HOUSTON — Harvey intensified into a hurricane Thursday and steered for the Texas coast with the potential for up to 3 feet of rain, 125 mph winds and 12-foot 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 stretch of coastline about 70 miles northeast of Corpus Christi. The region is mostly farm or ranchland dotted with waterfront vacation 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. Fueled by warm Gulf waters, it was projected to become a major Category 3 hurricane. The last storm of that category to hit the U.S. was Hurricane Wilma in October 2005 in Florida.

Superstorm Sandy, which pummeled 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.

As of late Thursday afternoon, Harvey was about 305 miles (490 kilometers) southeast of Corpus Christi, moving to the north-northwest at about 10 mph (17 kph). Sustained winds were clocked at 85 mph.

Harvey’s effect would be broad. The hurricane center said storm surges as much as 3 feet could be expected as far north as Morgan City, Louisiana.