Forecasters have warned the weather will be “unsurvivable” and capable of sinking entire communities, with mandatory evacuations in place for some areas.
Authorities implored coastal residents of Texas and Louisiana to flee, with the force already being felt hours before the full scale of the storm was due to hit.
One sheriff in the state of Louisiana warned that those choosing to stay "must understand that rescue efforts cannot and will not begin until after storm and surge has passed and it is safe to do so."
The Vermilion Parish Sheriff’s Office issued the grim warning that those ignoring evacuation warnings should "write your name, address, social security number and next of kin and put it a ziplock bag in your pocket."
Video and photos on social media showed torrents of rain flying sideways past street lights in Lake Charles, and streets covered with water closer to the coast. A sudden storm surge knocked over cameras meant to capture the hurricane’s effects.
With hours of violent weather ahead, officials said the extent of destruction would likely remain unclear until daybreak, when search and rescue missions will begin.
The storm rapidly intensified on Wednesday, gaining nearly 70% in power in just 24 hours and continuing to draw energy from the warm Gulf of Mexico waters.
University of Miami hurricane researcher Brian McNoldy said: “It looks like it’s in full beast mode, which is not what you want to see if you’re in its way,”
The National Hurricane Centre kept raising its estimate of Laura’s storm surge, from 10ft just a couple of days ago to twice that size - a height that forecasters said would be especially deadly.
“Some areas, when they wake up Thursday morning, they’re not going to believe what happened,” said Stacy Stewart, a senior hurricane specialist.
Whatever does not get blown down by the wind could easily be toppled by seawater pushing inland.
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.
The threat of such devastation posed a new disaster-relief challenge for a government already straining to deal with the coronavirus pandemic.
Texas Gov. Greg Abbott and Louisiana Gov. John Bel Edwards fretted, however, that the dire predictions were not resonating - despite authorities putting more than 500,000 coastal residents under mandatory evacuation orders.
Officials said at least 150 people refused pleas to leave and planned to weather the storm in everything from elevated homes to recreational vehicles.
“It’s a very sad situation,” said Ashley Buller, assistant director of emergency preparedness. “We did everything we could to encourage them to leave.”
Correspondent Geraint Vincent reports from Texas on Wednesday:
On Twitter, President Donald Trump also urged coastal residents to heed local officials.
Hurricane warnings were issued from San Luis Pass, Texas, to Intracoastal City, Louisiana, and reached inland for 200 miles.
Storm surge warnings were in effect from Freeport, Texas, to the mouth of the Mississippi River.
In the largest US evacuation during this pandemic era, more than half a million people were ordered to flee from their homes near the Texas-Louisiana state line, including the Texas cities of Beaumont, Galveston and Port Arthur, and the low-lying Calcasieu and Cameron parishes in south-western Louisiana, where forecasters said storm surge topped by waves could submerge entire towns.
A National Weather Service meteorologist in Lake Charles, Louisiana — in the bullseye of Laura’s projected path — took to Facebook Live to deliver an urgent warning for people living south of Interstate 10 in south-west Louisiana and south-east Texas.
“Your life will be in immediate and grave danger beginning this evening if you do not evacuate,” Donald Jones said.
Laura is expected to dump massive rainfall as it moves inland, causing widespread flash flooding in states far from the coast.
Flood watches were issued for much of Arkansas, and forecasters said heavy rainfall could arrive by Friday in parts of Missouri, Tennessee and Kentucky.
Laura is so powerful that it is expected to become a tropical storm again once it reaches the Atlantic Ocean, potentially menacing the north east.
Laura will be the seventh named storm to strike the U.S. this year, setting a new record for U.S. landfalls by the end of August.