After that, the storm is expected to continue west and possibly slam South Florida or another part of the continental United States. "There is an increasing chance of seeing some impacts from Irma in the Florida Peninsula and the Florida Keys later this week and this weekend," the National Hurricane Center said on Tuesday morning.
Authorities are preparing for Irma with Hurricane Harvey's devastation fresh in their minds.
Miami-Dade County Mayor Carlos Gimenez signed a declaration of emergency to cope with the threat. He said evacuations of special-needs residents will begin Wednesday, and others might be ordered to head out of town.
Gimenez ordered all county offices to be closed on Thursday and Friday. Schools will be closed on Thursday and Friday and all school activities have been canceled from Wednesday night through Friday.
Neigboring Broward County was coordinating plans for 43 shelters that can accommodate up to 33,000 people, county Mayor Barbara Sharief told reporters. Sheltering operations were expected to begin on Thursday, she said.
Broward School Superintendent Robert Runcie said closings will give authorities time to prepare shelters at the schools.
"Given the severity of this storm and in an abundance of caution, and to give our parents and our community sufficient time to adequately prepare, we will be suspending and closing schools on Thursday and Friday of this week," Runcie said.
Monroe County, in which Key West is located, issued a mandatory evacuation of visitors, tourists and non-residents starting 7 a.m. Wednesday. The county also announced a mandatory evacuation of residents starting at 7 p.m. Wednesday.
"There will be no roadblocks to prevent people from entering the Keys before the storm to take care of property and help evacuate family members," the county said. "But all residents should heed the resident evacuation orders."
The Keys SPCA said it is evacuating "all animals out of the Keys," and officials at county animal shelters "are directing residents to evacuate with their pets."
The storm's exact path still remains unclear. But after seeing news reports about water and food shortages in Texas last week as Harvey blew through, Floridians want to be prepared, and scrambled to assemble supplies.
Runs on bottled water, hardware stores
The Costco in Royal Palm Beach, Florida hadn't even opened yet, but already the line to get inside stretched around the parking lot.
"Good luck finding supplies for Hurricane Irma in So/Flo," Daniel Scroggins, who posted video of the scene, said Tuesday on Twitter.
The possibility of the storm led to a "mad run" on bottled water on Tuesday morning in Clearwater, Florida, according to resident Carrie Hart. She said that employees at a Publix supermarket tried to calm agitated shoppers after the store ran out of cases of water. She said the store expects to have more water tonight.
Another Publix in Davie, near Fort Lauderdale, posted a sign with bad news for any shoppers looking to buy water.
"Sorry!! No water at this time!" the sign read. "Waiting for deliveries. Estimated time of arrival is unknown!"
In Miami, Vanessa Mitchell posted a photo of a lengthy line stretching outside a Sam's Club.
Banks, too, saw a crush of people looking to withdraw cash. Miami resident Alex Batista said that a Chase Bank in Miami had long lines and no parking ahead of the storm.
"The banks are full of people getting money," Batista said. "It's crazy."
Still others flocked to Home Depot or other hardware stores in search of plywood, which can be used to board up windows.
US military preparations are under way.
More than 5,000 people -- military active duty personnel, civilians, contractors and families -- based at Naval Air Station Key West have received mandatory evacuation orders. The officials familiar with the evacuation plan say approximately 50 to 60 essential personnel will stay to man the installation's essential functions.
Navy officials told CNN that aircraft will be moving inland from Jacksonville and Mayport, Florida. Submarines are preparing to or have evacuated from Naval Submarine Base Kings Bay in Georgia, according to several Navy officials.
Navy spokesman Bill Dougherty said Naval Station Guantanamo Bay, Cuba, is making preparations for Irma and will shelter in place for the storm as it passes north of Cuba.
The USS Iwo Jima and USS New York were scheduled to leave Mayport, Florida as soon as Tuesday evening and head to Norfolk to load up with disaster supplies and be on standby for any request for assistance following Hurricane Irma, according to two US Navy officials
US Air Force spokesman Col. Patrick Ryder said the Air Force is preparing plans to "relocate the majority of F-16 aircraft from Homestead Air Reserve Base in southern Florida."
The "53rd Weather Reconnaissance Squadron based at Keesler Air Force Base, Mississippi, commonly known as the Hurricane Hunters, is scheduled to fly later this week to help monitor the storm's development," Ryder said.
Air Force search and rescue teams that were assisting in Texas have "returned to their home stations to recover and prepare for a potential response to Hurricane Irma," Ryder said.
The National Football League on Tuesday said the Miami Dolphins home game against the Tampa Bay Buccaneers, originally scheduled for 1 p.m. Sunday at Hard Rock Stadium, won't be played in Miami. The league is mulling whether to play the game Sunday at a neutral site or in Miami later in the season.
"In the interest of public safety in light of the current state of emergency, the NFL, in consultation with state and local officials as well as both clubs, has decided that playing an NFL game in South Florida this week is not appropriate," the NFL said.
Florida Gov. Rick Scott declared a state of emergency for the entire state of Florida and called on residents to prep for the worst. His office said that he has activated 7,000 guard members for Friday.
"Every family has got to get ready," he said Tuesday. "Three days of water per person, three days of food," as well as medicine, fuel, batteries, and more.
On Twitter, Sen. Marco Rubio called on Florida residents to consider plans to evacuate, find shelter, secure their homes and obtain the needed medicines ahead of the coming storm.
"At a MINIMUM #Florida will feel strong impact of #HurricaneIrma later this week. We know what we need to do to get ready," he wrote
Former Gov. Jeb Bush also took to Twitter to issue an all-caps warning about the hurricane.
"GET PREPARED SOUTH FLORIDA," he wrote
"Florida is known for preparing and doing a great job with disasters," Scott added. "We have no idea what's in store. We'll have to pray for the best. The hope would be this thing will dissipate and just go out into the Atlantic, but we've got to prepare for the worst."