"On the forecast track, the center of Alberto will move over the northern Gulf of Mexico overnight and cross the northern Gulf Coast in the warning area on Monday", the National Hurricane Center said.
Mississippi Governor Phil Bryant authorised the use of the National Guard, his office said in a statement. Alberto was expected to bring heavy rain and severe wind to Florida and other parts of the Gulf Coast.
It will bring heavy rain and winds of 100kmh (65mph) across southern states, the US National Hurricane Center said.
At 8 p.m. Saturday, the storm was about 105 miles north-northwest of the western tip of Cuba or about 150 miles southwest of the Dry Tortugas, according to the National Hurricane Center.
Alberto is moving north at 15 miles per hour with a minimum pressure of 1001 mb. He said Alberto's biggest threat will be its heavy rains, with forecasts of anywhere from four to 12 inches (10-30 centimeters) of rain in some areas.
The weather service posted a storm surge watch from Crystal River northward with 3 to 4 feet of surge possible Sunday - 5 to 6 feet at high tide.
Landfall is expected to be between Seagrove Beach and Destin in the Florida Panhandle.
"If you are in an area under a storm surge watch or warning and have not heard whether or not you should evacuate, contact your local emergency management and leave today according to their instructions", Knabb said. At the moment nearly all of the showers are on the east of Alberto's centre, with the heaviest rains falling across Cuba and Florida. In contrast to a tropical storm or hurricane, where the strongest winds are at the center, a subtropical storm can have the most powerful winds far from the core. Only a half-inch or less of rain is expected for Monday and the chance of rain will decrease from 70 percent to 40 percent as the day turns into evening.
Alberto is the first officially named storm of this hurrican season.
The atmosphere will continue to be somewhat conducive for rotating isolated thunderstorms, so some tornadoes are possible in South Florida. The storm was on track for a potential landfall in the Florida Panhandle, though the cone of uncertainty stretched well east and west of that mark.
Israeli forces kill three Palestinian militants in strike on observation post
The ministry identified those killed by tank fire as Hussein al-Amour, 25, and Abdul Haleem al-Naqa, 28. Since then, Israeli forces have killed at least 124 Palestinians and injured thousands more.