Hurricane Patricia became the strongest storm ever measured on the planet early Friday with experts warning it could trigger 39-foot waves along Mexico’s coast and “life-threatening” flash flooding.

Several million residents were told to prepare for the “worst-case scenario” as Patricia was expected to race ashore on Mexico’s Pacific coast late Friday afternoon or early evening. The tourist magnets of Puerto Vallarta and Manzanillo were directly in the Category 5 storm’s projected path.

Featuring 200 mph winds, the U.S. National Hurricane Center described Patricia the “strongest hurricane on record” in the Atlantic and eastern North Pacific Basins.

NBC News meteorologist Bill Karins warned that Patricia would be “the most devastating storm to ever hit Mexico” with “catastrophic damage” likely between Puerto Vallarta and Manzanillo.

While typhoons Nancy and Violet had stronger estimated winds, Patricia was the strongest storm ever actually observed, Karins added.

At 1:30 a.m. ET, Patricia was about 185 miles south-southwest of Manzanillo, and about 270 miles south of Cabo Corrientes.

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 produce deadly rip currents and “life-threatening flash floods and mudslides.”

It added: “Some fluctuations in intensity are possible today, but Patricia is expected to remain an extremely dangerous Category 5 hurricane through landfall.”

Up to 20 inches of rain was predicted for the Mexican states of Jalisco, Colima, Michoacan and Guerrero through Saturday, the NHC said.

The Mexican government declared a state of emergency.

The U.S. Consulate General in Guadalajara urged Americans in the hurricane warning area to “make preparations immediately to protect life and property.”

Rogelio Estreda, a representative for the Grand Fiesta Americana Resort in Puerto Vallarta, told NBC News that the site would be evacuated at 7 a.m. local time (8 a.m. ET).

“We are expecting something bad, but maybe nothing will happen,” Estreda said. “It can change at any time.”

Laura Diane Rebholz, who co-owns a modeling agency in Scottsdale, Arizona, told NBC News early Friday that she felt it was “safer to ride the storm out” at the Puerto Vallarta hotel where she’s vacationing.

“It’s almost as if it’s literally ‘the calm before the storm’,” she said. “It’s very much business as usual around the resort with staff seemingly unfazed by the hurricane.”

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.

Karins added that 10 inches of rain were already predicted for Texas over the next three days, warning that “what’s left of Patricia will make flooding in south Texas even worse” on Sunday.