Willa has maximum sustained winds of almost 260km/h with higher gusts, and is expected to hit the western coast of Mexico on Tuesday afternoon, the Miami-based US National Hurricane Center said.
Hurricane Willa was about 240 miles (390 kilometers) southwest of the port city of Manzanillo early Sunday with maximum sustained winds of 85 mph (140 kph).
Vicente and Willa together have made a record-breaking hurricane season in the Eastern Pacific Ocean.
Tropical Storm Vicente, located to the southwest of Willa, is "looking less organized", but is expected to produce heavy rains and flooding over southern Mexico.
By early Monday (Tuesday NZT), its core was about 310 kilometres southeast of Acapulco with top sustained winds of 75kmh.
Authorities in Nayarit, Sinaloa and Jalisco states closed schools in some municipalities and began setting up shelters.
The NHC said "a unsafe storm surge" was also likely to affect coastal areas by Tuesday evening, adding that rain and strong winds would be accompanied by "large destructive waves".
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Major resorts in Mexico are under a tropical storm warning, although it appears the worst of the hurricane might miss major resort areas including Mazatlan and Puerto Vallarta. Hurricane Willa, now a Category 4, is expected to land in the region late Tuesday or Wednesday. It was moving to the north at 5 miles per hour (8 kph).
Forecasters said it was expected to weaken into a tropical depression Monday night or early Tuesday while moving nearer to Mexico's southern Pacific shore.
The hurricane center said Willa was expected to strike near Mazatlan, a popular beach resort, and several other local tourist destinations also are in the storm's path. The hurricane center said it could produce 3 to 6 inches (7.5 to 15 centimeters) of rain in parts of Guerrero, Michoacan, Colima and Jalisco states.
It was expected to produce a life-threatening unsafe storm surge, which will push of ocean water into portions of the coast, and wind and rain.
Residents have been told to expect flooding and potential landslides.
This comes as Hurricane Michael devastated the south-east coast of the USA, leaving communities reeling from the storm "hell".