John first made landfall on Monday at major hurricane strength, triggering flooding and landslides that crushed homes while leaving at least five dead. Its remnants brought intense rains across several states, home to major ports and beach resorts, before strengthening again.
The storm moved back into the Pacific Ocean earlier on Wednesday and the NHC now forecasts John to again slam into the Mexican mainland early on Thursday.
"The ocean's surface temperatures are favourable to feeding this storm," civil protection meteorologist Fermin Damian Adame told a press conference, predicting a fresh round of "intense" rainfall through Friday.
Mexico's government on Wednesday issued a hurricane warning for the Pacific coast between Lazaro Cardenas, in Michoacan state, and Tecpan de Galeana in Guerrero.
Other towns east of Tecpan de Galeana found themselves under a Tropical Storm warning, including resort city Acapulco, which is still recovering from last year's Hurricane Otis.
The Miami-based NHC warned of likely "catastrophic" flash flooding and mudslides across four states along Mexico's Pacific coast.
John's initial hit cut power to around 100,000 homes, uprooted trees and power poles while ripping off roofs.
The five reported casualties included a 10-year-old boy and his mother, who were trapped in their home by a landslide in the village of Tlacoachistlahuaca, in Guerrero state, one of Mexico's poorest.
While John battered Guerrero's southern Costa Chica region on Tuesday, it is now forecast to hit its northern coastline, the Costa Grande. State authorities said around 4,000 homes in the resort city of Acapulco were at risk and urged people living in low-lying areas to immediately move to shelters.
Last year, Acapulco was devastated by Hurricane Otis, which killed more than 50 people and is estimated to have left some $US15 billion ($A22 billion) in damages, shocking forecasters by strengthening to a Category 5 hurricane in less than a day.
Scientists say hurricanes are strengthening faster due to higher sea temperatures resulting from human-caused climate change, leaving communities less time to brace for their impact.
(Reporting by ; Editing by David Alire Garcia, Jonathan Oatis and Sonali Paul)
