The storm was moving north at 6 miles per hour with winds that remained at 65 miles per hour.
Between 2 and 6 inches of rain could soak Alabama and western Georgia on Tuesday, and isolated deluges of 12 inches also are possible as the system heads into the Tennessee Valley on its way to the Ohio Valley and Great Lakes region.
"From an economic standpoint most of the damage is already done, probably $600 or $700 million worth of lost economic activity due to rain, warnings and preparations", said Chuck Watson, a disaster modeler at Enki Research in Savannah, Georgia. Lifeguards posted red flags along the white sands of Pensacola Beach, where swimming and wading were banned as Alberto disrupted long holiday weekend plans for millions.
Heavy rainfall and flash flooding were occurring over parts of the Florida Panhandle, the National Hurricane Center said. Gusty winds have knocked out some power and there is local coastal flooding on the very storm surge-prone coast south of Tallahassee.
Florida Gov. Rick Scott issued a declaration for all 67 counties in his state.
We will have to watch the far eastern side of Alabama today once we get a little heating and the atmosphere destabilizes because storms that form could produce strong wind gusts or even spin up tornadoes. They will develop northwest of the Tennessee Valley late Thursday into early Friday and move southeast as a large "complex" of storms.
As Subtropical Storm Alberto barrels towards the Florida coast, Atlanta can expect to see heavy showers and the possibility for severe weather later Monday afternoon. A tropical storm warning was discontinued from Florida's Anclote River to the Suwannee River.
Meanwhile, potentially life-threatening rough surf and rip currents continued on the northern Gulf Coast after Alberto rolled up big waves and tides along the coast.
The evacuations affect about 4,200 homes.
In Miami, organizers called off the sea portion of the Miami Beach Air & Sea Show on Sunday because of heavy rain and rough waters. Destin and Panama City Beach are within the watch area. Monday will see a few afternoon showers or storms and Tuesday will be dry with highs in the mid 80's both days. The storm had maximum sustained winds of 65 miles per hour (100 kph) as it approached the northern Gulf of Mexico. Last year, there were 17 named storms, 10 of which became hurricanes; the first named storm was Arlene - a rare preseason tropical storm that arrived in April.