Tropical Storm Barry could develop into a Category-1 hurricane as it makes landfall this week, bringing torrential rain.

Thousands of residents of the US state of Louisiana broke out sandbags or fled to higher ground Thursday as Tropical Storm Barry threatened to turn into the first hurricane of the season and blow ashore with torrential rains that could pose a severe test of New Orleans’ improved post-Katrina flood defences.

National Guard troops and rescue crews in high-water vehicles took up positions around the state as Louisiana braced for the arrival of the storm Friday night or early Saturday.

Barry could have winds of 110 kilometres per hour, very close to the threshold for a hurricane when it comes ashore.

But it is expected to bring more than 500 millimetres of rain in potentially ruinous downpours that could go on for hours as the storm passes through the metropolitan area of nearly 1.3 million people and pushes slowly inland.

Louisiana Governor John Bel Edwards, who declared an emergency earlier in the week as the storm brewed in the Gulf of Mexico, warned that the storm’s blow could form a dangerous combination with the already-high Mississippi River, which has been swelled by heavy rain and snowmelt upriver this spring.

“There are three ways that Louisiana can flood: storm surge, high rivers and rain,” Edwards said. “We’re going to have all three.”

He said authorities do not expect the Mississippi River to spill over its levees – something that has never happened in New Orleans’s modern history – but cautioned that a change in the storm’s direction or intensity could alter that.

Richard Vasquez packs his car with belongings as Tropical Storm Barry approaches land in Port Sulphur, Louisiana [Jonathan Bachman/Reuters]

As of Friday morning Barry was about 155km southwest of the mouth of the Mississippi, with winds around 85km/h. A hurricane warning was posted for a 160km stretch of Louisiana coastline just below Baton Rouge and New Orleans.

Southeast of New Orleans, authorities handed out sandbags and people piled into cars with their pets and began clearing out.

Plaquemines Parish, at Louisiana’s low-lying southeast tip, ordered the mandatory evacuation of as many as 10,000 people, and by midafternoon the area was largely empty.

Justice of the Peace David McGaha waited with his mother, his wife and their 15-year-old son and 11-year-old daughter for a ferry so they could evacuate to his mother’s house in Alabama.

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“If the river wasn’t so high, we’d probably stay. You have to worry about the water that’ll be pushing against those levees,” he said. “They made a lot of improvements to the levee, but they haven’t completed all the projects.”

Clarence Brocks, 65, a Plaquemines Parish native and volunteer fire chief who lost his home to Hurricane Katrina 14 years ago and had to start over from scratch, found himself packing up again.

“We’re in between two major bodies of water and the only thing protecting us is two, 18-foot (5.5 metres) levees, and one of them failed already for Katrina,” he said.

Torrential rains

The National Hurricane Center said as much as 500mm of rain could fall in parts of eastern Louisiana, including Baton Rouge, and the entire region could get as much as 250mm. The New Orleans area could get 250mm to 380mm through Sunday, forecasters said.

Meteorologist Benjamin Schott said the chief concern is not the wind. “Rainfall and flooding is going to be the number-1 threat with this storm,” he said.

New Orleans Mayor LaToya Cantrell said that the pumping system that drains the city’s streets is working as designed but that Barry could dump water faster than the pumps can move it.

“We cannot pump our way out of the water levels … that are expected to hit the city of New Orleans,” she warned.

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However, the city did not plan to order evacuations because Barry was so close and because it was not expected to grow into a major hurricane. Officials instead advised people to keep at least three days of supplies on hand and to keep their neighbourhood storm drains clear so water can move quickly.

Hurricane Katrina caused catastrophic flooding in New Orleans in 2005 and was blamed altogether for more than 1,800 deaths in Louisiana and other states, by some estimates.

In its aftermath, the Army Corps of Engineers began a multibillion-dollar hurricane-protection system that isn’t complete. The work included repairs and improvements to some 560 kilometres (350 miles) of levees and more than 70 pump stations that are used to remove floodwaters.

The National Weather Service said it expects the Mississippi to rise to 5.8 metres (19 feet) by Saturday morning at a key gauge in the New Orleans area, which is protected by levees 6 to 7.6 metres (20 to 25 feet) high.