The Catagory 4 storm, with winds of 130 miles per hour and moving up the Florida's Gulf coast - prompted one of the largest evacuations in US history.
"I will tell you in no uncertain terms - and I am not going to sugarcoat it - this is going to be a hard storm", Buckhorn said at the news conference.
Hundreds of thousands of homes and businesses have lost power, according to the Associated Press.
The massive storm triggered evacuation orders for 5.6 million people, and made two landfalls Sunday.
Almost 4.5 million homes and businesses across Florida lost power, and utility officials said it will take weeks to restore electricity to everyone.
State officials said more than 6 million Florida residents were left without power after Hurricane Irma, downgraded to a tropical storm on Monday, lashed much of the state and is now headed toward Georgia.
Irma has killed at least 20 people so far, leaving a trail of destruction in the Caribbean.
Many streets were underwater in downtown Miami and in other cities. "It was was like, 'Oh my God, what is going to happen?"'
"If Tampa stays on the east side of the eye, we expect the storm surge to be worse than if the eye moves inland and the west side of the storm hits it", he said.
For days, forecasters had warned that Irma was taking dead aim at the Miami metropolitan area and the rest of Florida's Atlantic coast. "We have to wait till all the elements pass through", he said.
Pay attention when watches or warnings are issued for your area
In the Jacksonville area, close to the Georgia line, storm surge brought some of the worst flooding ever seen there, with at least 46 people pulled from swamped homes.
North Carolina Gov. Roy Cooper also declared a state of emergency for all 100 counties in his state ahead of Irma.
"The center of Irma will move into southwestern Georgia later today, and move into eastern Alabama Tuesday morning", the National Hurricane Center said.
The challenges in the immediate aftermath of the storm will be many: Restoring across the state, removing debris from roads, dealing with possible fuel shortages, and making sure nursing home and hospital patients who were evacuated can safely return.
"We're asking folks to be patient and remain sheltered in place", said St. Augustine Fire Chief Carlos Aviles.
Forecasters said it could hit the heavily populated Tampa-St.
- Storm surge warnings in Florida, Georgia and SC mean "there is a danger of life-threatening inundation", the hurricane center said.
The Pasco Sheriff's office has felt it necessary to warn people not to shoot weapons at Hurricane Irma, after a Facebook event called "Shoot at Hurricane Irma" attracted interest from more than 50,000 people.
After a Florida evacuee delivered a 2-pound baby at 28-weeks at a south Georgia gas station late Friday, Dr. Mitch Rodriguez said the hospital is setting up other neonatal care areas in case more babies come early.