(CNN) Time is running out to evacuate and prepare for Hurricane Irma, officials warned Saturday night as violent winds and rains from the Category 3 storm began pounding the southern tip of Florida.

Irma's powerful winds and outer rain bands lashed the Florida Keys on Saturday as the massive storm slowly began turning from Cuba's northern coast up into the Florida Strait, the National Hurricane Center said.

Winds of 74 mph -- hurricane force -- reached Key West late Saturday, the National Weather Service said.

With maximum sustained winds of 120 mph, Irma is expected to strengthen once it moves away from Cuba, with the possibility of hitting the US mainland as a Category 4.

Irma's eye is expected to be cross the lower Florida Keys on Sunday morning before driving up the state's west coast in the afternoon, according to the hurricane center. Almost the entire state is under hurricane warning with concerns of devastating gales, heavy rain and life-threatening storm surge.

At least 36 million people were under hurricane warning Saturday night, the center said.

As Irma drew closer to the third most populous state, officials warned the 6.5 million Floridians under mandatory evacuation orders that they were down to their last hours to make a decision.

"If you have been ordered to evacuate, you need to leave now. This is your last chance to make a good decision," said Florida Gov. Rick Scott at a news briefing Saturday evening.

Federal Emergency Management Agency Administrator Brock Long was even more blunt when asked about people who didn't heed evacuation orders in the Keys.

"If you didn't evacuate the Keys, you're on your own until we can actually get in there and it's safe," he told CNN Saturday. "The message has been clear: The Keys are going to be impacted. There is no safe area within the Keys. And you put your life in your own hands by not evacuating."

The National Hurricane Center warned that preparations had be complete in southwest Florida in a matter of hours on Saturday evening.

1045p: Outer band from #Irma is pushing across the Keys. Hurricane Force Winds are occurring! Stay inside, away from windows! #FLkeys #flwx pic.twitter.com/n8ExvGP0Za — NWS Key West (@NWSKeyWest) September 10, 2017

Irma has proven catastrophic, killing 24 people this week in the Caribbean and leaving entire islands in ruins

The storm is massive as winds of at least tropical storm force covered 70,000 square miles -- larger than Florida's entire land area. Worsening conditions spawned tornado warnings in the Keys and northeastern Broward County, and knocked power for 168,000 residents in South Florida.

The major concern among officials was the storm surge , which is like a sustained high tide that can cause devastating flooding, warning that it could reach as high as 15 feet in some areas. A storm surge warning is in place for the Florida Keys, Tampa Bay, and an extensive stretch of coastline wrapping most of the way around the state. At 11 p.m. ET Saturday it was extended as far west as the Ochlockonee River.

"You can't survive these storm surges," Scott told CNN Saturday evening. "You've got to get out. You've got to evacuate. This storm is coming. Once the storm's here, we can't evacuate anybody."

Irma hit Cuba's Ciego de Avila province late Friday as a Category 5 hurricane before it weakened. The storm's violent gusts destroyed the instrument used to measure wind strength, Cuba's meteorological agency reported. Waves as high as 23 feet were recorded, and bigger ones remained a possibility as Irma plodded west, officials said.

Here are the latest developments:

-- Irma's center was 90 miles southeast of Key West at 11 p.m. ET Saturday and moving northwest at a relatively slow 6 mph.

-- Its forecast track had moved slightly westward meaning the storm was expected to move up Florida's west coast Sunday.

-- Hurricane warnings were extended north along Florida's west coast as far as Perry, and along the eastern coast to include Fernandina Beach, north of Jacksonville. A storm surge warning wraps around the state, from Brevard County to Tampa Bay.

-- More than 72,000 people have moved into more than 390 shelters across the state, according to a statement from the governor's office Saturday evening.

-- President Donald Trump tweeted that people should heed the advice of Florida's governor Saturday night.

-- As Irma barrels toward Florida, as many as 26 million people in the Dominican Republic, Haiti and Cuba could be exposed to destructive winds and torrential rain, the Red Cross said, with 1.2 million people already battered by the storm. Hurricane warnings were still in effect Saturday for parts of central Cuba.

