Shops are shut and protected with wood in Clearwater as hurricane Idalia intensified as it hurtles towards northwest Florida, threatening "catastrophic" impacts including dangerous storm surge, with officials forecasting it will slam the coast within hours as an extreme and historic Category 3 storm.

Hurricane Idalia barreled towards the west coast of Florida Tuesday, triggering mass evacuation orders and flood alerts as authorities warned the storm could strengthen to "extremely dangerous" levels before landfall.

The US National Hurricane Center (NHC) said Idalia, a Category 1 storm with winds of 140 kilometers per hour, was churning off Florida's southwest shore, bringing tropical storm conditions to western Cuba and flooding in Havana.

Warm waters in the Gulf of Mexico are expected to turbocharge Idalia into an "extremely dangerous major hurricane before landfall on Wednesday," the NHC said, warning of "storm surge inundation of 10 to 15 feet (three-five meters)" in coastal areas.

"Very few people can survive being in the path of a major storm surge, and this storm will be deadly if we don’t get out of harm's way and take it seriously," said Federal Emergency Management Agency chief Deanne Criswell.

Major hurricanes are Category Three or higher on the five-level Saffir-Simpson scale, with winds above 110 miles per hour that the NHC says could cause "devastating damage."

In the small coastal town of Steinhatchee, Robert Bryant was making final preparations to evacuate inland with his two cats and a dog.

"We are out on the water, so we are going to be the worst ones to get hit," said the 18-year-old student, whose home built on stilts lies close to the mouth of a river.

"Hopefully, it just blows over and we have a bit of wind... but you prepare for the worst and hope for the best," he told AFP.