ST. PETERSBURG, Florida - Beaches in Florida were largely empty ahead of Memorial Day as a slowly intensifying storm carrying brisk winds and heavy rain approached the U.S. Gulf Coast on Sunday. 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 are banned due to high surf and dangerous conditions.

Subtropical Storm Alberto -- the first named storm of the 2018 hurricane season -- prompted Florida, Alabama and Mississippi to launch emergency preparations Saturday. Rough conditions were expected to roil the seas off the eastern and northern Gulf Coast region through Tuesday.

"These swells are likely to cause life-threatening surf and rip current conditions," the National Hurricane Center (NHC) in Miami said in a statement. The governors of Florida, Alabama and Mississippi all declared states of emergency ahead of the storm.

The hurricane center said Sunday that a tropical storm warning was in effect from Bonita Beach, Florida, to the Mississippi-Alabama border.

On Sunday, gusty showers began lashing parts of Florida, and authorities were warning of the possibility of flash flooding. 

The National Weather Service (NWS) said Sunday that tropical storm warnings cover the entire Florida Gulf Coast and inland portions of the Florida Panhandle as well as parts of coastal Alabama. 

What is a subtropical storm?
The NHC defines subtropical storm as a "subtropical cyclone" in which the maximum sustained wind speed -- using the U.S. one-minute average -- is 39 mph or higher. Subtropical storms have cooler centers than tropical storms, but they can eventually develop into tropical storms and then into hurricanes.