FEMA Administrator Brock Long said at a press conference that the hurricane will leave portions of Florida without electricity for days, adding that over 100,000 residents of the state will need shelter.
"Hurricane Irma continues to be a threat that is going to devastate the United States in either Florida or some of the southeastern states," he said.
Florida state officials have already called for residents to head to shelters and obey all evacuations orders: more than 100,000 were told to evacuate from Miami-Dade County starting early Thursday.
According to reports, Hurricane Irma is likely to create the largest mass evacuation in US history: Miami-Dade county, along with nearby Broward and Palm Beach counties have a combined population of roughly 6 million people.
After the Hurricane Harvey, which hit the US state of Texas as a Category 4 storm last week in what turned out to be one of the costliest natural disasters in US history, Irma was upgraded to a Category 5 storm on Tuesday with winds in excess of 180 miles per hour.