The eye of category 2 Hurricane Sally made landfall near Gulf Shores, Alabama on Wednesday, the US National Hurricane Center (NHC) said, after heavy winds overnight.
Sally, packing maximum sustained winds of 105 miles per hour (165 km/h), is likely to cause extensive damage and life-threatening flooding along portions of the north-central gulf coast, the NHC said.
The agency earlier said the hurricane could bring more than 2 feet (60 cm) of rain to some areas as it moves inland, headed towards the Alabama-Florida border at 3 mph (5 kph).
Sally's northern eyewall had brought hurricane-force winds and rain from Pensacola Beach, Florida, westward to Dauphin Island, Alabama, for hours before its centre finally hit land.
Nearly 400,000 homes and businesses had lost electricity by early Wednesday, according to reports.
It is the eighth named storm to reach landfall on American shores this year, reportedly breaking a 104-year old record last set when seven storms landed in 1916’s storm season.
Ed Rappaport, deputy director of the hurricane centre, said Sally was a rare storm that could make history.
"Sally has a characteristic that isn't often seen and that's a slow forward speed and that's going to exacerbate the flooding," he said, likening the storm's slow progression to that of Hurricane Harvey, which swamped Houston in 2017.
Up to 30 inches (76 centimeters) of rain could fall in some spots, and "that would be record-setting in some locations," Rappaport said in an interview on Tuesday night.
Damage from Sally is expected to reach $2 billion (£1.5 billion) to $3 billion (£2.3 billion), said Chuck Watson of Enki Research, which tracks tropical storms and models the cost of their damage.
Additional reporting by Associated Press.
