The storm that was expected to make landfall later Monday had maximum sustained winds of 60 miles per hour and was moving north at 8 miles per hour.
"A flash flood watch starts tonight through at least Tuesday as at least 2-4" of rain are expected to fall with locally higher amounts.
Though the Atlantic hurricane season doesn't officially start until Friday, Alberto has become the first named storm this year, throwing disarray into long holiday weekend plans along Florida's Gulf Coast.
The center of Alberto is expected to cross into the warning area later Monday afternoon or evening before crossing into the Tennessee Valley on Tuesday and Wednesday.
A subtropical storm like Alberto has a less defined and cooler center than a tropical storm, and its strongest winds are found farther from its center.
Florida Gov. Rick Scott issued a state of emergency on Saturday for all 67 counties in his state. In Gulf County, T. H. Stone Memorial St. Joseph Peninsula State Park began evacuations Sunday morning.
Alabama and MS have activated their National Guards in preparation for Subtropical Storm Alberto, which is expected to make Landfall in Florida on Monday.
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The National Weather Service has issued a flash flood watch for an area stretching from MS to North Carolina that is home to millions of people.
Alberto also was expected to impact Cuba, with an additional 5 to 10 inches of rain projected to hit Central Cuba, forecasters said, with isolated storm totals of 20 to 25 inches of rain. Grant Brown, the city's public information officer, said they had already finished a number of preparations such as clearing culverts to prepare for big rains but Sunday had turned into a "really nice day".
Florida, Mississippi and Alabama have all declared states of emergency as Subtropical Storm Alberto bears down. It is still predicted to bring extremely heavy rain, gusty winds and, just east of where the center makes landfall, a rise in ocean water of up to several feet above normally dry land.
A CNN image released on May 26, 2018, shows where Subtropical Storm Alberto is likely to make landfall on Monday between New Orleans and Destin, Florida, before moving into MS and Alabama by Tuesday.
Meteorologists expect a turn toward the north-northwest at a slower speed into Sunday.
A Storm Surge Watch is in effect for.
The panhandle region from the Suwanee River to Navarre, Florida is in a Storm Surge Watch area.