Hurricane John was located 89km south of Punta Maldonado, in Mexico's Guerrero state, and blowing winds near 160.9km/h with higher gusts, the NHC said.
"John is expected to become a major hurricane as it approaches the coast of southern Mexico," NHC said in a statement.
The hurricane is set to make landfall late on Monday or early Tuesday on the southern Mexican Pacific coast, near the surfer getaway of Puerto Escondido in the state of Oaxaca, the NHC forecast showed.
Authorities announced school closures in parts of Oaxaca and neighbouring Guerrero as the storm approached.
The NHC urged residents to prepare for potentially deadly storm surges and flooding, while Oaxaca's governor said he had instructed authorities to co-ordinate with at-risk communities.
In Puerto Escondido, under a cloudy sky, people hurried to tow in boats and put away beach chairs on Monday.
Mexico's government has issued a hurricane warning for communities between Punta Maldonado and tourist hub Huatulco, including the famous beach resort of Acapulco, while a tropical storm warning stretches east of Huatulco to the major port of Salina Cruz.
Mexican state-run oil company Pemex's largest domestic refinery is in Salina Cruz, while Acapulco was battered by Hurricane Otis last year and recovery efforts are still ongoing.
NHC warned that heavy rainfall from John may cause "significant and possibly catastrophic, life-threatening flash flooding and mudslides," that will affect the Mexican states of Chiapas, Oaxaca and southeast Guerrero through Thursday.
