Hurricane Laura is expected to cause an "unsurvivable" storm surge, extreme winds and flash floods as it hits the US, the National Hurricane Center (NHC) says.
Laura was upgraded to a Category 4 storm as it approached the coasts of Texas and Louisiana on Wednesday.
The NHC warned local residents to "rush" to complete preparations.
Half a million have been told to leave. Laura and another storm, Marco, earlier hit the Caribbean, killing 24.
Marco has already struck Louisiana, bringing strong winds and heavy rain on Monday.
Initially it was feared that both storms would hit Louisiana as hurricanes within 48 hours of each other – an unprecedented event – but Marco was downgraded to a tropical storm.
Laura, on the other hand, has strengthened rapidly from a Category 3, gaining 70% in power in just 24 hours, to a Category 4, maximum sustained winds of 140mph (220km/h).
US President Donald Trump told those potentially affected by the storm to "listen to local officials" as the storm was "very dangerous and rapidly intensifying".
Evacuations are complicated by the Covid-19 pandemic. Texas Governor Greg Abbott urged families who could afford it to take refuge in hotels and motels to be distanced from others.
What can we expect from Hurricane Laura?
Early on Wednesday the NHC said satellite images had shown that Laura had undergone a remarkable intensification to become a "formidable hurricane".
In a series of tweets, it said Laura was expected to bring "life-threatening hazards" and an "unsurvivable storm surge" to parts of the coast of the Gulf of Mexico.
Well-built homes could incur major damage, trees could be snapped or uprooted and electricity and water would be unavailable for days or even weeks, it said.
"Hurricane force winds and widespread damaging wind gusts will also spread well inland into portions of eastern Texas and western Louisiana early Thursday," the NHC added.
Unsurvivable storm surge with large and destructive waves will cause catastrophic damage from Sea Rim State Park, Texas, to Intracoastal City, Louisiana, including Calcasieu and Sabine Lakes. This surge could penetrate up to 30 miles inland from the immediate coastline. #Laura pic.twitter.com/bV4jzT3Chd
— National Hurricane Center (@NHC_Atlantic) August 26, 2020
End of Twitter post by @NHC_Atlantic
The hurricane currently has maximum sustained winds of 125mph (201km/h). It could reach 145mph in the next few hours, with gusts of 170mph.
Storm surges of more than 20ft (6m) are possible. In an update at 14:00 local time, the NHC reported 3.2ft of inundation already on parts of the Louisiana coast.
"To think that there would be a wall of water over two storeys 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.
"The word 'unsurvivable' is not one that we like to use, and it's onRead More – Source