Forecasters say Dora has become the fourth hurricane in the Eastern Pacific season as it strengthens off the coast of Mexico.
The U.S. National Hurricane Centre in Miami said the storm was centred Tuesday night about 240 miles (386 kilometers) south of Acapulco, Mexico, and it was moving west-northwest at 17 mph (27 kph) with top sustained winds near 75 mph (120 kph).
Mexican authorities issued a tropical storm watch from Lazaro Cardenas to Cabo Corrientes, meaning tropical storm conditions are possible within 36 hours.
Its proximity to the coast also prompted heavy-rain warnings in the Pacific coast states of Guerrero and Oaxaca.
In Guerrero, the government was preparing nearly 900 shelters in flood-prone areas along the coast, which includes the resort city of Acapulco.