Hurricane Ida blasted ashore on Sunday as one of the most powerful storms ever to hit the US, blowing off roofs and reversing the flow of the Mississippi River.
The Category 4 storm rushed from the Louisiana coast towards New Orleans and one of the nation’s most important industrial corridors.
Ida’s 230kph made it the equal fifth-strongest hurricane to hit the mainland US.
The rising ocean swamped the barrier island of Grand Isle as Ida hit land just to the west at Port Fourchon.
Ida made a second landfall about two hours later near Galliano, churning through the far southern Louisiana wetlands, with more than 2 million people living in and around New Orleans and Baton Rouge under threat.
“This is going to be much stronger than we usually see and, quite frankly, if you had to draw up the worst possible path for a hurricane in Louisiana, it would be something very, very close to what we’re seeing,” Governor John Bel Edwards said.
People in Louisiana woke to a monster storm after Ida’s top winds grew by 72kph in five hours as the hurricane moved through some of the warmest ocean water in the world, in the northern Gulf of Mexico.
Wind tore at awnings and water spilled out of Lake Pontchartrain in New Orleans on Sunday, and boats broke loose from their moorings.
Engineers detected a “negative flow” on the Mississippi River as a result of storm surge, US Army Corps of Engineers spokesman Ricky Boyette said.
Mr Edwards said he watched a live video feed from around Port Fourchon as Ida came ashore.
“The storm surge is just tremendous," he said. "We can see the roofs have been blown off the port buildings in many places."
Officials said Ida’s swift intensification from a few thunderstorms to a huge hurricane in only three days left no time to organise a mandatory evacuation of New Orleans’ 390,000 residents.
Mayor LaToya Cantrell urged residents remaining in the city on Sunday to “hunker down.”
Marco Apostolico said he felt confident riding out the storm at his home in New Orleans’ Lower 9th Ward, one of the city’s hardest-hit neighbourhoods when levees failed and released floodwater during Katrina.
Mr Apostolico's home was among those rebuilt with the help of actor Brad Pitt to withstand hurricane-force winds. But the memory of Katrina still hung over the latest storm.
“It’s obviously a lot of heavy feelings,” he said. “And yeah, potentially scary and dangerous.”
The region being hit worst by Ida includes petrochemical sites and major ports, which could be significantly damaged.
New Orleans hospitals planned to ride out the storm with their beds nearly full, as similarly stressed medical centres elsewhere had little room for patients.
And shelters for those fleeing their homes carried an added risk of becoming flashpoints for new infections.
Forecasters warned winds stronger than 185kph threatened Houma, a city of 33,000 that supports oil platforms in the Gulf.
The hurricane was also threatening neighbouring Mississippi, where Katrina demolished oceanfront homes.
With Ida approaching, Claudette Jones left her home east of Gulfport, Mississippi, as waves started pounding the shore.
“I’m praying I can go back to a normal home like I left,” Ms Jones said. “That’s what I’m praying for. But I’m not sure at this point.”
Comparisons to the landfall of Katrina weighed heavily on residents bracing for Ida.
Katrina was blamed for 1,800 deaths as it caused levee breaches and catastrophic flooding in New Orleans.
Ida’s hurricane-force winds stretched 80km from the storm’s eye, or about half the size of Katrina, and a New Orleans’ official said that the city was in a “very different place than it was 16 years ago".
The levee system has been extensively overhauled since Katrina, Ramsey Green, deputy chief administrative officer for infrastructure, said before the worst of the storm hit.
Mr Green said if forecasts of up to 50 centimetres of rain proved true, the city’s underfunded and neglected network of pumps, underground pipes and surface canals probably would not be able to keep up.
About 530,000 customers were already without power late on Sunday afternoon, according to website PowerOutage.US.
The Louisiana Department of Environmental Quality was in contact with more than 1,500 oil refineries, chemical plants and other sensitive facilities and will respond to any reported pollution leaks or petroleum spills, agency spokesman Greg Langley said.
Mr Langley said the agency would use three mobile air-monitoring laboratories after the storm passed to sample, analyse and report any threats to public health.
Louisiana’s 17 oil refineries account for nearly one fifth of the US refining capacity and its two liquefied natural gas export terminals ship about 55 per cent of the nation’s total exports, the US Energy Information Administration says.
Louisiana is also home to two nuclear power plants, one near New Orleans and another 43km north-west of Baton Rouge.
US President Joe Biden approved emergency declarations for Louisiana and Mississippi before Ida’s arrival.
Mr Biden on Sunday said the country was praying for Louisiana and would put its “full might behind the rescue and recovery” after the storm passed.
Mr Edwards warned his state to brace for what could be weeks of recovery.
“Many, many people are going to be tested in ways that we can only imagine today,” he said.