The National Hurricane Center says Idalia is a tropical storm on a patch to cross the Gulf of Mexico and get stronger.
"Interests in Florida should monitor the progress of this system," forecasters advised.
They were preparing to issue storm surge and hurricane watches Sunday afternoon for portions of the Florida west coast.
"Strengthening is forecast, and Idalia is expected to become a hurricane by Tuesday."
Idalia is off the Yucatan Peninsula.
The center of Idalia is expected to move across Florida near Tallahassee Wednesday and then into Georgia, South Carolina and North Carolina before reemerging into the Atlantic; still as a tropical storm.
