Hurricane Idalia's maximum sustained winds increased to 80 miles per hour on Tuesday morning as the storm moved over the Gulf of Mexico, the National Weather Service said.

As of 8 a.m. ET on Tuesday, the storm was moving north at 14 mph and its eye was located about 320 miles southwest of Tampa, Florida. Idalia is expected to intensify further and become “an extremely dangerous major hurricane” before it makes landfall Wednesday morning in Florida’s Big Bend area, north of Tampa, according to the National Weather Service.

-ABC News' Max Golembo

Idalia strengthened from a tropical storm into a hurricane early Tuesday as maximum sustained winds reached 75 miles per hour, according to the National Weather Service.

Idalia is "expected to rapidly intensify into an extremely dangerous major hurricane" before making landfall in Florida on Wednesday morning, the National Weather Service said in its latest advisory. Winds are forecast to reach 120 mph by Tuesday night.

As of 5 a.m. ET on Tuesday, the eye of the storm was located about 85 miles north of the western tip of Cuba and about 370 miles southwest of Tampa, Florida. The storm was moving north at 14 mph, according to the National Weather Service.

A hurricane warning, which means hurricane conditions are expected within the warning area, remains in effect for the Cuban province of Pinar del Rio as well as parts of Florida, starting from the middle of Longboat Key northward to Indian Pass, including Tampa Bay. Other weather alerts were still in effect for more areas of Cuba and Florida, as well as Georgia and South Carolina.

-ABC News’ Max Golembo

Airlines and The Federal Aviation Administration said it's monitoring Tropical Storm Idalia as it heads to the U.S.

The FAA posted on social media that it's "closely monitoring" Idalia's path, but it isn't responsible for closing airports and canceling flights.

American Airlines issued a travel alert that allows its customers whose plans are impacted by Idalia to rebook their flights without incurring a change fee.

Southwest Airlines said it anticipates some changes to its schedule and an impact on its operation as the storm approaches. The airline has issued a travel advisory for its customers.

United Airlines has issued travel waivers for customers traveling to and from impacted airports. The company said it's working to ensure its team is prepared to deal with the expected severe weather.

Multiple counties in Florida are under mandatory and voluntary evacuations as Tropical Storm Idalia barrels toward the state.

People living in zones in Pinellas, Pasco, Citrus, Hillsborough, Levy, Manatee, Sarasota and Taylor counties must evacuate the area, according to multiple county officials.

County officials are also urging people living in mobile homes, low-lying areas and along the coast to leave the area as Idalia approaches.

Officials in Hernando, Jefferson, Manatee and Pasco counties have also issued voluntary evacuation orders for particular zones.

-ABC News' Michael Pappano, Jianna Cousin and Robinson Perez

As Idalia approaches, the Pasco County School District, north of Tampa, said its schools will be closed on Tuesday and Wednesday.

Schools in Hillsborough County, which encompasses Tampa, will also be closed Tuesday and Wednesday.