The Potomac River on Tuesday. (Calla Kessler/The Washington Post)

The District declared a state of emergency on Tuesday, following Maryland and Virginia as the region braces for Hurricane Florence to hit the East Coast later this week.

Mayor Muriel E. Bowser (D) joined city leaders including D.C. police, fire, transportation and public-works officials during a news conference to outline how authorities are preparing for the storm. District officials said they believe a main threat will be periods of heavy rain, starting Thursday night and into Friday.

“We will see torrential rain for at least three to four days,” said Christopher Rodriguez, director of the D.C. Homeland Security and Emergency Management Agency.

Florence expanded in size Tuesday, continuing on a beeline toward the East Coast as an “extremely dangerous” Category 4 hurricane with winds gusting to 140 mph, the National Hurricane Center said.

Forecasters expect the storm to make landfall between northern South Carolina and North Carolina’s Outer Banks as a strong Category 3, although shifts in the track are possible. Catastrophic flooding and destructive winds are likely in the eastern Carolinas.

If the storm drifts into Virginia, the Washington region will be particularly susceptible to flooding because of above-normal rainfall since May, including heavy rain in recent days. Trees will also be vulnerable in strong winds because the ground is already saturated. The Washington Post’s Capital Weather Gang said Tuesday that the storm’s extreme rainfall is likely to stay south of Washington.

D.C. Water crews are working overtime this week to clear drains along streets in anticipation of debris from the storm. About 500 D.C. Water employees are prepared to go to areas known to flood in heavy storms, officials said.

Workers also are putting up sea walls at the Blue Plains treatment center in Southwest Washington to avert problems with storm surge.

Pepco has 600 workers ready to respond to power outages and an additional 150 employees to handle downed trees. The utility is moving to bring in more workers from Chicago and Philadelphia, said David Velazquez, Pepco’s president and chief executive.

Metro is deploying more than 1,000 sandbags at stations prone to flooding, including Cleveland Park, Federal Triangle and Shaw, officials said. Metro crews were checking pumps and generators in the system to ensure they were ready to respond to outages.

A portion of the Mount Vernon Trail is flooded Tuesday in Alexandria. (Matt McClain/The Washington Post)

The rail system expects to operate normally, but officials warned they might need to make changes as the storm unfolds. If winds surpass 45 mph, changes to rail and bus schedules are likely. Customers who use the MetroAccess ride service were being advised to cancel trips for Thursday and Friday.

“They’re some of our most vulnerable customers, and we want to make sure they remain safe,” said Paul J. Wiedefeld, Metro’s general manager.

Bowser said that her staff is continuing to watch the forecasts and that “we know there will be flooding, heavy rains and wind.” The city last declared a state of emergency during preparation for a snowstorm in January 2016.

Bowser suggested residents make grocery-store purchases ahead of time and ensure they have necessary medications, flashlights and batteries on hand. She encouraged residents to use 311 to report flooding and other non-emergency needs.

She said some events, including Fiesta DC and the H Street Festival, will be rescheduled because of the weather.

At Frager’s Hardware on E Street SE near Capitol Hill, store manager Aisha Bryant said Tuesday that “absolutely we’ve seen an influx in customers” with Florence looming.

She said it started Monday morning with increased numbers of customers coming in to buy sandbags, tape to seal windows, flashlights and batteries. She said some customers were buying plywood to take to their beach homes in Maryland.

Bryant was planning to have additional staffing throughout the weekend as the storm unfolds — even though Florence was not bringing in customers at the rate of a snowstorm.

“We just received a truck yesterday,” she said, “and I am sure most of that will get sold pretty fast.”

Virginia declared a state of emergency over the weekend, and Maryland did the same on Monday.

Virginia Gov. Ralph Northam (D) ordered the evacuation of about 245,000 residents in low-lying portions of the Hampton Roads area and the Eastern Shore. President Trump declared a federal state of emergency for Virginia on Tuesday, following similar declarations for North and South Carolina.

In the District, a flood levee across 17th Street NW, just south of Constitution Avenue, could be used for the first time to protect the downtown area from flooding.

In Howard County, officials said they have been rolling out emergency preparedness plans this week and making sure crews, equipment, chain saws and other supplies are in place.

Ellicott City, a historic town in the county, is still recovering and rebuilding after it was hit hard by flooding earlier this year. Crews have worked to secure roads that are being rebuilt before the heaviest rain arrives.

Ryan Miller, director of emergency management for Howard County, said the advantage of a hurricane is that, unlike flash flooding that happened in the county last week, there is time to plan ahead.

“People had almost no time to prepare,” he said. “A tropical event like this is more widespread and more aerial.”

He said the weather forecast for Howard County was calling for between 2 and 6 inches of rain later this week. “As long as that comes over a day or day and a half,” Miller said, “you have time to react.”

Jason Samenow contributed to this report.