Tropical Storm Polo formed late Sunday off southwestern Mexico.
Polo had maximum sustained winds near 40 mph (65 kph) and was forecast to strengthen rapidly over the next few days, the U.S. National Hurricane Center in Miami said.
Polo was expected to become a major hurricane by midweek.
Polo was moving toward the east-northeast near 5 mph (7 kph). A turn toward the east was expected Tuesday and a sharp turn toward the north and northwest on Wednesday.
Heavy rain was forecast for the Mexican states of Michoacán, Colima and Jalisco through Thursday.
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