perturbation can cause it to switch north and south," Peters said. "When the system reaches the critical point and is so susceptible, a slight change - one more grain of rice, one more coin - can produce a massive response of the system. This phenomenon can be studied using statistical mechanics and critical phenomena."

The sun slowly evaporates water from the oceans, pumping water into the atmosphere. Much of that water vapor is stored and transported in the atmosphere before there is any rain, Neelin noted.

What finally triggers the rain?

Peters and Neelin were able to tie their findings back to seminal work in the 1970s at UCLA by Akio Arakawa, who sought to connect what is known about individual clouds to larger-scale atmospheric motions. As these motions increase water vapor in some regions, clouds begin to rise, heated by the condensing rainwater.

Arakawa postulated a balance between the clouds and the large scale. But if the clouds were reacting smoothly to the large-scale flow, rain would be much more predictable than it is.

"Arakawa's work was amazingly far-sighted," Neelin said, "but there's a new twist."

Complex interactions among the cloud motions organize the rainfall into clusters in space and a cascade of smaller and larger rain events. And these share the same mathematical structure as systems that physicists have studied.

"It's very hard to predict rainfall because of this type of interaction," Peters said. "In the last 20 years, scientists have become much better at predicting temperature and wind, but predicting precipitation has not improved much.

"Whenever you find different systems that are governed by the same mathematical laws, you are hitting on something fundamental. You have found a thread in the mathematical fabric of reality. This study raises the concept of 'self-organized criticality' to a higher status. It's not just a far-fetched possibility."

The research is federally funded by the National Science Foundation and the National Oceanic and Atmospheric Administration.

Peters began studying "avalanche distributions" in 2002, measuring how much rain falls in one storm. This led him to make predictions about the functional relationship between water vapor and rainfall.

"It's a self-organized critical system, from which we can make predictions," said Peters, who described physics as "beautiful."