The storm is around 190km southwest of the Pacific town of Cabo Corrientes and "is expected to be an extremely unsafe major hurricane when it reaches the coast of Mexico", the NHC said. Although the appearance has perhaps degraded somewhat since its peak Sunday night and early Monday morning, Willa still has a very well-defined eye surrounded by a ring of intense thunderstorms.
"While gradual weakening is forecast today, Willa is expected to be a unsafe major hurricane when it reaches the coast of Mexico".
Even at Category 4, Willa is "extremely unsafe", forecasters warned, adding that it was likely to strengthen slightly into a Category 5 Monday morning.
A satellite image by the National Oceanic and Atmospheric Administration shows Hurricane Willa in the eastern Pacific, on a path toward Mexico's Pacific coast on Monday. Hurricane Michael had 155 miles per hour winds when it slammed the Florida Panhandle earlier this month, killing at least three dozen people.
Its current intensity is about the same as Hurricane Michael's when it made landfall in Florida.
The hurricane centre said 15 to 30 centimetres of rain should fall - and some places could see up to 45 centimetres - on parts of western Jalisco, western Nayarit and southern Sinaloa states.
Willa had maximum sustained winds of 160 mph Monday morning and was centered about 135 miles southwest of Cabo Corrientes. That's just 1 miles per hour below the threshold for a Category 5 hurricane.
The hurricane was nearing the Islas Marias, a group of islands about 60 miles (96 kilometers) offshore that include a nature preserve and a federal prison.
In addition to Willa, Mexico is bracing for another tropical system right behind it, Tropical Storm Vicente. The center said Willa is expected make landfall late Tuesday or early Wednesday.
Below is a map of all major hurricanes that passed near Willa's location as it became a Category 5.
With Willa and Vicente, the 2018 Northeast Pacific hurricane season has hit a record for the most seasonal accumulated cyclone energy, Colorado State University meteorologist Philip Klotzbach said in a Twitter post.
Increasing numbers of major hurricanes, along with a greater propensity of storms to undergo "rapid intensification" are expected consequences of warmer ocean waters resulting from climate change.