Photos: Hurricane Irma tears through Caribbean Carmelo Mota, a builder, searches for tools in his destroyed home in Charlotte Amalie, US Virgin Islands, on Monday, September 18. Hurricane Irma devastated the US territory and other Caribbean islands in the region, leaving them exposed to new storms brewing in the Atlantic. Hide Caption 1 of 55 Photos: Hurricane Irma tears through Caribbean An aerial photo shows the devastation in Road Town, the capital of Tortola, the largest and most populated of the British Virgin Islands, on Wednesday, September 13. Hide Caption 2 of 55 Photos: Hurricane Irma tears through Caribbean UK Foreign Secretary Boris Johnson talks to a resident of Anguilla during a visit on September 13. Hide Caption 3 of 55 Photos: Hurricane Irma tears through Caribbean People collect food that was delivered by emergency workers in the Sandy Ground area of Marigot, St. Martin, on Tuesday, September 12. Hide Caption 4 of 55 Photos: Hurricane Irma tears through Caribbean Buildings are destroyed in St. Martin on September 12. Hide Caption 5 of 55 Photos: Hurricane Irma tears through Caribbean French President Emmanuel Macron shakes hands with St. Martin residents during a visit to the island on September 12. Hide Caption 6 of 55 Photos: Hurricane Irma tears through Caribbean French soldiers patrol St. Martin on September 12. Hide Caption 7 of 55 Photos: Hurricane Irma tears through Caribbean A person works to clean up a street September 12 after Hurricane Irma flooded parts of Havana, Cuba. Hide Caption 8 of 55 Photos: Hurricane Irma tears through Caribbean A man makes repairs in Havana on September 12. Hide Caption 9 of 55 Photos: Hurricane Irma tears through Caribbean This Marigot church was among the buildings destroyed in the storm. Hide Caption 10 of 55 Photos: Hurricane Irma tears through Caribbean Cubans affected by Hurricane Irma line up to collect drinking water in Isabela de Sagua, Cuba, on Monday, September 11. Hide Caption 11 of 55 Photos: Hurricane Irma tears through Caribbean Dutch King Willem-Alexander, front right, tours damage in St. Maarten on September 11. Hide Caption 12 of 55 Photos: Hurricane Irma tears through Caribbean A palm tree sticks out of a pool on the French side of St. Martin on September 11. Hide Caption 13 of 55 Photos: Hurricane Irma tears through Caribbean A woman stands next to her water-logged belongings that had been laid out to dry in front of her home in Isabela de Sagua on September 11. Hide Caption 14 of 55 Photos: Hurricane Irma tears through Caribbean People line up for supplies in St. Martin on September 11. Hide Caption 15 of 55 Photos: Hurricane Irma tears through Caribbean The skeleton of a boat drifts in St. Martin's Simpson Bay on September 11. Hide Caption 16 of 55 Photos: Hurricane Irma tears through Caribbean People salvage material from the remains of a house in Isabela de Sagua on September 11. Hide Caption 17 of 55 Photos: Hurricane Irma tears through Caribbean Members of the British Army provide support on Tortola, one of the British Virgin Islands. Hide Caption 18 of 55 Photos: Hurricane Irma tears through Caribbean A woman carries a dog at an airport checkpoint in St. Martin on September 11. Hide Caption 19 of 55 Photos: Hurricane Irma tears through Caribbean People wade through a flooded street as a wave crashes in Havana on Sunday, September 10. Hide Caption 20 of 55 Photos: Hurricane Irma tears through Caribbean Two men search through the rubble of their St. Martin restaurant on September 10. Hide Caption 21 of 55 Photos: Hurricane Irma tears through Caribbean People make their way through debris in the Cojimar neighborhood of Havana on September 10. Hide Caption 22 of 55 Photos: Hurricane Irma tears through Caribbean People board a plane leaving St. Martin on September 10. Hide Caption 23 of 55 Photos: Hurricane Irma tears through Caribbean A man wades through a flooded street in Havana on September 10. Hide Caption 24 of 55 Photos: Hurricane Irma tears through Caribbean An overview of Havana shows flooded streets on Saturday, September 9. Hide Caption 25 of 55 Photos: Hurricane Irma tears through Caribbean A woman surveys flooding in Havana on September 9. Hide Caption 26 of 55 Photos: Hurricane Irma tears through Caribbean A boat rests in a cemetery after Irma tore through Marigot, St. Martin. Hide Caption 27 of 55 Photos: Hurricane Irma tears through Caribbean Residents return home after Irma passed through Caibarien, Cuba, on September 9. Hide Caption 28 of 55 Photos: Hurricane Irma tears through Caribbean A man walks in Caibarien on September 9. Hide Caption 29 of 55 Photos: Hurricane Irma tears through Caribbean A man carries a child through a flooded street in Fort-Liberte, Haiti, on Friday, September 8. Hide Caption 30 of 55 Photos: Hurricane Irma tears through Caribbean A man walks on a St. Martin street covered in debris on September 8. Hide Caption 31 of 55 Photos: Hurricane Irma tears through Caribbean A damaged home is tilted onto its side on the Puerto Rican island of Culebra on Thursday, September 7. Hide Caption 32 of 55 Photos: Hurricane Irma tears through Caribbean A home is surrounded by debris in Nagua, Dominican Republic, on September 7. Hide Caption 33 of 55 Photos: Hurricane Irma tears through Caribbean Irma damage is seen in St. Martin's Orient Bay on September 7. Hide Caption 34 of 55 Photos: Hurricane Irma tears through Caribbean Employees from an electrical company work to clear a fallen tree in Sanchez, Dominican Republic. Hide Caption 35 of 55 Photos: Hurricane Irma tears through Caribbean A woman makes her way through debris in Fajardo, Puerto Rico, on September 7. Hide Caption 36 of 55 Photos: Hurricane Irma tears through Caribbean In this image made from video, damaged houses are seen in St. Thomas on September 7. Hide Caption 37 of 55 Photos: Hurricane Irma tears through Caribbean The storm left widespread destruction on the island of Barbuda on September 7. Hide Caption 38 of 55 Photos: Hurricane Irma tears through Caribbean A flattened home is seen in Nagua, Dominican Republic, on September 7. Hide Caption 39 of 55 Photos: Hurricane Irma tears through Caribbean Nagua residents ride through an area affected by the storm on September 7. Hide Caption 40 of 55 Photos: Hurricane Irma tears through Caribbean Trash and debris is washed ashore in Cap-Haitien, Haiti, on September 7. Hide Caption 41 of 55 Photos: Hurricane Irma tears through Caribbean People walk through damage in Marigot, St. Martin, on September 7. Hide Caption 42 of 55 Photos: Hurricane Irma tears through Caribbean People survey damage in Marigot on September 7. Hide Caption 43 of 55 Photos: Hurricane Irma tears through Caribbean Bluebeard's Castle, a resort in St. Thomas, was hit hard by Irma. St. Thomas resident David Velez sent this photo to CNN on September 7. Hide Caption 44 of 55 Photos: Hurricane Irma tears through Caribbean Irma ruined these vehicles in St. Thomas. Hide Caption 45 of 55 Photos: Hurricane Irma tears through Caribbean Waves smash into St. Martin on Wednesday, September 6. Hide Caption 46 of 55 Photos: Hurricane Irma tears through Caribbean A man looks at a vehicle turned upside down in the British territory of Anguilla. Hide Caption 47 of 55 Photos: Hurricane Irma tears through Caribbean An aerial view of St. Martin on September 6. Hide Caption 48 of 55 Photos: Hurricane Irma tears through Caribbean Damaged cars are seen on a St. Martin beach on September 6. Hide Caption 49 of 55 Photos: Hurricane Irma tears through Caribbean A boat is washed onto shore in St. Martin. Hide Caption 50 of 55 Photos: Hurricane Irma tears through Caribbean Cars are piled up in Marigot on September 6. Hide Caption 51 of 55 Photos: Hurricane Irma tears through Caribbean A man walks past damaged buildings in St. Martin on September 6. Hide Caption 52 of 55 Photos: Hurricane Irma tears through Caribbean A car is flipped onto its side in Marigot. Hide Caption 53 of 55 Photos: Hurricane Irma tears through Caribbean Broken palm trees are scattered on a Marigot beach on September 6. Hide Caption 54 of 55 Photos: Hurricane Irma tears through Caribbean Irma floods a beach in Marigot on September 6. Hide Caption 55 of 55

