Beaches in Florida were largely empty ahead of Memorial Day as a slowly intensifying storm carrying brisk winds and heavy rain approached the US Gulf Coast today.

The storm disrupted plans from Pensacola in the Panhandle to Miami Beach on Florida's southeastern edge.

Lifeguards posted red flags along the white sands of Pensacola Beach, where swimming and wading were banned due to high surf and dangerous conditions.

Subtropical Storm Alberto - the first named storm of the 2018 hurricane season that starts June 1 - prompted Florida, Alabama and Mississippi to launch emergency preparations yesterday.

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Rough conditions were expected to roil the seas off the eastern and northern Gulf Coast region until Wednesday.

"These swells are likely to cause life-threatening surf and rip current conditions," the National Hurricane Centre in Miami said.

In Miami, organisers called off the sea portion of the Miami Beach Air & Sea Show because of heavy rain and rough waters. And in the Tampa Bay area on the central Gulf Coast, cities offered sandbags for homeowners worried about floods.

Live video from webcams posted in Clearwater and Destin showed half-empty beaches, and whitecaps roiled the normally placid Gulf waters.

Jeffrey Medlin, meteorologist in charge at the National Weather Service's Mobile office, warned that even after the storm moves north there will still be swells coming up from the south that could cause dangerous rip currents.

Isolated tornadoes were possible across the region today and tomorrow.

- AP