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Alan and Dot Richardson, from England, wear ponchos as they walk along Bourbon Street in the French Quarter Friday, July 12, 2019, in New Orleans, ahead of Tropical Storm Barry. Image Credit: AP

Port Sulphur, United States: Millions of residents in Louisiana on Friday braced for impact from Tropical Storm Barry, which is threatening the southern US state and its largest city New Orleans with potentially disastrous rainfall and flooding.

Authorities ramped up evacuations, airlines cancelled flights and flood gates slammed shut as the National Hurricane Center forecast the strengthening storm would reach hurricane status Saturday and roar ashore along the state's central coast.

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

Thousands packed up and left their homes as flooding hit low-lying areas like Plaquemines Parish, where road closures left some communities isolated as they braced for Barry's arrival.

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A sign reading "Sorry We Have Abandoned Ship" is seen in the window as sandbags line store front of a restaurant in New Orleans, Louisiana, on July 12, 2019 ahead of Tropical Storm Barry. Image Credit: AFP

By late Friday the storm was packing winds of 100km/h, 14km/h shy of hurricane strength.

Dozens took shelter in Plaquemines's Belle Chasse 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.

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The Belle Chasse Auditorium is pictured its acting as a temporary shelter for 70 people from Tropical Storm Barry in Belle Chasse, Louisiana on July 12, 2019. Image Credit: AFP

Governor John Bel Edwards said New Orleans was well prepared to withstand the storm, but urged vigilance by residents there and 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.

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

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Map shows the accumulated precipitation forecast and rive gauge levels for the area affected by severe weather in Louisiana Image Credit: Agencies

With Barry threatening massive rainfall across several states including Louisiana, Mississippi, Alabama, Arkansas and Tennessee, states of emergency were being declared to help free up federal resources to address the storm.

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

Some Plaquemines residents were battening down to ride out the storm, despite mandatory evacuation orders.

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Residents clean out storm drains on Friday in the Mid-City neighborhool, notorious for its extreme flooding in New Orleans. Image Credit: Washington Post

"We've stayed for some pretty strong storms and we shouldn't have," said Keith Delahoussaye, a 60-year-old mechanic, at his trailer home in Port Sulphur.

But he was keeping a close eye on the level of the Mississippi River nearby. "If we see the water rising here, we'll leave."

Donald Brown, operations manager for Plaquemines Medical Center, said most people had evacuated.

"I'd say they fear it every time," said Brown, 58, who survived Katrina and was staying behind to look after the medical center. He said "it would be devastating" if the river breached the levee.

Dangerous mix of conditions

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

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

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Water floods a street ahead of Tropical Storm Barry in Cocodrie, Louisiana, U.S., on Friday, July 12, 2019. Barry is expected to come ashore along the Louisiana coast as a hurricane early Saturday, bringing a dangerous downpour that could inundate rivers, streets and homes Image Credit: Bloomberg

With storm surges of three to six feet projected, and 10 to 20 inches of rain forecast, the river could breach the 20-foot-high 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 has taken the "unprecedented" step of closing hundreds of flood gates, largely due to near-historic high levels of the Mississippi River.

Authorities closed highways in several locations along the coast as flood waters began creeping in.

In St. John's Parish next to New Orleans, some communities were under two or more feet of water, local television footage showed.

Grand Isle, one of the inhabited barrier islands east of the storm's likely path, suffered a full power outage Friday, bringing crucial pumping stations to a halt, authorities said.

Residents and business owners in New Orleans were laying down sand bags and boarding up windows. City officials have set up shelters for residents.

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Terrence Watkins and his son Kang, 3, load up with water and other supplies in New Orleans, Friday, July 12, 2019, as Tropical Storm Barry threatens Image Credit: AP

In 2005, Katrina - the costliest and deadliest hurricane in US history - submerged about 80 percent of New Orleans, causing some 1,800 deaths and more than $150 billion (Dh550 billion) in damage.

The city's main sports arena, the Superdome, was turned into an emergency shelter during Katrina, and the facility was due to host a concert by the Rolling Stones on Sunday, but it was postponed by a day due to Barry.

"We're here with you - we'll get through this together," the band said in a statement.