The National Weather Service warned late last night that a "potential moderate to major flood situation" exists here in the wake of heavy rains and melting snow that have transformed the area from snow-clogged to waterlogged.

The warning came as the weather service reported that about an inch of rain soaked the Potomac River basin yesterday. Heavy rains expected today could add another 1 1/2 inches, the weather service said, adding that melting snow could provide the equivalent of one more inch.

As a result, the weather service said, the river might reach or exceed flood stage by Monday in the Washington area.

The weather service extended through tonight a flash flood watch for all parts of the Washington Metropolitan area.

The watch means that a potential exists for flooding along small streams and that water may accumulate in poorly drained spots, including those where normal runoff channels are blocked by ice or snow.

Meanwhile, automobile traffic in the area was heavy yesterday as shopping centers filled with normal Saturday crowds. Cars were delayed by flooded intersections and by potholes, some hidden under murky water.

Flooding and repair of potholes caused at least temporary closing of roads throughout the area. Rock Creek Parkway and portions of Beach Drive were closed last night because of high water, and in Montgomery County part of Rte. 28 near Darnestown was closed along with Seneca Creek Road in Potomac. Ice jams were associated with high water in Rock Creek and Seneca Creek.

Part of Old Colchester Road in southern Fairfax County was closed under 4 feet of water and stretches of Glenn Dale Road and Northern Avenue in Prince George's County also were blocked. The Kenilworth Avenue overpass at Rte. 450 in the county was reopened after repair of a giant pothole.

Last night, D.C. and suburban fire departments reported receiving hundreds of calls from homeowners whose basements had flooded.

"We bring our portable pumps if the flood is bad," said Sgt. Robert Harding of the D.C. fire department. "Sometimes people have 4 and 5 feet of water; other people have just an inch, but that's a flood to them because it ruins their furniture."

Harding said his department had received about 100 such calls between 4 p.m. and 9:30 p.m., and suburban officials reported a similar pattern of calls.

During the day, area shopping centers and stores began filling as many shoppers made their first outings since Monday's snowstorm. Many stores were filled with merchandise from the George Washington's birthday sales postponed by the storm.

"Early this week it was like a ghost town," said Conan Paige, a caricature artist working in Landover Mall. "I'm glad to have them back," he said, gesturing at the Saturday shoppers.

The number of shoppers in the Prince George's County mall was about normal for a Saturday, most store managers who were interviewed said.In spite of the rain and the slush, people came out.

"I'm not really a sales shopping person," said Beverly Eicher of Bowie. "I'm really just here to get out of the house."

"I feel like the song says -- don't fence me in," said Maria Lopez of Capitol Heights. Lopez, 80, said she had been out last week "everytime I could get out of the house without falling in the snow."

With her street unplowed for three days, that had not been for or very often, she said. Lopez said she had planned a trip to California that had been postponed by the snow. Now she was out buying one more item to take on her trip to see friends and two grandsons.

"I made a silly decision not to go to California until the sun came out. Then the snow happened," she said. "It's like living in Alaska."

"It feels really good to be out," said Jean McGrew, shopping with her 4-year-old son and her 14-month-old daughter. She hadn't been out all week, except to walk to the store, she said. "When you start feeling cooped up and the kids start feeling cooped up, you start climbing the walls," said McGrew.

Many of the shoppers were women and couples with children. "It would have been better to come without the kids," said Bonnie Deibler of Charles County, shopping with a 2-month-old baby and an 8-year-old daughter.

It was a change for children, too, who had been confined to their houses. "I've been working, doing reading, math, and spelling," said Anhela St. Hill, 8, of Forestville. She also had been passing the time watching "Guiding Light and all those stories." she said. The snowbound days had often been "boring," her cousin, Clive Wynter, 11, of Southlawn, said.

Most of the merchants interviewed in the mall yesterday said they did not expect to recapture business lost when the George Washington's birth day sales day was snowed out.

In a bookstore, a customer stood by a paperback bookstand, reading a disaster story called "Blizzard."