Tropical Storm Harvey strengthened into a hurricane on Thursday as it headed to coastal Texas, where it was expected to bring life-threatening flooding, meteorologists said.

The National Hurricane Center said that Air Force hurricane hunter planes had spotted Tropical Storm Harvey strengthening in the Gulf of Mexico, with about 60-mile-per-hour winds more than 300 miles off the southern coast of Texas early Thursday. A hurricane warning, meaning hurricane conditions are expected, had been in effect from Port Mansfield in far southern Texas to Matagorda, about 100 miles southwest of Houston.

But later in the day, the planes spotted the system strengthening, and the estimated maximum sustained winds increased to close to 100 m.p.h. the hurricane center said early Friday. It was expected to make landfall Friday night or early Saturday.

Parts of Louisiana and the lower Mississippi Valley were also in the path of hurricane conditions from the storm system, the hurricane center said. Gov. John Bel Edwards of Louisiana said on Thursday that Hurricane Harvey was expected to hit Corpus Christi early Saturday as a Category 3 hurricane, but that its outer bands of rain could be felt earlier. The impact on Louisiana was expected early next week, he said.