Texas Governor Greg Abbott has warned that severe flooding is his main worry as the rains from Tropical Storm Harvey continue to lash the state.
Mr Abbott said the cities of Houston and Corpus Christi had already received as much as 20in (50cm) of rain.
He said another 40in of rain could be due before the storm subsides midweek.
Houston, the fourth biggest city in the US, could see 2in-3in (5cm-7.5cm) of a rain an hour on Saturday night, Mayor Sylvester Turner said.
About 4,500 inmates from prisons south of Houston are being evacuated to other prisons in east Texas because the Brazos River nearby is rising significantly, officials say.
The smaller of the city’s two airports, Hobby, has closed because of standing water on its runways, it was announced overnight.