-- Of the 24 deaths blamed on Irma, nine were in unspecified French territories, one in Barbuda, one in the British overseas territory of Anguilla, two in Dutch-administered St. Maarten, four in the British Virgin Islands, four in the US Virgin Islands and three in Puerto Rico.

Nervous Florida waits

Florida cities such as Naples, Sarasota, and Tampa are in or near the forecast path of the storm's eye.

"This will be a devastating storm for central Florida, Tampa, Fort Myers, Naples (and) all the way down to Key West," CNN meteorologist Chad Myers said Saturday afternoon.

Florida Power and Light estimated that 3.4 million of its customers could be without power at some point during Irma, which would make it largest number of outages they have ever had to deal with in history, company spokesman Chris McGrath told CNN Saturday.

"We think this could be the most challenging restoration in the history of the US," he said.

Miami began feeling some of Irma's winds Saturday, with high-rise construction cranes sent spinning in circles.

As much as 20 inches of rain was predicted through Wednesday across the Florida peninsula and southeast Georgia, and maximum wind gusts above 100 mph are expected in much of western Florida.

Cranes Already moving and hurricane Irma not Here Yet 😬 #irma #hurricaneirma #prayformiami #florida #miami🔌 #exclusiveconciergemia #cnnweather #yesweather A post shared by Exclusive Concierge (@exclusiveconciergemia) on Sep 9, 2017 at 5:13am PDT

Officials in other states also were preparing Saturday for Irma. South Carolina Gov. Henry McMaster issued a mandatory evacuation for some barrier islands, while Georgia Gov. Nathan Deal expanded the state of emergency to include 94 counties. Schools throughout Georgia, including the University of Georgia, planned to close Monday.

FEMA warned Alabama and North Carolina also to be on watch.

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