The Texas governor, Greg Abbott, has activated about 700 members of the state National Guard and put military helicopters on standby in Austin and San Antonio in preparation for search and rescues and emergency evacuations.
Harvey, which is due to make landfall late on Friday, is set to become the first hurricane to hit the Texas coast since Hurricane Ike in 2008.
The head of the National Weather Service said the storm posed “a grave risk to the folks in Texas”.
Louis Uccellini, NWS director, said Harvey would bring extremely heavy rainfall that causes inland flooding lasting through the middle of next week, a large storm surge and high winds.
US Immigration and Customs Enforcement officials were moving detained immigrants housed in the projected path of the hurricane. In the Gulf of Mexico, oil and natural gas operators had begun evacuating workers from offshore platforms.
Harvey intensified into a hurricane on Thursday and was heading for the Texas coast with the potential for up to 3ft of rain, 125mph winds and 12-ft storm surges in what could be the fiercest hurricane to hit the United States in almost a dozen years.
Forecasters labeled 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 100 miles inland.
Landfall was predicted for late Friday or early Saturday between Port O’Connor and Matagorda Bay, a 30-mile (48km) stretch of coastline about 70 miles northeast 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 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 Center 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.
Once it comes ashore, Harvey 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”.
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 Center 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”.
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.
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.
More about:
