Hurricane Laura has made landfall in south-western Louisiana as one of the most powerful storms to hit the state, with forecasters warning it could push a massive wall of water 65 kilometres inland from the sea.

The National Weather Service said the storm surge, possibly higher than a two-storey house, could be “unsurvivable,” acknowledging that as an unusually dire warning.

Laura crashed ashore about 1am local time on Thursday as a Category 4 storm, the second strongest on the five-step scale, packing winds of 240km/h in the small town of Cameron, Louisiana, the National Hurricane Centre (NHC) said.

By 4am it had been downgraded to a Category 3 storm with the centre about 50 kilometres north-north-west of Lake Charles, Louisiana, the NHC said.

Here are the Key Messages for Thursday morning for Hurricane #Laura. Catastrophic storm surge, extreme winds and flash flooding continues in portions of Louisiana. More: https://t.co/tW4KeFW0gB or your local weather forecast at https://t.co/SiZo8ohZMN pic.twitter.com/VSjWKiu45I

— National Hurricane Center (@NHC_Atlantic) August 27, 2020

Besides threatening life, the storm was barrelling toward the heart of the US oil industry, forcing oil rigs and refineries to shut down production.

“The eyewall of Laura will continue to move inland across southwestern Louisiana during the next several hours,” the NHC said in a early Thursday bulletin. “TAKE COVER NOW!”

Maximum sustained winds had slowed to 195km/h, but were still strong enough to blow out windows in Lake Charles’ 22-floor Capital One Tower, social media imaged showed.

Officials across the hard-hit area said it would be several hours before they could get out to begin search and rescue missions.

Downed trees blocking roadways were expected to be the biggest immediate challenge for rescuers.

BREAkING! Buildings gutted in Lake Charles LA in powerful eye wall of #HurricaneLaura @RadarOmega_WX @ChasinSpin pic.twitter.com/wP3fPyREIZ

— Reed Timmer, PhD (@ReedTimmerUSA) August 27, 2020

About 620,000 people were under mandatory evacuation orders in Louisiana and Texas, but officials acknowledged many people would choose to stay home.

In Vermilion Parish, just east of Laura’s landfall, the sheriff’s office gave them a stark warning.

“If you choose to stay and we can’t get to you, write your name, address, social security number and next of kin and put it a ziplock bag in your pocket,” read a statement on the sheriff’s Facebook page.

Hurricane-strength winds could blow as far as 320 kilometres inland to Shreveport, Louisiana, forecasters said.

“This is one of the strongest storms to impact that section of coastline,” said David Roth, a forecaster with the National Weather Service.

“We worry about that storm surge going so far inland there because it’s basically all marshland north to Interstate 10. There is little to stop the water.”

Hurricane Laura is the strongest storm in more than 150 years to hit Louisiana.
@GaryTuchmanCNN, on the ground in Lake Charles, Louisiana, says it “felt like we were experiencing an earthquake with dozens of aftershocks.” https://t.co/r2jslJviut pic.twitter.com/oTbSPaY8n8

— CNN (@CNN) August 27, 2020

The storm surge could penetrate inland from between Freeport, Texas, and the mouth of the Mississippi River, and could raise water levels as high as six metres in parts of Cameron Parish, Louisiana, the NHC said.

“To think that there would be a wall of water over two stories high coming on shore is very difficult for most to conceive, but that is what is going to happen,” said National Weather Service meteorologist Benjamin Schott at a news conference.

“The word ‘unsurvivable’ is not one that we like to use, and it’s one that I’ve never used before,” Mr Schott said of the storm surge.

Laura was also expected to spawn tornadoes on Thursday over Louisiana, far south-eastern Texas and south-western Mississippi, with widespread flooding likely from far eastern Texas across Louisiana and Arkansas.

-AAP