Tropical Storm Barry gathered strength on Friday as it chugged toward water-logged New Orleans, which girded 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 on Friday or early on Saturday when it nears Louisiana's coast, according to the National Hurricane Centre (NHC).
The NHC noted that sustained winds had increased to 80km/h, with higher gusts, and the storm will bring "life-threatening flooding" to coastal and river areas.
With Barry just 145km from the mouth of the Mississippi River, parts of Louisiana called for mandatory evacuations.
Governor John Bel Edwards said New Orleans was well prepared to withstand the storm, and the levees protecting it were not expected to be overtopped anywhere along the Mississippi River.
"But this is going to be a very, very significant rain event across most of Louisiana," he said on CNN. "This is going to impact just a huge swath of our state."
LaToya Cantrell, mayor of the city known for its Mardi Gras and jazz, warned residents to review their emergency plans and supply kits, and to stay updated on the forecasts.
US President Donald Trump declared a state of emergency and 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.
The Department of Homeland Security (DHS) announced it would temporarily halt immigration enforcement activity in areas subject to the state of emergency.
It said the Immigration and Customs Enforcement (ICE) agency, which is expected to begin sweeping operations nationwide to detain and deport illegal immigrants, would not target migrant families evacuating during the storm.
"Our highest priority remains the preservation of life and safety," the DHS said.
'A little nervous'
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 4.9m in New Orleans late on Thursday, one foot shy of flood stage.
With storm surges from the Gulf projected to reach two to four feet, the Mississippi has the potential to breach the 6m-high levee system protecting the city of 400 000.
The NHC said the center of Barry will be near the central or southeastern coast of Louisiana on Friday night or Saturday.
The NHC has upped its projections of Barry's rainfall totals to 25 to 50cm, with up to 63cm 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.
By late afternoon, strong steady winds were buffeting the city where some store owners laid sandbags or boarded up window fronts, while tourists lounged in cafes or snapped photographs of the swollen river.
"I'm a little nervous," admitted Lorraine Jones, who was visiting from Charlotte, North Carolina to attend a sorority convention.
"Right now I feel safe, but if push comes to shove, we'll make a move," she told AFP.
In 2005, Katrina - the costliest and deadliest hurricane in US history - submerged about 80% 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 about 1 800 deaths and inflicting more than $150bn 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.
Residents in some parts of New Orleans, which is under a tropical storm warning, waded through calf-deep floodwaters following Wednesday's intense rains and cleared 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.
The governor has authorized the mobilisation of up to 3 000 members of the National Guard.
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