Hurricane Willa was just 3.2 kmh shy of being designated a devastating Category 5 storm.
Hurricane Willa gained dangerous strength early on Monday as it churned toward Mexico's Pacific coast, an area frequented by tourists, with torrential rain and major flooding in the offing, the US National Hurricane Center (NHC) said.
The Miami-based NHC upgraded Willa to a powerful Category 4 storm on the 5-step Saffir-Simpson scale as it unleashed maximum sustained winds of 250 kmh (155 mph) - just 3.2 kmh shy of being designated a devastating Category 5 hurricane.
Willa was expected to hit Mexico's west coast on Tuesday afternoon or evening.
The hurricane "continues to rapidly strengthen," the NHC said in an advisory, adding that Willa would probably trigger a life-threatening surge in Pacific tides.
By early Monday, the storm was located about 285 km (175 miles) southwest of Cabo Corrientes on the coast of Jalisco state.
The NHC estimated that total rainfall would range from 12.5 cm to 38.1 cm across stretches of Jalisco, Nayarit and Sinaloa states, home to the white sand beach resorts of Puerto Vallarta and Mazatlan.
Meanwhile, Tropical Storm Vicente, which formed in the Pacific off southern Mexico on Saturday, began to weaken while hovering along the southern Mexican coast on Sunday and by early Monday was located about 310 km (195 miles) southeast of Acapulco.
It had maximum sustained winds of 75 kph (45 mph) and the centre of the storm would likely remain just offshore or near the southern coast of Mexico through Tuesday morning, according to the NHC.
Vicente is seen weakening to tropical depression status by Monday night, but could still cause significant rainfall.