Hurricane Patricia developed rapidly into a “potentially catastrophic” category 5 storm that could sweep waves of almost 40 feet onto Mexico’s Pacific coast Friday, forecasters said.

The Mexican government declared a state of emergency, warning residents to prepare for the “worst-case scenario” as Patricia threatened to race ashore as early as Friday afternoon or evening with the potential to be the most powerful Pacific hurricane on record.

Image: Storm clouds in Puerto Vallarta

Image: Storm clouds in Puerto Vallarta

Several million residents — along with the tourist magnets of Puerto Vallarta and Manzanillo — were directly in the storm’s projected path, the U.S. National Hurricane Center and Mexican emergency agencies said.

Patricia strengthened from a tropical storm to a category 5 monster — the strongest on the scale — in less than 24 hours, but “don’t get hung up on the category,” said Danielle Banks, a meteorologist for The Weather Channel.

“Regardless of that, we’re talking about massive rainfall that could bring deadly mudslides and flash flooding” across the Mexican Pacific coast, she said Thursday night.

At 11 p.m. ET, Patricia was about 200 miles south-southwest of Manzanillo, and about 295 miles south of Cabo Corrientes. It was moving swiftly to the northwest with maximum sustained winds estimated at 160 mph.

Hurricane warnings stretched from San Blas to Punta San Telmo, an area that includes Puerto Vallarta and Manzanillo. CONAGUA, the Mexican national water commission, predicted waves up to 39 feet at landfall.

The National Hurricane Center said it was expected to remain “an extremely dangerous major hurricane” when it hits the coast, producing deadly rip currents, “life-threatening flash floods and mudslides.”

Patricia would be only the second category 5 hurricane to hit the entire Pacific coast since full recordkeeping began in 1949. An unnamed storm struck in late October 1959 near Manzanillo, killing an estimated 1,800 people — 800 of them from mudslides alone.