Milton strengthened to the most powerful category of storms as it churned through the southwest Gulf of Mexico, about 1180 km from Tampa. It was packing sustained winds of up to 257 km/h, the National Hurricane Center said.
The storm was expected to turn northeast on Tuesday and head toward the populous Tampa-St Petersburg area, making landfall late on Wednesday as a Category 3 storm before racing across the Florida Peninsula into the Atlantic, the centre said.
Hurricane Milton was upgraded to a Category 5 as it churned through the southwest Gulf of Mexico. (AP PHOTO)
Milton could produce storm surge as high as three metres, 225km/h wind gusts and rainfall totals of 38 cm in some spots along the Florida coast, it said.
"It's going to be powerful, so please take the appropriate precautions," said Florida Governor Ron DeSantis during a news briefing after issuing a state-of-emergency declaration for 51 counties.
"This has the potential to have a lot of damage."
President Joe Biden also declared an emergency for Florida, allowing federal disaster-relief operations to commence.
Relief efforts are already underway throughout the US Southeast in the wake of Helene, a Category-4 hurricane that killed more than 200 people across six states. It was the deadliest named storm to hit the US mainland since Hurricane Katrina killed nearly 1400 people in 2005.
Floridians are still working to clear the debris from Hurricane Helene, which hit two weeks ago. (AP PHOTO)
With Milton bearing down on Florida, forecasts of a supercharged Atlantic hurricane season were starting to look more on target than they did at the beginning of September, typically the peak time for the formation of named storms.
Milton is the season's ninth hurricane, but six have formed since September 9. Milton is the second Atlantic storm to reach Category 5 this season after Beryl, which in July became the earliest storm to reach that distinction.
On Monday, 6.5 million people living from Tallahassee to Miami were under hurricane, storm-surge and flood advisories. Local officials issued evacuation orders for parts of several counties and planned to issue more later on Monday, urging residents to prepare to flee if needed.
"If they have issued an evacuation order, I beg you, I implore you, to evacuate," Florida Division of Emergency Management director Kevin Guthrie said.
Florida was pre-emptively bringing truckloads of food, water, generators and gasoline to areas expected to be hit, while officials were preparing to open shelters. Heavy-duty vehicles were deployed to remove storm debris and 5000 National Guard troops were on stand-by, DeSantis said.
The storm will bring intense rain Monday and Tuesday to swathes of Mexico's eastern coastline and torrential downpours over the states of Campeche and Yucatan, Mexico's weather authority said, as Milton approached the port of Progreso.
In Yucatan, Governor Joaquín Diaz Mena suspended classes from Monday afternoon as Milton intensified. Authorities in the city of Merida urged residents to stock up on supplies.
