Mexican authorities have urged people to move to shelters while officials in Texas distributed sandbags and warned of flash floods as Tropical Storm Hermine headed toward the north-western Gulf coast.
It is the second major storm to hit the area this season. Hurricane Alex roared ashore in late June, killing at least 12 people as remnant rains drenched a wide swathe of north-eastern Mexico for days.
Hermine could approach hurricane strength before making landfall early on Tuesday in a sparsely populated area about 50 miles south of Matamoros, a city bordering Brownsville, Texas, according to the National Hurricane Centre in Miami.
A hurricane watch was issued for the area from Rio San Fernando, Mexico, northward to Baffin Bay in Texas.
The cattle-ranching region is one of the most dangerous in Mexico's turf war between two drug cartels. It is the same area where 72 migrants were killed two weeks ago in what it believed to be Mexico's worst drug gang massacre.
Mexican emergency officials urged people living in low-lying coastal areas to move to shelters.
"We urge the general population to be on alert for possible floods and mudslides," said Salvador Trevino, director of civil defence in Tamaulipas state, where Matamoros is located.
On the Texas coast, emergency officials readied pumping equipment and distributed sandbags in Cameron County, said John Cavazos, the county's emergency management co-ordinator. He said they are also suggesting that people in recreational vehicles in county parks along the coast should move.
Officials are worried about flooding because the ground is already saturated from earlier rains. Some areas could get up to 12in of rain, he said.
"Anyone living in an area that's known to flood, they need to take some precautions," Mr Cavazos said.