Maximum sustained winds increased to near 160 miles (260 kilometers) per hour, according to the NHC, which put Otis in the most powerful category of the five-step Saffir-Simpson scale.
“Catastrophic damage likely where the core of the hurricane moves onshore,” the NHC warned.
“Otis is forecast to remain a Category 5 hurricane through landfall, and rapid weakening is then forecast due to the higher terrain of Mexico,” it added.
As of Tuesday evening, the storm’s center was located about 55 miles from Acapulco, and a hurricane warning was in effect for the coastline from Punta Maldonado to Zihuatanejo.
President Andres Manuel Lopez Obrador made an appeal on social media for people to move to emergency shelters and away from rivers, streams and ravines.
Soldiers were seen patrolling the beach of Acapulco, where visitors earlier made the most of the calm before the storm.
“We won’t be running any tours today,” boat operator Carolina Torres said, voicing hope that Otis might weaken before making landfall.
“If it hits us, that’s very serious for us,” she added.
Rainfall of up to 20 inches (51 centimeters) was expected across Guerrero and parts of neighboring Oaxaca state, the NHC said.
“This rainfall will produce flash and urban flooding, along with mudslides in areas of higher terrain,” it warned.
“A potentially catastrophic storm surge is expected to produce life-threatening coastal flooding,” it added.
Hurricanes hit Mexico every year on both its Pacific and Atlantic coasts, usually between May and November, though few make landfall as a Category 5.
Earlier this week, Tropical Storm Norma left three people dead, including a child, after making landfall for a second time in the northwestern state of Sinaloa.
Norma came ashore for the first time on the Baja California peninsula on Saturday before heading back out to sea, later barreling into the mainland.
Earlier this month, two people died when Hurricane Lidia, an “extremely dangerous” Category 4 storm, struck the western states of Jalisco and Nayarit.
And in August, storm Hilary, which at one point was also a Category 4 hurricane, caused one death and damaged infrastructure as it hit Baja California.
Scientists have warned that storms are becoming more powerful as the world gets warmer with climate change.