The US state of Texas is bracing for potentially catastrophic flooding and winds as Hurricane Harvey intensified on Thursday and cruised toward a late Friday impact near the coastal city of Corpus Christi.

The National Hurricane Centre described Harvey's sudden strengthening as "astounding." The storm is expected to strike as a Category 3 hurricane - meaning with winds greater than 178 km/h per hour - making it the most powerful storm to make landfall in the United States since Hurricane Wilma in 2005.

Despite the increasingly alarming forecasts, officials in Corpus Christi as of Thursday evening had held off on ordering mandatory evacuations of the city, which includes a great deal of low-lying land and a barrier island. "I'm not going to risk our police and fire people trying to drag somebody out of the house if they don't want to go," Mayor Joe McComb said at an afternoon news conference.

The surprise hurricane is poised to be the first major test of disaster response for the Trump administration, whose appointee to lead the Federal Emergency Management Agency - William "Brock" Long - was confirmed in June.

"With Harvey now strengthening at a faster rate than indicated in previous advisories, the intensity forecast has become quite concerning," the National Hurricane Centre wrote in a Thursday morning advisory. "Harvey has intensified quickly this morning, and is now forecast to be a major hurricane at landfall, bringing life-threatening storm surge, rainfall, and wind hazards to portions of the Texas coast."

Harvey had disintegrated into a tropical depression as it crossed the Yucatan Peninsula into the western Gulf of Mexico this week. But it reorganised itself over the hot Gulf waters, forming a new, 24-km-wide eye, and rapidly evolved into a hurricane by midday on Thursday, US time.

It then drifted northwest at 16-km an hour and was forecast to slow down a bit, giving it time to siphon energy from the steamy Gulf. When it comes ashore, forecasters said, it could have sustained winds of 200 km/h per hour, with a 4-metre storm surge.

Worse, it is projected to stall on the Texas coast for several days, which could dump historic quantities of rain, with some places seeing as much as 89 cm, the hurricane centre said.

The storm is forecast to meander to the east, deluging Houston and possibly New Orleans next week.

Officials in Corpus Christi scrambled on Thursday to respond to the sudden hurricane threat but decided against mandatory evacuations. Instead, officials instructed residents on the barrier island and low-lying areas inland to evacuate on a voluntary basis.

"We are up to and almost at the threshold of mandatory evacuations, but we are not going to cross that line right now," McComb said. "We are going in the strongest possible terms to encourage the residents in the low-lying areas, as they say, 'Get out of Dodge.' "

Nueces County Judge Samuel Neal, who is overseeing the county's emergency response, did not rule out mandatory evacuations but said such a move would not be done lightly.

"We will do it if we feel it's necessary," he said. "This would create a major, major impact on the way a lot of people do business."

Texas Governor Greg Abbott declared a preemptive state of disaster in 30 counties, including Harris County, home to Houston, the fourth most-populated city in the country. Charles Bujan, mayor of the barrier-island city of Port Aransas, Texas, ordered all citizens to evacuate except those working as emergency responders.

Long has stressed in interviews with The Washington Post that state and local officials need to improve their emergency readiness and recognise that it is not the federal government's responsibility alone to respond to natural disasters.

Long has also urged citizens to understand that they will often be their own first responders in a crisis.

"People need to be the help before the help arrives," he said earlier this month.

Long met with New Orleans Mayor Mitch Landrieu and Louisiana Governor John Bel Edwards on Tuesday and discussed preparations for hurricane season and the August 5 flooding in New Orleans.

"Preparedness is a partnership between the local, state and federal level," Long said. "Here, there is great concern over the city of New Orleans's ability to pump water out of the so-called bowl."

The city has 120 water pumps, but currently only 105 are operational, said Tyronne Walker, communications director for Landrieu. He said the city has brought in 26 generators to provide electricity during an emergency.

"Right now, there's no reason to panic, but you know, everybody should just be focused on getting their plans in order," Walker said. "While we're in a stronger position than we were in the last drainage incident, we're still vulnerable."

Harvey would be the first hurricane to hit Texas since Ike, a high Category 2 storm, came ashore in September 2008 in Galveston and caused tens of billions of dollars in property damage.

In Corpus Christi, some residents on Thursday left work early to begin preparing their homes, while others headed out of town or contemplated hitting the road before the storm arrived.

"Everybody's just trying to get away from this area right now," said Ricky Nesmith, the kitchen manager at Blackbeard's On the Beach.

Nesmith said a full staff came into the restaurant Thursday morning, but most workers left early to get their homes ready. The looming storm has not slowed business, Nesmith said, saying that large groups kept the restaurant busy Thursday before the owner decided to close up shop early.

Bill Sissamis, who owns the Silverado Smokehouse, a barbecue restaurant about three kilometres from the water, said that he would stay open as long as the weather allows.

"We do tend to get a lot of false alarms here," said Sissamis, 54, of previous storm warnings, adding, "The city goes a little bit nuts."

The Gulf of Mexico is vitally important for the nation's oil infrastructure.

A Citigroup report to investors said more than 85 per cent of Texas's refining capacity is located inside the highest precipitation zone for the storm.

Washington Post