Low-lying areas were bracing for flooding Monday, as workers cleared sewer grates and residents packed their belongings as mandatory evacuations began in a flood-prone area of Fairfax County.

The National Weather Service issued a flood watch for the Washington region, and the weather grew so treacherous that D.C. officials suspended passing out sandbags as they had on Sunday because they didn’t want residents venturing outside in Hurricane Sandy.

By midafternoon, Rock Creek had crested at six feet, meaning it was about to flood with the worst of the wind and rain still ahead.

As Battalion Chief Sherrod Thomas of the D.C. Fire Department drove along Beach Drive, he looked at the swiftly moving creek that in parts had already overcome the banks and was edging close to the narrow, twisting road and said, “You could go white-water rafting.”

In the Huntington area of Fairfax County, residents on two streets, Arlington Terrace and Fenwick Drive, were told to clear out as evening fell.

Police and firefighters were to begin going door to door by 6 p.m., and planned to shut off utilities soon after, said Capt. Kendall Thompson, with the Fairfax County Fire and Rescue Department. He said the order could widen to include neighboring streets.

A bus was on its way to evacuate residents without cars. Those with nowhere else to go were being directed to a county shelter at the Lee District recreation center at 6601 Telegraph Rd. Pets are allowed.

Although residents hoped to avoid evacuations, many were anticipating the order, as the neighborhood has flooded repeatedly in recent years. The streets in Huntington weren’t underwater early Monday night, but they were expected to flood later.

“The water is rising,” Thompson said. “This is the time for folks to get out.”

Rebecca Lombardi and her husband, Nico Mocci, spent the weekend getting ready — boarding up and sandbagging their basement windows and carrying belongings upstairs. Along with their cat, Henry, they plan to stay with friends, just as they did in 2011, when they came home to four feet of standing water.

“We’re just hoping it doesn’t go past the basement,” Lombardi said. “We just want to get this over with so we can come home and get started with the cleanup and the insurance.”

For the past two days, crews with the D.C. Department of Public Works have been in some neigh­borhoods, especially flood-prone Bloomingdale, clearing leaves off the roads and sidewalks, said Lon Walls, a fire department spokesman.

Mayor Vincent C. Gray (D) toured the neighborhood Sunday, giving out sandbags and encouraging people to be safe.

Residents of rowhouses lined sandbags along low-lying window sills. One house already decorated for Halloween placed a cardboard sign in the hands of a scarecrow in a hockey mask. “Die Sandy Die,” it read.

In Prince George’s County, workers put up sandbags and wooden barricades around the main government office building in Upper Marlboro, which sits near the Patuxent River and a pond. The building was flooded last year during Tropical Storm Lee, and recently the county installed its new 311 call-center offices in the same space. Officials said they are better prepared this year.

“We are doing everything we can,” said Tom Himler, an aide to County Executive Rushern L. Baker III (D). “The majority of the wall is up, and we are adding structure to it by putting up boards and trying to make it firmer.” Staff members will be standing by 24/7, he said, wet vacs at the ready.

By midafternoon, flooding had shut down only one road, Governor Bridge Road near the Anne Arundel border, Himler said.A storm surge of up to four feet is predicted at the city’s harbor, a level that officials said could submerge a dinghy dock and parking lot but should stop short of the iconic Market House and other shops and restaurants behind it that fell victim to Isabel in 2003.

In downtown Annapolis, where merchants still sell photos of the flooding left by Hurricane Isabel nine years ago, city officials were not expecting anything as bad.

Annie Gowen, Peter Hermann, Robert Samuels, Miranda Spivack and John Wagner contributed to this report.