Milton strengthened to the most powerful category of storms as it churned through the southwest Gulf of Mexico, about 735 miles (1,183 km) from Tampa. It was packing sustained winds of up to 160 mph (257 kph), 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 center said.
Milton could produce storm surge as high as 10 feet (3 m), 140-mph wind gusts and rainfall totals of 15 inches (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 1,400 people in 2005.
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 Sept. 9.
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 preemptively 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 5,000 National Guard troops were on standby, 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 1 p.m. on Monday as Milton intensified. Authorities in the city of Merida urged residents to stock up on supplies.
