By Brendan O'Brien and Rich McKay

Aug 28 (Reuters) - Tropical Storm Idalia was expected to intensify into a major hurricane on Monday as it barreled toward Florida's Gulf Coast, where authorities urged millions of residents to prepare to evacuate ahead of an expected landfall early Wednesday morning.

Idalia, with maximum sustained winds of 65 mph (105 kph), was churning in the Caribbean on Monday about 90 miles off Western Cuba, moving north at 8 mph, the National Hurricane Center said in an advisory at 7 a.m. EDT.

The storm's growing intensity and its current northerly track put some 20 million Floridians under hurricane and tropical storm watches.

Idalia is predicted to be Category 3 on the Saffir-Simpson Hurricane Wind Scale when it makes landfall in northern Florida's Big Bend area, where the panhandle transitions into the peninsula, the hurricane center said.

"All Floridians, you need to be executing your plans," Florida Governor Ron DeSantis said during a news conference on Monday. "This is going to be a major hurricane. This is going to be a powerful hurricane and this is absolutely going to impact the state of Florida."

Local officials along the state's Gulf Coast were preparing to issue evacuation orders on Monday, DeSantis said, noting that Floridians in the area should prepare to lose power.

"Keep in mind, if you are told to evacuate, you do not need to drive hundreds of miles, you do not need to leave the state of Florida. You basically need to go to higher ground," he said, adding that many school districts were also planning to cancel classes.

By Tuesday, Florida's Gulf Coast, southeast Georgia and the eastern North and South Carolina should expect torrential rains of 4 to 8 inches (10 to 20 cm) that could cause scattered flash and urban flooding to begin.

Along with the heavy rain, winds of more than 110 miles per hour could result in life-threatening storm surge, the center warned.

Like many beachfront communities along the coast, the city of Bradenton opened sandbag stations on Monday and urged its 55,000 residents to stay vigilant.

"Let's be prepared - secure items that could become airborne, gas up your car, have cash and bottled water on hand," it said in a post on the social media platform X.

EMERGENCY SHELTERS

In other communities, such as Hillsborough County, officials declared an emergency and set up shelters where those in need can ride out the storm.

DeSantis issued a state of emergency for 46 Florida counties covering most of the northern part of the state. Some 5,500 members of the National Guard were mobilized, with 2,400 high-water vehicles and a dozen aircraft deployed for rescue and recovery efforts.