Hurricane Harvey has intensified into potentially the most powerful hurricane to hit the US mainland in more than a decade, as authorities warn locals to shelter from what could be life-threatening winds and floods.
Harvey was set to make landfall late on Friday or early on Saturday in the middle Texas coast, slamming into Corpus Christi first and potentially looping back over the Gulf of Mexico before hitting Houston, some models showed.
Some of the biggest US crude refineries are in the storm's projected path. Oil and gas operations have already been disrupted and US petrol prices have spiked.
Residents and emergency vehicles were streaming out of Corpus Christi, heading inland to emergency shelters or ferrying patients to safety under roiling skies and bright orange flares from the city's refineries, all of which had halted production.
Petrol stations and grocery stores all along the south Texas coast were packed as residents readied their cars and pantries for any shortages following the storm.
Harvey strengthened into a category 2 storm with winds of 177km/h as it moved northwest about 225km off Corpus Christi, the US National Hurricane Center said.
The NHC expects Harvey to move slowly and linger over Texas for days. Its latest tracking model shows the storm moving back along the Texas coast early next week after sitting west of Houston, giving the nation's fourth most populous city a double dose of rain and wind.
"Now is the time to urgently hide from the wind. Failure to adequately shelter may result in serious injury, loss of life, or immense human suffering," the National Weather Service said.
Up to 97cm of rain is expected over parts of Texas, and sea levels may surge as high as 3.7 metres. Flood warnings are in effect for Louisiana and northern Mexico.
The storm's approach triggered evacuations in south Texas communities and central coast residents were voluntarily leaving the area. Cities cancelled classes on Friday and Monday at dozens of schools along the south Texas coast, home to 5.8 million people from Corpus Christi to Galveston.
Louisiana and Texas declared states of disaster, authorising the use of state resources to prepare. President Donald Trump has been briefed and is ready to provide resources if needed, the White House said on Thursday.