Laura now a Category 4 hurricane as it approaches Texas, Louisiana

Laura rapidly gained strength Wednesday, growing into a menacing Category 4 hurricane with the potential for a six-metre high storm surge that forecasters said would be "unsurvivable" and capable of sinking entire communities. Authorities implored coastal residents of Texas and Louisiana to flee.

Officials have called Laura "a formidable hurricane" that threatens to smash homes and sink entire communities. It has undergone a remarkable intensification, and storm surge as high as 6.10 metres could occur in some coastal areas, the National Weather Service said.

"Conditions will rapidly deteriorate along the coast in a few hours," the agency said in a bulletin. "Preparations to protect life and property need to be completed."

Top winds of 233 kilometres per hour are now predicted before landfall, pushing water onto more than 724 kilometres of coast from Texas to Mississippi.

In the largest U.S. evacuation of the pandemic, more than half a million people were ordered Tuesday to leave an area of the Gulf Coast along the Texas-Louisiana state line.

"Heed the advice of your local authorities," said Joel Cline, tropical program co-ordinator at the National Weather Service. "If they tell you to go, go! Your life depends on it today. It's a serious day, and you need to listen to them."

WATCH l The view of Laura from overhead:

Laura grew nearly 70 per cent in power from the previous day, churning through warm Gulf of Mexico waters.

At 2 p.m. ET on Wednesday, Laura was located 320 kilometres from both Lake Charles, La., and Galveston, Texas. It was packing maximum sustained winds of 220 km/h and was moving northwest at 26 km/h.

The hurricane was predicted to make landfall in the United States late Wednesday or early Thursday.

"Some areas, when they wake up Thursday morning, they're not going to believe what happened," said Stacy Stewart, a senior hurricane specialist with the National Hurricane Center.

A Category 4 hurricane can cause damage so catastrophic that power outages may last for months in places, and wide areas could be uninhabitable for weeks or months, posing a new disaster relief challenge for a government already straining to deal with the coronavirus pandemic, with some 180,000 killed in the U.S.

"We need to be concerned about the federal capacity to respond to a major hurricane disaster, particularly in light of failings that are all too obvious in the public health area," said Kathleen Tierney, former director of the Natural Hazards Center at the University of Colorado. "I really worry: Who's minding the store?"

President Donald Trump on Tuesday afternoon used Twitter to urge coastal residents to heed local officials, warning that Laura "is a very dangerous and rapidly intensifying hurricane."

The Federal Emergency Management Administration in Washington has also communicated preparations and warnings online.

Hundreds of thousands told to leave

More than 385,000 residents were told to evacuate the Texas cities of Beaumont, Galveston and Port Arthur.

Another 200,000 were ordered to leave the low-lying Calcasieu and Cameron parishes in southwestern Louisiana, where forecasters said as much as four metres of storm surge topped by waves could submerge whole communities.

In Galveston and Port Arthur, many people boarded buses to Austin and other inland cities. "If you decide to stay, you're staying on your own," Port Arthur Mayor Thurman Bartie said.

Laura also is expected to dump massive rainfall over a short period of time as it moves inland, causing widespread flash flooding in states far from the coast.

Flash flood watches were issued for much of Arkansas, and forecasters said heavy rainfall could move to parts of Missouri, Tennessee and Kentucky late Friday and Saturday.

Urging people in southwest Louisiana to evacuate before it's too late, Louisiana Gov. John Bel Edwards said they need to reach wherever they intend to ride out the storm by noon Wednesday, when the state will start feeling the storm's effects.

Shelters follow coronavirus protocols

Officials urged people to stay with relatives or in hotel rooms to avoid spreading the virus that causes COVID-19. Buses were stocked with protective equipment and disinfectant, and they would carry fewer passengers to keep people apart, Texas officials said.

Shelters opened with cots set farther apart to curb coronavirus infections. Evacuees were told to bring a mask and just one bag of personal belongings each.

"Hopefully, it's not that threatening to people, to lives, because people are hesitant to go anywhere due to COVID," Robert Duffy said as he placed sandbags around his home in Morgan City, La. "Nobody wants to sleep on a gym floor with 200 other people. It's kind of hard to do social distancing."

The hurricane is threatening a centre of the U.S. energy industry. The government said 84 per cent of Gulf oil production and an estimated 61 per cent of natural gas production were shut down. Nearly 300 platforms have been evacuated.

While oil prices often spike before a major storm as production slows, consumers are unlikely to see big price changes because the pandemic decimated demand for fuel.

Laura killed nearly two dozen people on the island of Hispaniola, including 20 in Haiti and three in the Dominican Republic, where it knocked out power and caused intense flooding.

It's taking aim at the U.S. coast just days before the Aug. 29 anniversary of Hurricane Katrina, which breached the levees in New Orleans, flattened much of the Mississippi coast and killed as many as 1,800 people in 2005. 

Galveston, Houston and other parts of Texas, meanwhile, were blasted three years ago by Hurricane Harvey in August 2017, with an estimated economic toll from damage, at $125 billion US, said to have been rivalled only by Katrina.