Tropical Storm Barbara picked up steam off Mexico's southern Pacific coast on Wednesday, threatening to make landfall with hurricane strength in a rural fishing region.
Authorities closed four ports and urged residents to stay indoors, although television images showed people walking outside in coastal towns of the state of Oaxaca as Barbara approached the Gulf of Tehuantepec.
The storm packed maximum sustained winds of 100 kilometers (65 miles) per hour as it swirled some 89 kilometers (55 miles) southeast of Salina Cruz at 1500 GMT, according to the US National Hurricane Center.
The center said Barbara was forecast to grow into a hurricane before making landfall and then weaken rapidly after its eye crosses the coast.
Hurricane warnings were issued as Mexico's National Water Commission warned that the storm was expected to hit the coast at around midday and dump heavy rain across the region.
Barbara's winds could affect the neighboring states of Chiapas, Tabasco, Campeche, Yucatan, Quintana Roo, Veracruz and Guerrero, the commission said.
The storm was moving towards the northeast at 13 miles per hour (21 kph), the US hurricane center said. Tropical storm force winds extend outward up to 110 kilometers (70 miles) of the storm's center.
Barbara is forecast to dump up to 20 centimeters (eight inches) of rain over parts of Oaxaca and Chiapas, with as much as 30 centimeters (12 inches) possibly falling in isolated areas of southeastern Oaxaca, the center said
"These rains could cause life-threatening flash floods and mud slides," it warned.
Tropical storm conditions "are expected to reach the coast within the warning area this morning, making outside preparations difficult or dangerous."
Once the storm moves inland, however, it is forecast to weaken, and "is expected to dissipate within the next day or so."
In March 2012, two girls died and 25,000 homes were affected when Hurricane Carlotta tore across Oaxaca.