Hurricane Idalia strengthened over the Gulf of Mexico on Tuesday on its relentless crawl toward Florida's Gulf Coast, forcing evacuations in low-lying coastal areas expected to be swamped when the powerful storm hits on Wednesday.

Idalia was carrying maximum sustained winds of 150 kph by early Tuesday afternoon, but its intensity will ratchet higher before it slams ashore in the early hours of Wednesday, according to the Miami-based National Hurricane Center (NHC).

By that time, it was forecast to reach Category 3 strength – classified as a major hurricane, with maximum sustained winds of at least 179 kph – on the five-step Saffir-Simpson wind scale.

Idalia's most dangerous feature, however, appeared to be the powerful surge of wind-driven seawater it is expected to deliver to barrier islands and other vulnerable areas along the coast.

Florida Governor Ron DeSantis urged residents in low-lying communities to heed orders to seek higher ground, warning that the storm surge could cause life-threatening floods.

"The time is running out very, very rapidly," he said at an afternoon news briefing.

Mary Wolcott Martino, 79, an editor and travel writer in St Petersburg, was packing up as fast as she could on Tuesday afternoon, grabbing items that cannot be replaced such as hard drives containing family photos.

"We're leaving in five minutes, headed out the door now," she said.

She and her husband did not board up their home as they did ahead of last September's Hurricane Ian, saying they are more concerned about flooding than high winds this time.

The NHC said Idalia's center would likely hit Florida's coastline somewhere in the Big Bend region, where the state's northern panhandle curves around into the Gulf side of the Florida Peninsula. The area, roughly between the inland cities of Tallahassee and Gainesville, is much more lightly populated than the Tampa-St. Petersburg area to the south. (Reuters)