Cuba was being pounded by rain along its western coast, raising the threat of flash floods and mudslides.
The slow-moving system is expected to cause wet misery across the eastern U.S. Gulf Coast over the holiday weekend.
It is not organized enough yet to be a tropical storm, and is considered subtropical.
Florida's Governor, Rick Scott, has also declared a state of emergency.
The forecast track of Subtropical Storm Alberto has shifted farther east, the National Hurricane Center said Saturday night. Alabama Governor Kay Ivey issued a state of emergency covering 40 counties beginning at 6 a.m. on Sunday.
Gov. Phil Bryant signed a proclamation Saturday morning declaring a state of emergency ahead of Alberto's landfall. A storm surge watch has been discontinued west of the Florida/Alabama border.
Flash floods, thunderstorms expected going into Memorial Day
Forecasters warn to watch for flooding, especially in urban areas, and potentially strong winds this afternoon. Akron's record for Monday is 93 degrees set in 1911, according to the National Weather Service .
Authorities say conditions are especially risky with flooding rains coming overnight and on a holiday weekend when many people have outdoor plans.
"The combination of storm surge and the tide will cause normally dry areas near the coast to be flooded by rising waters moving inland from the shoreline", the National Hurricane Center in Miami said.
A tornado "or two" may occur over the Florida Keys and parts of southwestern Florida later Saturday afternoon or evening. As of late Sunday night, it had 65 mph winds, and the center was roughly 100 miles south of Apalachicola, Florida. As it travels up the warm waters of the Gulf, it could well become a full tropical storm. Expect rounds of heavy rain through Tuesday with widespread rain totals of 2 to 3 inches expected with locally higher amounts possible. A gradual strengthening was expected as the storm moves north.
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Subtropical Storm Alberto is expected to hit the United States northern Gulf Coast on Monday, the National Weather Service has declared. Subtropical storms can develop into tropical storms, which in turn can strengthen into hurricanes.
An official from the National Weather Service warns that even after Subtropical Storm Alberto passes, there's still a risk for rip currents. There were no immediate reports of emergencies.
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