More than five million residents in Florida have been ordered to evacuate as Hurricane Irma roars towards the US coast during the next 24 hours.

Warnings that the Category 4 hurricane, with sustained winds of up to 258km per hour, could regain strength as a Category 5, signal potentially ‘devastating’ consequences for Florida.

A further two hurricanes – Jose a Category 4 and Katia a Category 2 – threaten the eastern Caribbean, which has already felt the full impact of Irma, killing 21 people and leaving catastrophic devastation in its wake.

US President Donald Trump said in a videotaped statement that Irma was “a storm of absolutely historic destructive potential” and called on people to heed recommendations from government officials and law enforcement.

In Palm Beach, Trump’s waterfront Mar-a-Lago estate was ordered evacuated.

A message to my fellow Americans🇺🇸#IrmaHurricane2017
🎥https://t.co/jyg6AlyAAb
🎥https://t.co/iPujtHzejr pic.twitter.com/HCQ9kmKc4E

— Donald J. Trump (@realDonaldTrump) September 8, 2017

The ‘worst storm in a century’ is expected to hit Florida in 24 hours but local Florida public alert warnings issued on Saturday at 10am (AEST) said “potentially catastrophic Hurricane Irma” could strike Florida Keys within the next 12 hours.

“All significant hurricane impact threats are likely this weekend, including life threatening storm surge flooding and damaging winds, potentially extremely high rainfall amounts, as well as a threat of tornadoes. Most likely arrival time of tropical storm-force winds will be early Saturday afternoon. Preparations to protect life and property should be completed,” it read.

According to the Federal Emergency Management Agency (FEMA), the United States has experienced only three Category 5 storms since 1851, and Irma is far larger than Hurricane Andrew in 1992.

Few cars to be seen in downtown Miami’s normally grid-locked CBD near the Freedom Tower as Hurricane Irma aims its sights on millions of homes and businesses. Photo: AAP

“We are running out of time. If you are in an evacuation zone, you need to go now. This is a catastrophic storm like our state has never seen,” Governor Rick Scott told reporters, adding that the storm’s effects would be felt from coast to coast in the state.

A total of 5.6 million people, or 25 per cent of the state’s population, was ordered to evacuate Florida, the Associated Press reported, citing Andrew Sussman, the state’s hurricane program manager.

An evacuation map issued by the Miami-Dade Mayor Carlos Gimenez shows the extent of the impact zone once Irma makes landfall.

Officials also began posting warnings on social media to alert millions of residents that time was running out to find shelters away from the peninsular as families were posting messages urging loved ones to leave the area.

The calm before the storm at Bonita Springs, Florida. Photo: Getty

WHAT WE KNOW: THREE HURRICANES IN THE ATLANTIC

• Hurricane Irma: One of the most powerful Atlantic storms in a century has spread devastation in St Martin, where it killed eight people, and Barbuda in the Caribbean and is now heading for Florida. It was Category 5 storm, currently downgraded to Category 4
• Hurricane Jose: In the open Atlantic, about 670km east of the Caribbean’s Lesser Antilles islands; the National Hurricane Centre (NHC) upgraded its designation to Category 4 on Friday morning
• Hurricane Katia: Expected to hit the state of Veracruz on Mexican Gulf Coast by early Saturday; 255km east of the port of Tampico, with maximum sustained winds of 155km/h, making the storm a Category 2 hurricane by mid-morning on Friday; could cause landslides and flooding, and urged people living below hills and slopes to be prepared to evacuate.

— with RAW