Hurricane Sally has uprooted trees, flooded streets and cut power to hundreds of thousands of homes and businesses, bringing "historic and catastrophic" flooding to the Alabama-Florida coast.
Sally, which made landfall early on Wednesday near Gulf Shores, Alabama, as a Category 2 storm, was donwgraded in the afternoon to a tropical storm as maximum sustained winds dropped to 113 km/h.
Some parts of the Gulf Coast had been inundated with more than 46 cm of rain over the previous 24 hours, with more precipitation expected as the storm's winds slow further, the US National Hurricane Center said.
Residents along the Alabama and Florida coasts were caught off guard by damage from the slow-moving storm.
"Normally it goes away but with this one it was first the anxiety of it coming and then when it finally came, it didn't move," said Preity Patel, 41, in Pensacola.
"It was just constant rain and wind."
The coastal Florida community suffered up to 1.5m of flooding and travel was cut by damaged roads and bridges.
More than 500,000 homes and businesses across the area were without power as the storm knocked over stately oak trees and tore power lines from poles.
A section of the Pensacola Bay Bridge, known also as the Three Mile Bridge, was missing a "significant section", Florida Governor Ron DeSantis said.
The storm was moving at 8km/h toward the Alabama-Florida border but was predicted to pick up speed, the NHC said.
"The rain is what stands out with this one: it's unreal," said Cavin Hollyhand, 50, who left his home on a barrier island and took shelter in Mobile, Alabama.
Some isolated areas could see up to 90 cm of rain before Sally is done, the NHC said.
Upon landfall at Gulf Shores, Sally's winds were clocked at 170 km/h. Along the coast, piers were ripped away by the storm surge and winds.
Alabama Governor Kay Ivey told residents not to go outside to check on damage unless necessary and to stay away from live power lines and fallen trees.
In Pensacola, where wind gusts were clocked at 130 km/h, images on social media showed major floods. Witnesses reported hailstorms in the city and the NHC warned of possible tornados.
Sally is the 18th named storm in the Atlantic this year and eighth of tropical storm or hurricane strength to have hit the United States.
Damage from Sally is expected to reach up to $US3 billion ($A4.1 billion).
Australian Associated Press