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Hurricane Patricia became the strongest storm ever measured on the planet early Friday, with experts warning it could trigger 40-foot waves along southwestern Mexico and "life-threatening" flash flooding.

More than 7 million residents — and an estimated tens of thousands of U.S. citizens visiting or living there — were told to prepare for the "worst-case scenario" as the ferocious storm was expected to race ashore on Mexico's Pacific coast between 6 to 10 p.m. ET Friday.

At 5 p.m. ET, Patricia was about 60 miles west of Manzanillo, and about 110 miles south-southeast of Cabo Corrientes.

The U.S. National Hurricane Center said Patricia was expected to make landfall "in the next several hours." A hurricane warning was in place for San Blas to Punta San Telmo.

The tourist magnets of Puerto Vallarta and Manzanillo were directly in the Category 5 storm's projected path, and Puerto Vallarta's airport was closed Friday out of precaution as some stranded vacationers described their inability to fly out of a "nightmare."

By 5 p.m. winds had weakened slightly to 190 mph, the Hurricane Center said. Winds of 200 mph were measured earlier, and the Hurricane Center labeled Patricia as the "strongest hurricane on record" in the Atlantic and eastern North Pacific Basins.

Mexico has not formally requested help from the U.S., but State Department spokesman Mark Toner told reporters Friday that America "stands up to offer any assistance that we can in the aftermath of what at least appears to be a pretty epic event in terms of the intensity and size of the storm."

Category 5 EPAC hurricane #Patricia was just sampled by #NOAA43 (@NOAA P3). This is the eye radar img 10/23/2015 pic.twitter.com/QRhZLEUigx — NOAAHurricaneHunters (@NOAA_HurrHunter) October 23, 2015

NBC News meteorologist Bill Karins warned that Patricia would be "the most devastating storm to ever hit Mexico" with "catastrophic damage" likely between the posh resort of Puerto Vallarta and the bustling port city of Manzanillo.

While typhoons Nancy and Violet had stronger estimated winds, Patricia was the strongest storm ever actually observed, Karins added. Patricia already has "put on quite a show" in how rapidly and unexpectedly it has strengthened, he said.

Typhoon Haiyan, which devastated the Philippines in 2013, made landfall with 190 mph winds.

While Patricia weakened slightly to 190 mph winds since the afternoon, "Patricia is expected to remain an extremely dangerous category 5 hurricane through landfall," the NHC said. “Preparations to protect life and property should be rushed to completion.”

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 about 40 feet at landfall.

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The National Hurricane Center said the hurricane 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."

Historic #Patricia God help these people in its path. pic.twitter.com/hLgDM3Xsz1 — Jim Cantore (@JimCantore) October 23, 2015

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."

Patricia would be only the second Category 5 hurricane to hit the entire Pacific coast since full record-keeping 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.

12H 23/1800Z 18.8N 105.4W 180 KT 205 MPH! What that says is in 12 hours when this is coming ashore it will be at 205 MPH!!!!!!!!!!!!!!!! — Stephanie Abrams (@StephanieAbrams) October 23, 2015

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."

But Australian newlywed Natalie Griffin said Friday that she and her husband were trying to catch an early flight out of Puerto Vallarta after five days of vacationing. At the hotel on Thursday, she said, guests were told that they could be evacuated by bus to Guadalajara, Jalisco's capital city located further inland.

Griffin said she decided to take her chances at the airport early Friday, but flights out were looking grim.

"We were all excited as we thought we were about to board, and now they have said the airport is closed but they want to get special permission to fly this plane out," she told NBC News. "Everyone wants to know if we are flying or not so we can make plans to leave the area."

Among those hunkering down include Atlanta-born Ian Hayden Parker, who founded the Vallarta Daily News in Puerto Vallarta, which has established 18 hurricane shelters.

"We live in a resort town, but outside of the tourist zone, there is still a lot of poverty and people without computers, Internet or phones," said Parker, who hopes to continue to inform locals about the storm's impact.

After residents were urged to begin storm preparations Thursday, Parker said, "many people followed our advice and are now just playing the wait and see game."

Men fill small bags with sand from the beach as they prepare for the arrival of Hurricane Patricia in Puerto Vallarta, Mexico, on Oct. 23, 2015. Rebecca Blackwell / AP

For some tourists, the impending hurricane created an unexpected upheaval in their plans — and forced them to pack up quickly and head for safer ground.

Jason Sapp and Teri Batterfield were set to get married Friday night on the beach outside of the Hyatt Ziva in Puerto Vallarta. Instead, they and about 25 other guests were on a bus heading to Guadalajara — a ride they were told would take 10 hours because of all the traffic from people evacuating the coastal area.

Daughter Jessica Sapp, of the San Diego area, told NBC News that it's been a "nightmare" for the couple and the other guests who flew in on Thursday for the wedding. The Hyatt, she added, has been shut down.

"They were putting boards on the windows and everybody had to evacuate," she said. "They had a little bit of food for us, but everything is shutting down."

Fiona Bronte, 24, said she first checked into the Vidanta resort in Puerto Vallarta on Saturday with her grandparents and parents. Guests at her hotel were awaiting anxiously Friday afternoon to be redirected to a shelter — but their patience was fraying.

"Everyone is starting to get worried and trying to stock up on food and water," the San Francisco woman said, adding, "People are remaining calm at the moment, but nerves are starting to run high."