MEXICO CITY (Reuters) - Tropical Storm Willa gained strength on Saturday in the Pacific Ocean off Mexico and is forecast to become a hurricane by Sunday, the U.S. National Hurricane Center (NHC) said.

The storm was about 280 miles (451 km) southwest of the major port of Manzanillo, and had maximum sustained winds of 60 mph (97 kph). It is likely to reach the coast of western Mexico by Wednesday before it accelerates toward the northeast, the NHC said

Another tropical storm, Vicente, formed in the Pacific off southern Mexico on Saturday, churning toward the coast but was only expected to strengthen slightly over the next couple of days,

Vicente was about 125 miles (201 km) southeast of the port of Salina Cruz, where a major refinery owned by national oil company Pemex is located, with maximum sustained winds of 50 miles per hour (80 kph).

The storm was moving at about 9 miles per hour (14 kph) toward the west and expected to turn northwest early next week and make landfall near the Mexican states of Guerrero and Michoacan as early as Tuesday.

“Some slight strengthening is possible today and Sunday,” the hurricane center said.