Irma weakened slightly overnight but remained a dangerous and deadly hurricane taking direct aim at Florida, threatening to march along the peninsula's spine and deliver a blow the state hasn't seen in more than a decade.
Irma was a Category 4 storm with maximum sustained winds of 240 kph and is forecast to remain at that strength when it comes ashore someplace south of Miami on Sunday. The storm killed at least 20 people in the Caribbean and left thousands homeless as it devastated small islands in its path.
Florida Gov. Rick Scott urged people in coastal and low-lying areas to heed evacuation orders. Across Florida and Georgia, about 1.4 million people were ordered to leave their homes, clogging interstates as far away as Atlanta.
Scott said people fleeing could drive slowly in the shoulder lane on highways, but he hasn't reversed the southbound lanes. Several small communities around Lake Okeechobee in the south-central part of Florida were added to the evacuation list because the lake may overflow, the governor said.
"You don't have to go a long way. You can go to a shelter in your county," Scott said. "This storm is powerful and deadly. We are running out of time."
The latest forecast shifted the most powerful part of the storm to the west of the Miami metropolitan area that his home to some 6 million people, but hurricane-force winds are still likely there.
"Irma is likely to make landfall in Florida as a dangerous major hurricane, and will bring life-threatening wind impacts to much of the state regardless of the exact track of the centre," the hurricane center said in its forecast.
The last major hurricane - a storm with winds of at least 180 kph - to hit Florida was Wilma in 2005. Its eye cut through the state's southern third as it packed winds of 193 kph.
Five people died.
Forecasters predicted a storm surge of 6 to 12 feet above ground level along Florida's southwest coast and in the Keys. As much as a foot of rain could fall across the state, with isolated spots receiving 20 inches.
Gas shortages and gridlock plagued the evacuations, turning normally simple trips into tests of will. Interstates 75 and 95 north were bumper-to-bumper, while very few cars and tractor-trailers drove on the south lanes.
Most powerful storm ever recorded in the Atlantic
The National Hurricane Center issued hurricane warnings for the Keys and parts of South Florida and Lake Okeechobee. It added a storm surge warning and extended watch areas wrapping around much of the peninsula.
With winds that peaked at 300 kph, Irma was the most powerful hurricane ever recorded in the Atlantic.
Irma's weakening comes at a cost. When that happened, its hurricane-force wind field expanded greatly, to about 180 kilometeres wide, said Jeff Masters, meteorology director at the private service Weather Underground.
"It's a big storm," Masters said. "It's not as big as Katrina, but it is definitely a large hurricane now."
Even as forecasts showed the storm's centre could enter Georgia far inland after churning up the Florida peninsula, Gov. Nathan Deal urged nearly 540,000 coastal residents to evacuate, noting Irma's path remains unpredictable.
Forecasts show it could enter the state Monday anywhere from the Atlantic coast to the Alabama state line.