Thia NOAA-NASA GOES Project satellite image shows storm activity off the south-east coast of the US.

HARVEY intensified into a hurricane today and steered for the Texas coast with the potential for up to nearly one metre of rain, 200km/h winds and four metre storm surges in what could be the fiercest hurricane to hit the United States in almost a dozen years.

Forecasters labelled Harvey a “life-threatening storm” that posed a “grave risk”.

Millions of people braced for a prolonged battering that could swamp dozens of counties more than 160km inland.

Landfall was predicted for late Friday or early Saturday local time between Port O’Connor and Matagorda Bay, a 48km stretch of coastline about 70km north-east of Corpus Christi.

The region is mostly farm or ranchland dotted with waterfront holiday homes and has absorbed numerous Gulf of Mexico storms for generations.

Harvey grew unexpectedly quickly Thursday from a tropical depression into a Category 1 hurricane. Fuelled by warm Gulf waters, it was projected to become a major Category 3 hurricane. The last storm of that category to hit the US was Hurricane Wilma in October 2005 in Florida.

Superstorm Sandy, which pummelled New York and New Jersey in 2012, never had the high winds and had lost tropical status by the time it struck. But it was devastating without formally being called a major hurricane.

“We’re forecasting continuing intensification right up until landfall,” National Hurricane Centre spokesman Dennis Feltgen said.

Typical Category 3 storms damage small homes, topple large trees and destroy mobile homes. As in all hurricanes, the wall of water called a storm surge poses the greatest risk.

As of late Thursday afternoon, Harvey was about 490 kilometres south-east of Corpus Christi, moving to the north-northwest at about 17km/h. Sustained winds were clocked at 137km/h.

Harvey’s effect would be broad. The hurricane centre said storm surges as much as one metre could be expected as far north as Morgan City, Louisiana, some 640km away from the anticipated landfall.

And once it comes ashore, 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.

National Weather Service Director Louis Uccellini said scientists were “looking at a potentially impactful storm over a two-, three-, four-day period.”

Residents fill sand bags as they prepare for Hurricane Harvey in Corpus Christi, Texas. Picture: AP Photo/Eric GaySource:AP

No widespread evacuations were immediately ordered. Officials in Port Aransas and Aransas Pass, two small coastal towns near the projected landfall area, asked the 12,000 residents to leave and warned those who stayed behind that no one could be guaranteed rescue.

“We are closing down,” said Bethany Martinez, a front desk clerk at a Holiday Inn Express at Port Aransas. The 74-room hotel a couple of blocks from the Gulf of Mexico was about two-thirds full before all guests were cleared out. This would be the first hurricane for Martinez, who is pregnant and has two boys, 5 and 6. They were with grandparents in Austin.

Asked about her demeanour, she replied: “Afraid.”

Harvey would be the first significant hurricane to hit Texas since Ike in September 2008 brought winds of 177km/h to the Galveston and Houston areas and inflicted $22 billion in damage. It would be the first big storm along the middle Texas coast since Hurricane Claudette in 2003 caused $180 million in damage.

It’s taking aim at the same vicinity as Hurricane Carla, the largest Texas hurricane on record. Carla came ashore in 1961 with wind gusts estimated at 280km/h and inflicted more than $300 million in damage. The storm killed 34 people and forced about 250,000 people to evacuate.

President Donald Trump on Twitter asked people to get ready for the hurricane and posted links to websites for the Federal Emergency Management Agency, the National Hurricane Centre and a Homeland Security site with tips for emergency preparedness.

White House spokeswoman Sarah Huckabee Sanders said Trump was “briefed and will continue to be updated as the storm progresses.”

Many people heeded the warnings.

James Redford carries a sheet of plywood as he helps board up windows in preparation for Hurricane Harvey in Corpus Christi, Texas. Picture: AP Photo/Eric GaySource:AP

In Houston, one of the nation’s most flood-prone cities, Bill Pennington was philosophical as he prepared his one-story home for what he expected would be its third invasion of floodwaters in as many years and the fifth since 1983.

“We know how to handle it. We’ll handle it again,” Pennington said he told his nervous nine-year-old son.

Dozens were in lines early Thursday at a Corpus Christi Sam’s Club, at home improvement stores and supermarkets. The city also was passing out sandbags. Alex Garcia bought bottled water, bread and other basics in the Houston suburb of Sugar Land after dropping his daughter off at college. He said grocery items were likely more available in Houston than back home in Corpus Christi, where Garcia, a beer distributor salesman, said stores were “crazy.”

“We’ll be selling lots of beer,” he laughed.

Kim Fraleigh, of Sugar Land, stocked up with five cases of water, three bags of ice and other supplies at a supermarket.

“We’ve got chips, tuna, dry salami, anything that does not require refrigeration,” she said.

Joey Garcia, director of the HEB store, said more than a semi-trailer load of water was sold Wednesday, and he expected to two more trailers on Thursday. In Galveston, where a 1900 hurricane went down as the worst in US history, City Manager Brian Maxwell said he was anticipating street flooding and higher than normal tides.

“Obviously being on an island, everybody around here is kind of used to it.”

— Associated Press writers Frank Bajak in Houston; Seth Borenstein and Catherine Lucey in Washington; Diana Heidgerd, Jamie Stengle and David Warren in Dallas; and videographer John Mone in Sugar Land contributed to this story.