Hurricane Milton intensified rapidly on Monday, with dangerous winds and storm surges forecast for Mexico's Yucatan Peninsula before the massive storm is set to slam into Florida by Wednesday.
The storm comes close behind deadly Hurricane Helene, which hit the same areas, and some Florida residents have been ordered to evacuate again.
The US National Hurricane Center said Milton was "an extremely dangerous Category 4 hurricane," on a scale of five, packing maximum sustained winds of 150 miles (240 kms) an hour.
It warned of storm surges raising water levels by five feet (1.5 meters) along the Yucatan Peninsula and large, destructive waves on the coast.
Rainfall of 10 inches (25 cm), with localized spots of up to 15 inches (38 cm), are expected to cause havoc in Florida, bringing flash flooding in urban areas.
Emergency workers are still racing to provide relief in the aftermath of Helene, which killed more than 225 people in several states across the US southeast.
Researchers say climate change likely plays a role in the rapid intensification of hurricanes, because there is more energy in warmer oceans for them to feed on.
Florida Governor Ron DeSantis has declared 51 of his state's 67 counties under a state of emergency for Milton, predicting the storm could have "major impacts."
President Joe Biden was briefed and said in a statement his administration was readying "life-saving resources."
Hurricane Helene hit the Florida coastline as a Category 4 storm, dumping torrential rainfall, and later causing massive flooding in remote inland towns in states further north, including North Carolina and Tennessee.
The storm, which struck Florida on September 26, was the deadliest natural disaster to hit the mainland United States since 2005's Hurricane Katrina, with the death toll still rising.
Communities, particularly those in mountainous areas, have been left without power and drinking water.
AFP