Hurricane Michael, a Category 1 storm, has been gaining strength as it moves through the Gulf of Mexico threatening Cuba and Florida.
It is expected to become a Category 3 storm by the time it makes landfall on the Florida Panhandle sometime Wednesday.
"#Michael could be one of the worst hurricanes to strike the Florida Big Bend and Florida Panhandle region," Rick Knabb, the hurricane expert at The Weather Channel, tweeted on Monday. "We only have today and Tuesday to complete life-saving preparations at the coast and inland. Evacuate as instructed."
The National Hurricane Center said Michael will carry sustained wind speeds of 175 kilometers per hour (109 mph) by late Tuesday.
Robbie Berg, a hurricane specialist at the Miami-based center, said the hurricane will spend two to three days over the Gulf of Mexico, growing in strength because of the warm water temperatures and atmospheric conditions favorable to storms.
"There is a real possibility that Michael will strengthen to a major hurricane before landfall,'' Berg said.
Michael was already lashing western Cuba by late Monday. The hurricane center said the storm could dump as much as 30 centimeters (12 inches) of rain, potentially triggering flash floods and mudslides in mountainous areas.
The center said Michael will move northward across the Yucatan Channel later Monday, and across the eastern edge of the Gulf of Mexico Monday evening through Wednesday.
"All indications are that it's going to be severe,'' said Tallahassee City Commissioner Gil Ziffer, adding that if the storm hits Florida's capital, there would be significant tree damage and power outages. "Hopefully, we will have no one hurt and no loss of life."
Florida Gov. Rick Scott has declared a state of emergency for 35 counties, a move that frees up resources for storm preparation and activates 500 members of the Florida National Guard.
Residents of barrier islands, mobile homes and low-lying coastal areas in the Big Bend area received mandatory evacuation orders Monday. Forecasters said the area could receive 3.5 meters (11 feet) of storm surge.