Tropical Storm Carlos formed off Mexico's Pacific coast on Wednesday, threatening to lash the region with heavy rain and grow into a hurricane over the weekend.

Carlos packed maximum sustained winds of 65 kilometers (40 miles) per hour as it swirled some 370 kilometers (230 miles) south of Acapulco, Guerrero state, according to the US National Hurricane Center.

The storm could dump heavy rains over the impoverished southern states of Guerrero and Oaxaca during the next two days, the center said.

The center's five-day forecast map shows Carlos turning into a hurricane on Saturday but not making landfall, instead grazing past the coast.

Guerrero was the worst hit region in September 2013 when twin hurricanes Ingrid and Manuel almost simultaneously hit both coasts of Mexico, leaving 157 people dead.

Carlos would be the third hurricane of the Pacific storm season, followed by Andres, which never made landfall, and Blanca, which weakened to a tropical storm when it reached Baja California peninsula on Monday.