The second major thunderstorm in two days battered the Washington area yesterday, causing a second round of power outages, fallen trees and flooded streets only 19 hours after the one Saturday evening.

Municipal and utility cleanup crews were still repairing downed electrical lines and pumping out waterlogged parts of the Metro subway system yesterday when the second storm hit with similar anger and punch. Its power varied, but on average the storm unleashed two inches of rain in an hour starting around 2 p.m.

The National Weather Service predicted that that may not be the last of it. The same kind of thunderstorm probably will hit the area today and tomorrow, said Weather Service specialist Joe Cefaratti.

"It's just so hot and humid, it builds up," he said.

Temperatures are expected to go up to the low or mid-90s today, and about 90 tomorrow.

Yesterday, like the day before, the extent of the rainfall -- and of the damage -- varied widely by location because of the quirkiness of heavy rain on summer days with high humidity. No death or severe injury was reported as a result of either storm.

Potomac Electric Power Co. and Virginia Power officials said at 5 p.m. yesterday that about 10,000 homes were without electricity -- 6,000 in Maryland and D.C., mostly around Bethesda, Rockville and Hyattsville; and 4,000 in Virginia, mostly in Alexandria and Arlington and Fairfax counties.

In Montgomery County, fire officials said two houses and a home furnishings business were damaged by fire caused by lightning yesterday.

Late yesterday afternoon, the Weather Service briefly issued a tornado warning for eastern Loudoun County, around the Arcola area, and Fairfax County. Loudoun Sheriff's Department officials said a small tornado apparently passed through that section around 4 p.m.

The sheriff's office said that no tornado was seen but that residents reported hearing what they believe was the whoosh of a tornado. Officers there and in Fairfax said their counties incurred extensive damage to trees and property from wind and rain.

The Weather Service said some parts of southwestern Loudoun received more than four inches of rain in an hour yesterday after about 2 p.m. -- the average for a month -- while parts of Prince George's County received the equivalent of a light hosing down, about one-twentieth of an inch.

On Saturday night, weather equipment at National Airport found that rain drenched that area with 1.7 inches in one hour starting around 7 p.m., Weather Service officials said.

For utility and repair crews, these two storms were reminiscent of the back-to-back snowstorms this past winter. Water and sewer crews spent much of yesterday pulling refuse from backed-up sewer lines that had brought standing water to nearby intersections -- only to see a repeat downpour.

Meteorologists explained that the last two days of rain are not the result of any large weather system or line of thunderstorms moving through the area. Instead, the storms were caused in part by a buildup of heat and humidity, which drives the moisture into the atmosphere, forming rain clouds that eventually burst.

The lack of wind that usually accompanies thunderstorms means that the rain clouds stand still as they empty themselves, said Weather Service meteorologist Kevin McCarthy.

"It all falls in one spot, essentially, instead of moving on," he said.

Also, the Weather Service said that helps explain why certain areas, such as sections of Arlington and parts of Virginia and Maryland far from the District, had little rain Saturday while sections of the District were under water.

On Saturday, the "isolated cells" of thunderstorms seemed to center on urbanized areas that tend to be several degrees warmer than underdeveloped and rural sections. The storm clouds gathered over these citified areas, with the heat shimmering off pavement and buildings, and then drenched them.

On Saturday, many of the complaints about standing water and power outages centered in and around the District.

Some of the worst flooding in the area occurred in the Trinidad section of Northeast Washington, where residents waded through thigh-high water.

The University of the District of Columbia announced the temporary closing of its Carnegie Library downtown, where five to eight feet of water stood in two basement levels yesterday. The water immersed a security officer's car parked in an underground driveway and may have damaged books in the graduate library in the 84-year-old building's basement, said university spokesman John Britton.

In the Metrorail system, Saturday's rain came so fast that it poured through entrances and openings and flooded tracks on downtown sections of the Red and Yellow lines. That caused the rail line to shut down automatically in those stations for safety reasons.

By yesterday, the only impassable section of subway was on the Yellow line around the National Archives station. Service resumed midafternoon yesterday after crews spent the day pumping water from the tunnel there. Their work was hampered by floating wood and gravel brought by the water from a street-level construction site near the subway entrance.

One reason rainwater collected in certain downtown Metro stations -- including Farragut North, Metro Center, Archives and Judiciary Square -- is that they are at a lower elevation than most stations, said Metro spokesman Mary Bucklew.

Nancy Moses, spokesman for Pepco, said that Saturday night the utility experienced a variety of problems that cut power to 8,900 homes in the District and Maryland.

Most customers had power restored by 10:30 p.m. Saturday night.

But hundreds of area residents experienced flickering power in their homes most of yesterday, only to watch it go down again with yesterday's rain.

Staff researcher Blaine P. Friedlander and staff writers Mary Jordan and Retha Hill contributed to this report.