Tropical storm Hermine suddenly formed on Labour Day and made landfall Monday night on the coast of northeastern Mexico, about 40 km south of Brownsville, Texas.
A hurricane watch was in effect on Monday evening from Mexico to Texas.
Hermine could cause dangerous storm surge, violent flash floods, and deadly mudslides, according to the U.S. National Hurricane Centre in Miami. It could also spawn isolated tornadoes on the Texas coast.
Heavy rain will fall on some of the same areas affected by Hurricane Alex in June.
Hermine is forecast to move inland over southern Texas on Tuesday. The storm should then track into central Texas on Wednesday.
With files from The Associated Press