Tropical Storm Otis strengthened early Tuesday as it approached Mexico’s southern Pacific coast and was forecast to become a hurricane before making landfall near the resort of Acapulco late Tuesday or early Wednesday.

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The U.S. National Hurricane Center said that Otis was about 155 miles south-southeast of Acapulco on Tuesday morning with winds of 70 mph. It was moving north-northwest at 8 mph.

There was a hurricane warning in effect from Lagunas de Chacahua to Zihuatanejo.

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Otis was expected to dump five to 10 inches of rain on the southern state of Guerrero with as much as 15 inches possible in some areas. That raised the possibility of mudslides in Guerrero’s steep mountainous terrain.

In the Atlantic, Hurricane Tammy continued moving northeastward over open water with winds of 75 mph after sweeping through the Lesser Antilles over the weekend. Tammy was located about 580 miles south-southeast of Bermuda. The storm was expected to weaken by Thursday, according to the U.S. National Hurricane Center.