Hurricane Irma continued to barrel toward the U.S. on Saturday, with massive evacuation efforts underway as forecasts showed the storm likely making landfall southwest of Miami.

Irma intensified to a Category 5 storm late Friday, battering Cuba, before weakening back to a Category 4 early Saturday, according to the National Hurricane Center.

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The storm has alternated between a Category 4 and 5 storm, though winds have remained well over 100 miles per hour as it has pummeled a series of Caribbean islands, leaving more than 20 dead, while approaching the U.S.

The storm was 245 miles southeast of Miami early Saturday morning. About 5.6 million people – more than a quarter of Florida's population – were asked to evacuate in the state ahead of the storm, according to The Associated Press. Thousands more were under evacuation orders in South Carolina and Georgia.

A storm surge warning was in effect for Tampa Bay and the Florida Keys, with dangerous water levels expected. Officials urged residents to rush preparations, with a hurricane warning in effect for much of southern Florida, including the Keys, Lake Okeechobee and Florida Bay.

"Storm Surge is the unforgiving hazard..run from the water..get into a facility that can handle forecasted winds," Federal Emergency Management Agency (FEMA) administrator Brock Long tweeted early Saturday.

The National Weather Service urged Key West residents to evacuate as the storm approached, warning on Twitter, "this is as real as it gets."

"Nowhere in the Florida Keys will be safe," the weather service wrote.

Hurricane #Irma continues to move west/northwest off the coast of Cuba. This storm continues to be extremely dangerous!!! #FLkeys pic.twitter.com/vfF1B1oggE — NWS Key West (@NWSKeyWest) September 9, 2017

Florida's largest utility said Friday that about 9 million of the state's roughly 21 million residents could lose power due to the storm, with Irma's intense winds expected to down power lines.

Economic losses from the hurricane could push the price tag of the storm to as high as $200 billion, Bloomberg reported, citing estimates from one disaster modeler. In 2005, Hurricane Katrina left $160 billion in total losses in 2017 dollars.

"This hurricane is a beast," CBS Miami meteorologist Lissette Gonzalez said, according to the Miami Herald.

The newspaper reported that outer bands of Irma were hitting south Florida early Saturday, with blustery conditions felt in Miami's western suburbs.

Irma is expected to make landfall early Sunday in the Keys, and state and local officials have spent days urging residents in the storm's path to make preparations and evacuate.

"This is a storm of absolutely historic destructive potential," President Trump said in his weekly address released Friday.

"I ask everyone in the storm’s path to be vigilant, and to heed all recommendations from government officials and law enforcement. Nothing is more important than the safety and security of our people."