Morning tidal flooding left King and Union streets underwater Monday in Alexandria. (Matt McClain/The Washington Post)

Virginia Gov. Ralph Northam on Monday ordered the evacuation of about 245,000 residents in flood-prone areas as Hurricane Florence strengthened on its path toward the East Coast.

Residents were to be evacuated by 8 a.m. Tuesday in the Hampton Roads area — including portions of Virginia Beach, Hampton and Norfolk — as well as part of the Eastern Shore. Northam (D) said he had requested 21 swift-water rescue teams from other states and activated 1,500 of the state’s 6,000 National Guard members, with the rest on standby.

“I’m making this announcement now to give residents, families, schools and businesses time to prepare,” Northam said.

The Category 4 storm rapidly intensified Monday, with winds gusting to 140 mph. The National Hurricane Center said Florence is expected to strengthen to 150 mph before making landfall somewhere on the southeast or Mid-Atlantic coast Thursday night.

Forecasters said the hurricane could slow or stall out over the Mid-Atlantic later this week, which could lead to significant rain and bring wind that could down trees.

Refusing to comply with the evacuation order would be a misdemeanor, but officials said they do not have the resources to go door-to-door. As for where evacuees should go, Northam said, “The simplest answer is, go to higher ground and inland.”

Officials said local governments would have information on shelters throughout the state. Residents in coastal areas can check to see which zone they live in by going to

“If it stalls, we’re expecting significant rainfall,” Northam said. “The largest threat to life from hurricanes is not the high winds. Flooding is the deadliest result of these storms.”

The Navy on Monday ordered warships in Norfolk to sea to avoid the storm.

Across the region, officials began implementing plans to prepare for what forecasters say could be only the 11th major hurricane to hit the southeast coast since 1851.

In the District, a flood levee built to protect parts of downtown could be deployed for the first time as the region braces for the possibility of heavy rain from Florence.

National Park Service spokesman Mike Litterst said the agency is monitoring forecasts and stream gauges to determine whether conditions warrant deploying the levee across 17th Street NW, just south of Constitution Avenue. It would be the first time officials have used the levee.

Rain from Florence would arrive at a time when the ground already is saturated in the region. Flood warnings were issued Monday for much of the Potomac River upstream from the District, and segments of the Chesapeake & Ohio Canal were closed because of high water after weekend storms.

Hains Point in East Potomac Park was inaccessible because of flooding. The Park Service posted a photo Monday on social media showing egrets, herons and mallards standing on posts as walkways along the Tidal Basin and Jefferson Memorial were underwater.

On the Mall, officials have conducted four practice runs to put the city’s newest levee in place.

The levee, in part, is made of two concrete and stone walls on either side of 17th Street. In the event of a flood, a barrier of metal posts and panels would be installed with a crane to span the street and connect with the two walls.

“We’ve double-checked everything . . . and it’s all ready to go,” Litterst said.

The post and panels are stored at a Park Service maintenance facility in the District’s Brentwood neighborhood and would be brought to the site. The levee system, finished in 2014, is designed to keep floodwaters from flowing north on 17th Street from the Tidal Basin.

The levee was built after the Federal Emergency Management Agency determined a flood could inundate a crescent of downtown Washington from 17th Street and Constitution Avenue east to the Capitol and south toward Fort McNair. The National Weather Service, Park Service and Army Corps of Engineers are continuing to consult on whether it will be deployed.

In Maryland, Gov. Larry Hogan (R) signed an executive order Monday issuing a state of emergency for what he called potential “historic, catastrophic and life-threatening” flooding. He said during a news conference that officials should know more in the next two days about Florence and its possible effects.

“There is still some uncertainty of the path of Hurricane Florence,” Hogan said, “but our state is taking every precaution and I urge all Marylanders to do the same.”

Russell J. Strickland, executive director of the Maryland Emergency Management Agency, said the executive order puts the National Guard and state agencies on alert. He said the state is particularly concerned about low-lying areas, including Ellicott City and Frederick, which experienced flooding earlier this year.

D.C. officials Monday were discussing preparations for the storm, according to a spokeswoman for Mayor Muriel E. Bowser (D).

According to The Washington Post’s Capital Weather Gang, forecast models suggest more than a foot of rain could fall over higher elevations of the Carolinas and Virginia, which would generate dangerous flooding downstream. The heaviest rainfall could begin Friday or Saturday and continue into the following week.

Parts of the region experienced flooding Monday after heavy weekend rain.

Several roads in the Frederick area were closed. The Monocacy River bridge along Route 355 near Frederick had water approach record levels, leaving residents worried about the impact of more rain from Florence. The storm also altered Amtrak schedules south of Washington beginning Wednesday.

Officials with Maryland Natural Resources Police warned that rivers and streams are swollen and advised the public to stay away from rushing water. At Assateague State Park on Maryland’s Eastern Shore, beach access was closed because of dangerous surf and waves more than 12 feet higher than normal.

In Alexandria’s low-lying Old Town neighborhood, sandbags were being distributed to businesses and residents.

A pedestrian walks on a sidewalk along King Street after morning tidal flooding Monday in Alexandria. (Matt McClain/The Washington Post)

Raytevia Evans, a spokeswoman for the Alexandria Fire Department, said officials spoke with members of several city agencies to make preparations before Florence’s rain arrives. She said officials would keep swift-water rescue teams on call and develop plans to use schools and recreation facilities as shelters if flooding occurs.

Crews were watching Weather Service predictions, Evans said, adding that the city does “absolutely expect we’ll get a whole lot of rain.”

“We’re making sure people have a plan now,” she said. “We’re letting people know to communicate with their families, their employees, and prepare their businesses and homes for possible impact.”

Ovetta Wiggins, Fenit Nirappil and Michael Brice-Saddler contributed to this report.