The major tourist communities of Destin and Panama City Beach are within the watch area.
Alberto is moving north at 9 miles per hour.
Mandatory evacuations were ordered for small barrier islands in one northern Florida county, and voluntary evacuations were issued for another.
A storm surge watch remained in effect for much of Northern Florida, from the Suwannee to Navarre in the Panhandle.
After reaching the coast, the storm will bring powerful winds and heavy rains as it moves into the Tennessee Valley on Tuesday and Wednesday, the hurricane center said.More news: Israel keeps Gaza residents 'caged in a toxic slum': UN's Zeid
A Tropical Storm Warning remains in effect from the Suwannee River in Florida to the Alabama/Florida border.
Frequent updates on Subtropical Storm Alberto, the tornado risk in north Florida, and the ongoing heavy rain episodes in South Florida are available on the Florida Storms Facebook and Twitter accounts, or via the mobile app Florida Storms. Alberto will bring widespread heavy rain in the Panhandle of Florida and Alabama later today and tonight. This is more rain than Havana expects in the first six months of the year, and is likely to produce life-threatening flash floods and mudslides. The storm had maximum sustained winds of 65 miles per hour as it approached the northern Gulf of Mexico. Alberto will become the ninth named storm to make landfall on the continental United States before June 1 on record. After reaching top winds of 45 mph and spending 30 hours as a tropical storm, Arlene died.
"Hazardous storm surge is possible along portions of the central and eastern Gulf Coast beginning Sunday".
A graphic from the National Weather Service shows Subtropical Storm Alberto's projected path as of 5 p.m. ET on May 28, 2018. While coastal areas may get from 3 to 6 inches of rain, the Florida panhandle into much of Alabama and western Georgia may see 4 to8 inches with isolated storm totals of 10 to 12 inches. However, once Alberto is inland and deprived of the warm waters that fuel tropical weather systems, the storm was expected to steadily weaken. Florida Governor Rick Scott declared a state of emergency in all 67 of Florida's counties on Sunday and the office of Mississippi Governor Phil Bryant issued a statement announcing that he had authorized the use of the national guard.
Authorities in Howard County, Maryland, said a 39-year-old man was missing after flash flooding from a separate storm tore through the historic downtown of Ellicott City on Sunday.