High winds and tides sent water surging onto the banks of the Potomac River and the shores of Chesapeake Bay last night, while continuing heavy rains added to the threat of additional, possibly more severe flooding later this week.

Roads were closed on both sides of the Potomac, several dozen Southern Maryland residents fled their homes, and businessmen began heaping up sandbag barriers as flooding was reported last night in low-lying areas from Alexandria to Annapolis.

Several days of east wind "has pushed the water up" the bay and its tributaries and "hasn't let the tide take it out," said Lt. Cassin Gittings of the Annapolis police.

Meanwhile, heavy rains falling over the headwaters of the Potomac could swell the river to as much as six feet above flood stage at the foot of Wisconsin Avenue NW by Thursday, said Leo Harrison, a hydrologist and river forecaster with the National Weather Service.

Within the last two or three days as much as six or seven inches of rain has fallen in the upper reaches of the river, raising the possibility that river levels could match those attained after Tropical Storm Agnes in 1972, Harrison said.

In the 24 hours ended at 7 p.m. yesterday, the official measuring station at National Airport recorded 0.84 inches of rain. At Dulles the figure was 0.97 inch. But at Roanoke the figure was 6.34 inches.

"I'm looking at cars just floating away down the street," said Dan Culkin, a Roanoke area businessman. A Salem, Va., rescue squad member said people were climbing telephone poles on Main Street to avoid rapidly rising water.

As much as 20 inches of rain fell in parts of Virginia during the last few days, the weather service said, and Gov. Charles S. Robb declared a limited state of emergency.

Harrison said the Shenandoah River reached record levels in Waynesboro, Va., last night and was expected to reach 11 feet above flood stage in Front Royal today. Evacuations were under way last night in parts of the Shenandoah Valley.

Harrison said the wind, which was gusting to more than 30 miles an hour here last night, and the rain, which alternated between drizzle and downpour, were the products of a powerful atmospheric system that has been pumping moisture northward from the Gulf of Mexico.

People living on boats in the Fort Washington Marina in Prince George's County were evacuated, and Fairfax County authorities were preparing for possible evacuations in the Belle Haven area.

High water surged over the Annapolis City Dock yesterday, inundated St. George island in St. Mary's County, and shut down part of the Alexandria waterfront as well as long stretches of the George Washington Memorial Parkway south of Alexandria. Portions of Ohio and Anacostia Drives in the District also were closed.

About 55 people were evacuated in St. Mary's, and about two dozen Calvert County residents took shelter in a high school. Annapolis merchants sandbagged their premises and the Weather Service called people near the Potomac warning of possible further problems.