“This is your last chance to make a good decision,” Rick Scott, governor of Florida, warned residents in the state’s evacuation zones, which encompassed a staggering 6.4 million people, or more than 1 in 4 people.
For days, the forecast had made it look as if the Miami metropolitan area of 6 million people on Florida’s Atlantic coast could get hit head-on by the long-dreaded Big One.
But that soon changed. Meteorologists predicted Irma’s center would blow ashore Sunday morning in the perilously low-lying Florida Keys, then hug the state’s west coast, plowing into the Tampa Bay area by Monday morning.
The latest forecasts suggests the storm has shifted to a more westerly course, putting directly in the path of its bruising winds and deadly surges of sea water.
Mr Scott, governor of Florida, told anyone in an evacuation zone to leave their homes before the storm hit on Sunday morning,
"You need to leave — not tonight, not in an hour, right now," he said, as the window closed on getting to safety.
The new course brings a degree of relief for easterly cities such as Miami, which had feared a direct hit. Instead the western Gulf coast is now in greatest danger.
Forecasters predict winds of more than 177 kph when Irma reaches the Florida Keys at around dawn on Sunday.
The hurricane was stalled over the northern Cuba coast on Saturday evening as much of Florida watched its path, waiting for it to make its expected northward swing and begin its assault on the US mainland.
Although it has weakened in the past 24 hours to a category three storm, authorities say it remains a deadly threat.
Its path means Tampa Bay, which was last hit by a hurricane in 1992, could experience a storm surge of as much as two and half metres.
Mr Scott said the surge could be the most deadly part of the storm.
“Do not think the storm is over when the wind slows down,” he said during a regular briefing for journalists. “The storm surge will rush in and it could kill you.”
About 6.3 million people, more than a quarter of Florida’s population, have been ordered to evacuate their homes.
More than 50,000 of those moved into schools, community centres and churches to weather the storm in shelters.
The warnings extend north into Georgia, Alabama, North Carolina and South Carolina.
Utility companies fear millions of people will lose power.
Florida Power and Light Company expects 3.4 million of its 5 million customers to be affected and the power cuts could last weeks in someplaces.
Rob Gould, its public information officer, said: “Likely on the east coast, we will see a restoration, but on the west coast, a complete rebuild,” he said. “We anticipate this restoration effort will be measured in weeks, not days.”
The late change in course may prove a let off to cities in the east, but local authorities took pains to prevent complacency setting in.
Sheets of rain arrived in the late afternoon as skies blackened over Fort Lauderdale. Several tornado warnings were issued across the state and sightseers who had strolled beside the beach watching the gentle swell turn to angry surf headed for their cars and home.
"Irma is a huge and dangerous storm system that continues to pose a threat to all of South Florida," said Barbara Sharief, mayor of Broward County, which covers the area north of Miami, including Fort Lauderdale.
She said the county expected as much as 38 cm of rain and storm surge of more than a metre.
A curfew went into effect at 4pm to keep roads clear for emergency services.
The White House said Donald Trump was being kept abreast of developments.
“This is a storm of enormous destructive power, and I ask everyone in the storm’s path to heed ALL instructions from government officials,” he wrote on Twitter.
The header picture on his account was also updated to show him being briefed on Hurricane Irma’s path.