Tropical Storm Barry gathered strength late Thursday as it chugged toward water-logged New Orleans, with the city and Louisiana coast girding for heavy rains, storm surge and flooding that pose a threat reminiscent of 2005's deadly Hurricane Katrina.

The weather system, which has already caused major flooding in the low-lying city, is expected to reach hurricane strength Friday or early Saturday when it nears Louisiana's central or southeast coast, according to the National Hurricane Center (NHC).

The NHC noted that sustained winds had increased to near 50 miles (81 kilometers) per hour, with higher gusts, and the storm will bring "life-threatening flooding" to coastal and river areas.

With Barry just 90 miles (145 kilometers) from the mouth of the Mississippi River, Governor John Bel Edwards declared a state of emergency and two parishes, as counties are known in Louisiana, called for mandatory evacuations in some locations.

The southern state was bracing for what Edwards said could be "an extreme rain event" that impacts "a huge portion of the state."

New Orleans Mayor Latoya Cantrell warned residents to review their emergency plans and supply kits, and to stay updated with the latest forecasts.

President Donald Trump tweeted his concern to "everyone on the Gulf Coast," imploring them to prepare their homes for the storm and heed the directions of federal, state and local officials.

"Please be prepared, be careful, & be SAFE!" he wrote.

Louisiana is facing an extraordinarily dangerous confluence of conditions, according to experts.

The level of the Mississippi River, already swollen from historic rains and flooding upstream in the nation's Midwest, was at 16 feet (4.9 meters) in New Orleans late Thursday, one foot shy of flood stage.

The city of 400,000 is protected by a 20-feet-high levee system. With storm surges from the Gulf projected to reach two to four feet, the Mississippi has the potential to breach the levees.

The NHC said the center of Barry will be near the central or southeastern coast of Louisiana Friday night or Saturday.

The storm has slowed down in the Gulf, to about three miles (five kilometers) per hour, suggesting a longer drenching may be in store for Louisiana.

The NHC has upped its projections of Barry's rainfall totals from 10 to 15 inches to 10 to 20 inches (25 to 50 centimeters), with up to 25 inches in some areas, including just south of Louisiana's capital city Baton Rouge.

"Flash flooding and river flooding will become increasingly likely, some of which may be significant," the NHC said.

Despite the dire warnings, downtown New Orleans exuded a mix of preparation and relaxation as skies cleared for much of Thursday.

Some store owners laid sand bags or boarded up window fronts, while tourists lounged in cafes, snapped photographs of the swollen river and bought street art.

"I'm a little nervous," admitted Lorraine Jones, who was visiting from Charlotte, North Carolina to attend a sorority convention with thousands of fellow members.

"Right now I feel safe, but if push comes to shove, we'll make a move," she told AFP.

On Wednesday, officials said the 118 pumps spread throughout the city were operating at "optimum capacity."

In 2005, Katrina -- the costliest and deadliest hurricane in US history -- submerged some 80 percent of New Orleans as its flood defenses gave way.

Best remembered for the devastation wreaked on the city known as The Big Easy, Katrina also pounded other parts of Louisiana as well as Mississippi and Alabama, leading to some 1,800 deaths and inflicting more than $150 billion in damage.

If the storm becomes a hurricane as anticipated, it would be the first of the Atlantic season, which runs from June through November.

Crews from the state's transportation department erected barricades in New Orleans and cleaned out ditches and other debris ahead of the expected deluge.

A hurricane warning, which means hurricane conditions are expected within the area, is in effect for a wide swath of the Louisiana coast from Grand Isle, a populated barrier island, west to Intracoastal City.

Residents in some parts of New Orleans, which is under a tropical storm warning, were seen wading through calf-deep floodwaters following Wednedsay's intense rains, and clearing debris from their lawns.

Pedi-cab driver Grace Hack, 25, said she was keeping one eye on the river level and struggling to decide whether to stay or retreat to Atlanta with friends.

"It seems like today is the window for decision-making and evacuation," she said.

Mandatory evacuations were ordered for parts of Jefferson and Plaquemines parishes, and voluntary evacuation was in effect in Grand Isle, according to news website NOLA.com.

The governor has authorized the mobilization of up to 3,000 members of the National Guard.