Hurricane Laura made landfall early on Thursday in south-western Louisiana as one of the most powerful storms to hit the state.
Forecasters are warning it could push a massive wall of water 65 km inland from the sea.
Social media video showed a wall of water tearing down a street in Lake Charles, Louisiana, as a road sign shook in the wind.
The National Weather Service said the storm surge, possibly higher than a two-storey house, could be “non-survivable”, acknowledging it as an unusually dire warning.
Laura crashed ashore around 6 am GMT as a Category 4 storm, the second strongest on the five-step scale, packing winds of 240 kph in the small town of Cameron, Louisiana, the National Hurricane Centre said.
By 9 am, it had been downgraded to a Category 3 storm with the centre about 50 km north–northwest of Lake Charles, Louisiana, the hurricane centre said.