Thousands in US state break out sandbags or flee to higher ground as storm develops into a powerful hurricane.

Tens of thousands of people in the US state of Louisiana and its largest city New Orleans braced themselves for Tropical Storm Barry to hit on Saturday amid fears it could turn into a powerful hurricane.

People boarded up buildings, stocked up on water, and readied for torrents of rain and punishing wind as Barry threatened millions as it churned a path ashore and tested efforts to guard against flooding since Hurricane Katrina devastated New Orleans 14 years ago.

Nearly 50,000 people were without power.

Authorities ramped up evacuations, airlines cancelled flights, and floodgates slammed shut as the National Hurricane Center (NHC) forecast the strengthening storm would reach hurricane status on Saturday and roar ashore along the state’s central coast.

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

The large storm system currently in the Gulf of Mexico is bringing heavy rains, a potential storm surge, and flooding that pose a threat reminiscent of 2005’s deadly Hurricane Katrina.

The costliest and deadliest hurricane in US history, Katrina submerged about 80 percent of New Orleans, causing some 1,800 deaths and more than $150bn in damage.

By early Saturday the storm was packing winds of 100 kilometres an hour, just shy of hurricane strength, according to NHC.

‘Life-threatening’

Dozens took shelter in an auditorium while others headed inland to stay with friends or relatives and avoid what the NHC called “life-threatening flooding” to coastal and river areas.

Governor John Bel Edwards said New Orleans was well prepared to withstand the storm, but urged vigilance by residents across the state, as authorities called on people to stay off the streets.

“No one should take this storm lightly, and I urge everyone to remain informed,” Edwards said on Twitter.

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New Orleans Mayor LaToya Cantrell urged people to stay indoors, but the city’s famed party atmosphere was far from extinguished just hours ahead of the storm.

With Barry threatening massive rainfall across several southern states, federal emergency declarations were issued to help free up resources to address the storm.

“We could be looking at widespread major flooding across several river basins,” NHC said.

Mississippi swelling

The level of the Mississippi River, already swollen from historic rains and flooding upstream, was at 4.9 metres in New Orleans, about 30cm shy of flood stage.

River levels were expected to peak at 5.2 metres, according to a forecast late Friday by the National Oceanic and Atmospheric Administration.

But with storm surges of one to two metres projected, and 25-50cm of rain forecast, the river could still breach the 6.1 metres levee system protecting the city of 400,000.

“Much of the Gulf Coast, especially Louisiana, are already at extremely high water levels and so the heavy rains and any potential storm surge will lead to dangerous flash flooding,” Jill Trepanier, an expert at Louisiana State University, said in a statement.

Mike Yenni, president of Jefferson Parish near New Orleans, said the community had taken the “unprecedented” step of closing hundreds of floodgates, largely because of near-historic high levels of the Mississippi River.

Residents and business owners in New Orleans were laying down sandbags and boarding up windows while city officials set up shelters for residents.