Forecasters said Alberto could bring dangerous high water to southern coastal states when it douses an area from Mississippi to western Georgia with up to 12 inches (30 cm) of rain and possible tornadoes.
But Alberto's top winds weakened as the storm advanced, dropping to 45 miles an hour (75 km per hour) as the center of the storm made landfall near Panama City, Florida, the National Weather Service said.
Heavy rainfall and flash flooding were occurring over parts of the Florida Panhandle, the National Hurricane Center said.
In Panama City, Assistant Fire Chief Gary Swearingen said the rain and wind knocked down trees but did not require any rescues.
Alberto, the first named Atlantic storm of 2018, spun up days before the formal June 1 start of the hurricane season.
Authorities in Florida's Franklin and Taylor counties issued mandatory evacuation orders for thousands of coastal residents.
The storm will bring powerful winds and heavy rains as it moves into the Tennessee Valley on Tuesday and Wednesday, the hurricane center said. The storm hit on the last day of the Memorial Day weekend, complicating holiday travel.
In Maryland, officials in Howard County said a 39-year-old man was missing after flash flooding from a separate storm tore through the historic downtown of Ellicott City on Sunday. The man was swept away as he tried to help rescue people from floodwaters.
The area had barely recovered from a devastating flood about two years ago that killed two people and damaged dozens of buildings.
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)
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