"You need to leave -- not tonight, not in an hour, right now," Governor Rick Scott warned residents in the evacuation zones ahead of the storm's predicted arrival tomorrow morning.
For days, the forecast had made it look as if the Miami metropolitan area of 6 million people could get hit head-on with the catastrophic and long-dreaded Big One.
"For five days, we were told it was going to be on the east coast, and then 24 hours before it hits, we're now told it's coming up the west coast," said Jeff Beerbohm, a 52- year-old entrepreneur in St. Petersburg.
"As usual, the weatherman, I don't know why they're paid."
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Tampa has not been struck by a major hurricane since 1921, when its population was about 10,000, National Hurricane Center spokesman Dennis Feltgen said. Now the area has around 3 million people.
With the new forecast, Pinellas County, home to St. Petersburg, ordered 2,60,000 people to leave.
Irma has left more than 20 people dead in its wake across the Caribbean, ravaging such resort islands as St. Martin, St. Barts, St. Thomas, Barbuda and Antigua.
Irma weakened slightly in the morning but was expected to pick up strength again before hitting the Sunshine State.
Meteorologists predicted its center would blow ashore tomorrow in the perilously low-lying Florida Keys, then hit southwestern Florida and move north, plowing into the Tampa Bay area.
This morning, the state was already beginning to feel Irma's muscle. Nearly 25,000 people had lost power, mostly in the Miami area, as the wind began gusting.
In Key West, 60-year-old Carol Walterson Stroud sought refuge in a senior center with her husband, granddaughter and dog. The streets were nearly empty, shops were boarded up and the wind started to blow.
"Tonight, I'm sweating," she said. "Tonight, I'm scared to death."
In one of the biggest evacuations ever ordered in the US, about 6.3 million people in Florida -- more than one-quarter of the state's population -- were warned to leave, and 5,40,000 were directed to clear out from the Georgia coast.
Major tourist attractions, including Walt Disney World, Universal Studios and Sea World, all prepared to close today.
The Miami and Fort Lauderdale airports shut down, and those in Orlando and Tampa planned to do the same later in the day.
With winds that peaked at 300 kmph, Irma was once the most powerful hurricane ever recorded in the open Atlantic.
But given its mammoth size and strength and its projected course, it could still prove one of the most devastating hurricanes ever to hit Florida and inflict damage on a scale not seen here in 25 years.
Ray Scarborough and girlfriend Leah Etmanczyk left their home in Big Pine Key and fled north with her parents and three big dogs to stay with relatives in Orlando.
Scarborough was 12 when Hurricane Andrew hit in 1992 and remembers lying on the floor in a hall as the storm nearly ripped the roof off his house.
"They said this one is going to be bigger than Andrew. When they told me that, that's all I needed to hear," said Scarborough, now a 37-year-old boat captain. "That one tore everything apart."