The center reported that the storm was nearing hurricane strength with maximum sustained winds of 65 miles (105 km) per hour and was located about 70 miles (112 km) south of the port of Salina Cruz in southern Oaxaca state.

Salina Cruz is home to Mexican state oil monopoly Pemex's largest refinery.

Barbara churned northeast at about eight miles per hour, and is expected to swing through the Isthmus of Tehuantepec, Mexico's narrowest point, once it makes landfall later on Wednesday.

The center issued a hurricane warning from Oaxaca's Puerto Angel to Barra de Tonala, and a tropical storm warming from Barra de Tonala to Boca de Pijijiapan in Chiapas state.

Barbara is forecast to reach hurricane strength before it hits Mexico's southern coast, the center added.

Between 4 and 12 inches (10 to 30 cm) of rain is expected over eastern Oaxaca through western Chiapas once Barbara reaches land, along with a storm surge of between 3 to 5 feet (1 to 1.5 meters) above normal tide levels.