The Category 1 storm, with wind speeds of around 75 miles per hour (120 kilometers per hour), strengthened to a hurricane overnight and is expected to make landfall by afternoon or early evening, the National Hurricane Center (NHC) said.
It could bring storm surge of up to five feet and as many as 18 inches of rain to parts of south Texas, the NHC said, warning of dangerous flash flooding.
Hanna was about 100 miles east south-east of Corpus Christi, Texas, at 7:00 am local time (1200 GMT), and was moving west at about nine miles per hour, an NHC advisory said.
Expected to make a slight turn later in the day, it "should make landfall along the Texas coast within the hurricane warning area this afternoon or early this evening," the advisory continued.
Storm warnings were already in effect along Texas's Gulf Coast early Saturday. In Corpus Christi officials closed libraries and museums as the city braced for the storm, local media reported.
Hanna will roar ashore as Texas is already facing a huge surge in coronavirus infections, with officials instituting a state-wide mask mandate to try to curb the spread of the disease.