US President Donald Trump is encouraging the public to prepare for Hurricane Harvey before it makes landfall in Texas.
The president took to Twitter to encourage "everyone in the path of #HurricaneHarvey to heed the advice & orders of their local and state officials".
His tweet included a link with more information on the storm.
Harvey was upgraded to a dangerous Category 3 hurricane and is expected to make landfall on Friday night or Saturday morning on the middle Texas coast.
The US National Hurricane Centre said Harvey has maximum wind speeds of 120mph as the powerful storm approaches the Texas coast.
Forecasters are labelling it a "life-threatening storm".
The storm quickly grew on Thursday from a tropical depression into a Category 1 hurricane, and then developed into a Category 2 storm early Friday.
By Friday afternoon, it had become a Category 3 storm.
The last Category 3 storm to hit the US was Hurricane Wilma in October 2005 in Florida.
The slow-moving Harvey is fuelled by warm Gulf of Mexico waters.
Texas governor Greg Abbott has warned that Hurricane Harvey "is going to be a very major disaster".
Mr Abbott said he had asked Mr Trump for a federal disaster declaration.
Aside from savage winds and storm surges, the system was expected to drop prodigious amounts of rain.
The resulting flooding, one expert said, could be "the depths of which we've never seen".
The slow-moving hurricane could be the fiercest storm to hit the US in almost a dozen years, with the potential for winds hitting 125mph and storm surges of 12ft.
"We're forecasting continuing intensification right up until landfall," NHC spokesman Dennis Feltgen said earlier.
All seven Texas counties on the coast from Corpus Christi to the western end of Galveston Island have ordered mandatory evacuations of tens of thousands of residents from all low-lying areas.
In four of those counties, officials ordered their entire county evacuated and warned those who stayed behind that no one could be guaranteed rescue.
Voluntary evacuations have been urged for Corpus Christi and for the Bolivar Peninsula, a sand spit near Galveston where many homes were washed away by the storm surge of Hurricane Ike in 2008.
Texas officials expressed concern that not as many people are evacuating compared with previous storms.
"A lot of people are taking this storm for granted thinking it may not pose much of a danger to them," Mr Abbott earlier told Houston television station KPRC.
"Please heed warnings and evacuate as soon as possible."
Mr Abbott has activated about 700 members of the state National Guard ahead of Harvey making landfall.
The NHC said storm surges as much as 3ft could be expected as far north as Morgan City, Louisiana, 400 miles away from the anticipated landfall.
Once it reaches land, the storm is expected to stall, dumping copious amounts of rain for days in areas like flood-prone Houston, the nation's fourth most-populous city, and San Antonio.
State transportation officials are considering when to turn all evacuation routes from coastal areas into one-way traffic arteries heading inland.