Floridians clogged highways in a desperate scramble to get out of Hurricane Irma's crosshairs, as the deadly Category 4 storm took direct aim at the southern United States, shifting westward Friday as the worst case scenario became the most-likely situation.

Irma, downgraded from Category 5, was still regarded as the most powerful in the Atlantic in recorded history, bringing sustained winds of 150 mph – not as strong as the 185 mph of previous days, but forceful enough to cause severe damage to homes and other structures.

"This is a storm of absolutely historic destructive potential," President Trump said in a videotaped statement.

He urged all those in the storm's path to "heed all recommendations" from officials.

"Hurricane Irma continues to be a threat that is going to devastate the United States in either Florida or some of the southeastern states,'' FEMA Administrator Brock Long said at a news briefing on Friday. He warned that parts of Florida would be out of electricity for days if not longer and that more than 100,000 people may need shelter.

Florida Gov. Rick Scott said during a Friday news conference the Florida Keys would begin to feel Irma's effects by Saturday morning. He said Irma, on its current path, was more deadly than Hurricane Andrew, a Category 5 storm that killed 65 in August 1992.

"All Floridians should be prepared to evacuate," Scott said, though he acknowledged the evacuations were causing a logjam on the state's roads. Scott said as of 6 p.m. Thursday 8.4 million gallons of fuel had been shipped into Port Everglades and more than five million gallons of fuel was shipped into Port Tampa Bay.

But Scott emphasized the seriousness of getting to a safe location.