Hurricane Laura is expected to cause a catastrophic storm surge, extreme winds and flash floods as it hits the US, the National Hurricane Center (NHC) says.
Laura is currently a Category 3 storm and is due to strengthen to Category 4 before reaching Texas and Louisiana later 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 with 48 hours of each other – an unprecedented event – but Marco was downgraded to a tropical storm.
Laura, on the other hand, has strengthened rapidly to a Category 3, gaining 70% in power in just 24 hours.
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.
The hurricane currently has maximum sustained winds of 125mph (201km/h). It could reach 145mph in the next few hours, with higher gusts.
It is expected to reach landfall near the Texas-Louisiana border shortly after midnight local time on Thursday (05:00 GMT).
It is also expected to generate tornadoes over Louisiana, Texas and Mississippi on Wednesday night.
More than 385,000 residents have been ordered to leave the Texas cities of Beaumont, Galveston and Port Arthur, while an additional 200,000 were told to evacuate Calcasieu Parish in south-western Louisiana.
Port Arthur, Texas, is home to the nation’s largest oil refinery, and workers have been taken off at least 281 offshore drilling platforms in the Gulf of Mexico, officials said on Monday.
What happened in the Caribbean?
Marco and Laura brought high winds and rough seas to the Caribbean, leaving at least 24 people dead, including a baby and an eight-year-old child in Haiti.
Heavy rains have also battered parts of Cuba, the Dominican Republic, and the US territory of Puerto Rico. President Trump declared a state of disaster in the territory on Saturday.
In Cuba, authorities have evacuated at least 160,000 people from several coastal regions, while thousands have been evacuated in the Dominican Republic.
In Jamaica, there were reports of landslides and flooded roads.