Irma Strengthens Back Into Category 5 Hurricane; Warning Zone Expanded For Florida.

MIAMI (CBS) -- Hurricane Irma continues to spin furiously toward Florida with winds reaching 155 mph. And officials say 5.6 million people have been asked to evacuate the state.

Irma became a Category 4 storm Friday morning, but it still poses a major threat to Florida's Atlantic Coast as all of South Florida is under a hurricane warning. By Friday night, the National Weather Service said the powerful storm strengthened back into a Category 5 hurricane as it made landfall near Cuba.

Time is running out for Floridians to evacuate before Irma roars ashore this weekend. At least 1.4 million live in mandatory evacuation zones and those zones could spread all the way to the state's northern borders.

The National Weather Service in Key West warned residents, "This is as real as it gets, nowhere in the Florida Keys will be safe."

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FEMA predicts Irma will devastate the southern United States.

"I can guarantee you that I don't know anybody in Florida that's ever experienced what's about to hit South Florida," said FEMA Administrator Brock Long.

People at Miami and Fort Lauderdale's airports have only hours to catch the last flights out.

"Hopefully the storm will stay on track and it won't hit until Saturday and we'll be out of here," said one passenger.

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Those still in Miami are getting more worried about the powerful storm.

"We are filling sand bags to prevent water from getting into the house," said Terrence Sedler. "I always knew there was going to be hurricanes, but never expected one to hit right away. It's my first one."

Those who are staying say they're ready to meet Irma face-to-face.

"Basically, the whole of southern Florida is going to get hit. Whether I went west to Sarasota or Fort Myers or downtown, there's going to be surge," said John Griffin. "There is going to be flooding, so I figure I'd stay put. I'm on the seventh floor of my apartment and have impact-proof windows, so hopefully I'll get a view. At worst the power will go out, so I'll have to sweat it out."

Storm surges are a big concern with Irma.

FEMA estimates about 2.5 million homes are in flood-hazard zones.

A steady stream of bumper-to-bumper traffic is heading north, but the rush to evacuate is stalled in some areas because gas stations are quickly running out of fuel.

"For gas stations in evacuation zones, we need you to stay open as long as you can so people can get out," said Florida Gov. Rick Scott.

Frantic residents are scooping up water and last-minute supplies to ride out the storm.

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FEMA estimates the hurricane could send more than 100,000 people to shelters. People are taking heed of evacuation warnings as they are heading to shelters throughout Miami-Dade and Broward Counties.

Irma battered Turks and Caicos overnight and it's headed for Cuba before it turns north for its collision course with the U.S. mainland. The storm has killed at least 21 people so far.