Washington rode a weather roller coaster yesterday that swept it through rain, flooding and a blanket of evening rush-hour fog that further slowed traffic already impeded by streets filled with slush, potholes and puddles.

The overnight low was to get no lower than the 40s, but the National Weather Service said the pattern would change drastically - and for the worse - by afternoon today.

By then, forecasters said, the mercury would dip below freezing and the area would be buffeted by winds as high as 50 miles an hour. There also was a prospect of some snowfall, he said, that would coat mounds of slush turned back into ice.

The pattern began yesterday with 15 hours of rain that dumped more than an inch on the area by mid-after noon.

The rain, ice and melting snow combined also to snarl morning rush hours traffic. Minor flooding was reported throughout the area during the day and night.

The rain also caused the roof of the Rockville Mall to leak so badly that the shopping center was closed by city officials for "reasons of life safety.

A more critical situation confronted the Racquet Club, an indoor tennis club attached to the Mall, whose steel roof beams started buckling under the heavy wight of snow, ice and water. In an effort to prevent the roof from collapsing, firemen shoveled away some of the ice and snow, which reached a depth of two feet in some parts. By nightfall, a watch on the structure still was being maintained.

In the area, rain and melting snow clogged many sewer drains, leaving huge puddles at street corners. The weather also increased the number of potholes, slowing traffic and causing several minor accidents, according to area police.

"The sewer system is trying to take in two rainfalls in one day," said Harold Stern, chief of the District's Bureau of Water and Sewer Services. "Each dropping of rain today is melting some of the snow that's sitting out there. We're putting two rains down the hole at the same time."

Stern said the rain and melting snow are creating the worst drainage situation for the city since the blizzard of 1966.

Stan Ather, D.C. street maintenance engineer, said the "freezing and thawing cycle," of the city's weather has caused the largest number of potholes to crack open on city street in years.

"We've got 21 trucks on the streets filling holes today," Ather said. "We've got 75 men out there. It's an all-out effort. We're using 100 tons of asphalt a day and that's just on emergencies to avoid accidents and having people bang their cars."

Area police officials said there were few accidents during yesterday morning's rush hour. But they added that traffic remained heavy and major roads were clogged by slow-moving cars well into the afternoon.

"Seems to me like a lot of people took their cars this morning because of the rain," said one traffic official in Washington. "All day we've had heavy traffic crawling down K Street, M Street and Pennslyvania Avenue into the city."

At the corner of 21st and L Streets NW, Philip Earlie, an accountant with the Department of Housing and Urban Development was waiting for a tow truck to pull his white Porsche out of a pothole.

"I just bought it, I just bought it," he said, looking at his left wheel which was stuck in the gaping hole. "I think it'll be all right, but . . . you'd think the city or somebody would throw some asphalt in these things."

Morning rush hour commuters who chose public transportation also had their problems yesterday. Metro had one major breakdown on both the Red and Blue Lines.

A National Airport-bound train refused to move from the Foggy Bottom station for more than an hour and other trains were detoured around it on the opposite track. That caused about a five-minute delay to Blue Line passengers, Metro estimated.

On the Red Line, a train broke down just short of Union Station en route to Rhode Island Avenue and had to be pushed out of the tunnel by another train. That resulted in 20-minutes delays, according to Metro.

At the corner of 20th and M streets an oil truck, rushing through a changing stoplight, drenched Domimic Andosar, a bartender, with cold slush and dirty water.

"You can't walk here today," he said. "This is the third time this week they've doused me."

Sylvia Coombes, who was walking out of the George Washington University Clinic on Pennsylvania Avenue, took tip-toe steps along an key stretch of the street.

"I'm afraid of falling, always when the streets are like this," she said. "If you fall it's in the winter. I'm going home. I wouldn't have come out if I didn't have to go to the clinic."

In the suburbs streets also were icy and officials there were on the alert for further flooding.

Manassas Airport was closed during the afternoon for a few hours because of flooding of a nearby creek.

Schools in Loudoun County were closed yesterday because if flooding on back roads.