Hurricane Humberto has crossed the south-east Texas coast, with winds as strong as 135km/h.
It had been upgraded from a tropical storm earlier today, but is only rated Category One, the weakest on the hurricane scale.
Humberto was expected to dump 12.5-25cm of rain across southern states including Louisiana, Arkansas and Mississippi.
The storm could also trigger isolated tornadoes in Texas and Louisiana.
Louisiana Governor Kathleen Blanco had already declared a state of emergency as a precautionary measure.
In Texas, Governor Rick Perry activated search and rescue teams, including six Blackhawk helicopters, 50 military vehicles and 200 soldiers.
Humberto is expected to weaken as it moves further inland over the next 24 hours.
