Idalia strengthened to a Category 2 storm with 100 mph winds Tuesday afternoon as it barreled toward Florida's Gulf Coast.
Multiple counties are under a state of emergency and residents living in vulnerable areas are being warned to pack up and leave.
Florida Gov. Ron DeSantis warned residents in the impacted areas to leave immediately.
"You really got to go now. Now is the time," he said. "If you stay hunkered down tonight, it's going to be too nasty tomorrow morning to be able to do it."
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The governor warned that first responders won't be able to get those who stay until after the storm has passed.
As the governor spoke, Idalia was 190 miles southwest of Tampa and 300 miles south of Tallahassee.
Earlier, Idalia was churning in the Gulf of Mexico but was projected to come ashore as a Category 3 system with sustained winds of up to 120 mph in the lightly populated Big Bend region, where the Florida Panhandle bends into the peninsula.
The National Weather Service in Tallahassee called Idalia "an unprecedented event" since no major hurricanes on record have ever passed through the bay that abuts the Big Bend region.
To make matters worse, a rare blue supermoon could raise tides above normal. The moon is expected to be close to the Earth the same day Idalia is expected to make landfall.
"I would say the timing is pretty bad for this one," said Brian Haines, the meteorologist in charge at the National Weather Service office in Charleston.
Per Fox Weather, Tampa and Tampa Bay are expected to see a storm surge up to 7 feet, inundating major areas. A life-threatening storm surge of up to 15 feet is now forecast from Aucilla River to Yankeetown.
A tornado watch has also been issued for parts of Florida, including Fort Myers and Tampa, affecting at least 7 million residents, until 6 a.m. EDT.
Some of the Idaila's out bands have already started impacting the western Florida coast, triggering Tornado Warnings, Fox Weather reported.
Pentagon Deputy Press Secretary Sabrina Singh said Tuesday that the Florida National Guard has been "fully activated" with more than 3,000 Guardsmen mobilized across the state and an additional 1,800 on the way.
"The department stands ready to assist the state of Florida, FEMA and state and local officials in any recovery efforts that are needed," Singh said.
DeSantis, meanwhile, said a state of emergency has been expanded for 49 counties.
Tolls were waived on highways out of the danger area, shelters were open and hotels prepared to take in evacuees. More than 30,000 utility workers were gathering to make repairs as quickly as possible in the hurricane's wake.
In Tarpon Springs, a coastal community northwest of Tampa, 60 patients were evacuated from a hospital out of concern that the system could bring a 7-foot storm surge.
St. Petersburg Mayor Ken Welch urged residents not to be complacent.
"It is my hope and prayer that you have your emergency plan in place and you are executing that plan," Welch said at a news conference. "Time is running short to make sure you are prepared for this storm."
With a large stretch of Florida's western coast at risk for storm surges and floods, evacuation notices were issued in 21 counties with mandatory orders for some people in eight of those counties. Many of the notices were for low-lying and coastal areas and for people living in mobile and manufactured homes, recreational vehicles or boats, and for people who would be vulnerable in a power outage.
Many school districts along the Gulf Coast were to be closed through at least Wednesday. Several colleges and universities also closed, including the University of Florida in Gainesville.
Two of the region's largest airports stopped commercial operations, and MacDill Air Force Base on Tampa Bay sent several aircraft to safer locations. The Busch Gardens Tampa Bay theme park also planned to close.
Idalia will be the first storm to hit Florida this hurricane season, but it is only the latest in a summer of natural disasters, including wildfires in Hawaii, Canada and Greece, the first tropical storm to hit California in 84 years, and devastating flooding in Vermont.
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The National Weather Service says area of flash and urban flooding are expected across portions of the west coast of Florida, the Florida Panhandle and southern Georgia Tuesday into Wednesday, spreading into portions of the eastern Carolinas Wednesday into Thursday.
South Carolina Henry McMaster has declared a state of emergency in anticipation of Idalia.
Texas Gov. Greg Abbott said he has deployed resources and personnel to assist Florida ahead of Idalia.
Idalia comes as much of Florida is still recovering from Hurricane Ian last year, which was responsible for nearly 150 deaths. The Category 5 hurricane damaged 52,000 structures, nearly 20,000 of which were destroyed or severely damaged.
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The National Oceanic and Atmospheric Administration recently said the 2023 hurricane season would be far busier than initially forecast, partly because of extremely warm ocean temperatures. The season runs through Nov. 30, with August and September typically the peak.
The Associated Press contributed to this report.