Hurricane Harvey has slammed into Texas with winds of up to 130mph (215 km/h) battering coastal areas.
The storm, which has even stronger gusts, is the most powerful to hit the US mainland for 12 years. Up to 40in (1m) of rain is expected to fall.
President Donald Trump has signed a disaster proclamation for the state, which will free up federal aid for the worst-affected areas.
Texas Governor Greg Abbott has warned of record flooding in multiple regions.
Harvey made landfall around 22:00 local time (03:00 GMT) as a Category 4 storm, and has already brought down trees and cut power in the city of Corpus Christi. Thousands of residents have boarded up their homes and fled.
About three hours after it struck, the storm was downgraded to Category 3 after winds dropped slightly to 125mph (201km/h). It remains life-threatening, and capable of causing major damage to buildings.
The mayor of coastal Rockport, Patrick Rios, urged the few remaining people in the 10,000-strong town to leave immediately.
The city manager said people were trapped in a housing centre for elderly people, where the roof had collapsed and rescue teams were unable to gain access, according to CBS.
Long queues of traffic have been heading out of the storm’s path.
Governor Abbot said it was obvious Texas was confronting a “very major disaster”, and more than 1,000 National Guardsmen were to help with the disaster relief operation.
High winds began lashing the coast at nightfall on Friday as coastal water levels rose at the same time.