The threat for strong storms and widespread heavy rain decreases for Wednesday.
Alberto was 50 miles (80 kilometers) south of Panama City, Florida, and moving north at 8 miles per hour, the National Hurricane Center in Miami said in an advisory at 11 a.m. A falling tree killed two TV journalists who were covering the storm in North Carolina.
He added: "North Carolina needs to take Alberto seriously".
Alberto also disrupted Memorial Day plans in Alabama, Florida and MS as it moved into the Southeast.
Geoffrey Tennant, a fire chief in North Carolina said he spoke to the crew only 10 minutes before they were struck and killed by the falling tree.
Jason Powell said he was seeking to keep his children entertained until Alberto blows past his Panhandle vacation spot. And, it said, heavy rains are also expected in many areas.
Alberto formed several days ahead of the June 1 official start of the six-month Atlantic hurricane season.
The emergency declarations free up state resources to respond to the storm if necessary.
Mississippi Governor Phil Bryant authorised the use of the National Guard, his office said in a statement.
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In Maryland, a state of emergency was issued after extreme flooding and heavy rain hit the state.
"On the forecast track, the center of Alberto will move over Alabama later tonight and Tuesday", the NHC said.
Alberto will probably weaken through the day as it moves northward into the Tennessee Valley and then to the Ohio Valley, finally withering into a "remnant low pressure storm" by this evening, with winds at around 40km/h, Mr Roth said.
It is forecast to drop as much as 30cm (12in) of rain across MS to western Georgia and to bring storm swells of about 60-120cm (2-4ft) to low-lying areas. The Florida Keys and Florida peninsula could receive up to 5 inches of rain in some areas. The rains may produce flash flooding, the National Hurricane Center said. And in the Tampa Bay area on the central Gulf Coast, cities offered sandbags for homeowners anxious about floods.
Authorities did not immediately attribute any deaths or injuries strictly related to Alberto. There will also be a chance of showers and thunderstorms on Saturday afternoon, forecasters wrote.
Additionally, NOAA predicted there would be anywhere from five to nine hurricanes and one to four major hurricanes before the season's end in November.
While Alberto nears the shore, the warning area has been scaled back from the Suwannee River back west to the Florida-Alabama state line.
If that forecast holds, it would make for a near-normal or above-normal season.