ST. PETERSBURG, Fla. — 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 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 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 in Miami said in a statement.

In Miami, organizers called off the sea portion of the Miami Beach Air & Sea Show Sunday 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.

Gusty showers were to begin lashing parts of Florida on Sunday, and authorities were warning of the possibility of flash flooding.

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

In Gulf Shores, Alabama, webcams showed beaches beginning to fill up as the storm's track shifted slightly east away from the region, but red flags on the beach warned beachgoers to stay out of the rough water. Grant Brown, the city's public information officer, said they had already finished a number of preparations such as clearing culverts to prepare for big rains but Sunday had turned into a "really nice day."

With conditions expected to worsen overnight officials are encouraging people planning to check-out Monday to give themselves extra time.

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. Just because it's "nice and sunny" after the storm passes, Medlin says there's still a risk for swimmers.

"People have drowned by going out to the water too soon," he said.

Isolated tornadoes were possible across the region on Sunday and Monday.

Under overcast skies and occasional drizzle, several Gulfport, Mississippi, residents lined up to fill 10- and 20-pound (5- and 9-kilogram) bags with sand they will use to block any encroaching floodwater expected as a result of Alberto.

Tommy Whitlock said sandbagging has become a usual event in his life since he lives next to a creek.

"I'm doing this because every time we have a hard rain, it floods at my house," Whitlock said. "We get water from other neighborhoods, and water can get up to a foot deep in some places."

Alberto is expected to strengthen until it reaches the northern Gulf Coast, likely on Monday night.

The NWS said waves as high as 18 feet (5.5 meters) could pound the popular Gulf beaches in Baldwin County, Alabama, and northwestern Florida on Monday. A high surf warning was in effect through 7 p.m. Tuesday local time.

At 2 p.m. EDT Sunday, the National Hurricane Center in Miami said Alberto was about 135 miles (220 kilometers) west of Tampa, Florida, and moving north at 13 mph (20 kph). The storm had top sustained winds of 50 mph (85 kph).

A subtropical storm like Alberto has a less defined and cooler center than a tropical storm, and its strongest winds are found farther from its center. Subtropical storms can develop into tropical storms, which in turn can strengthen into hurricanes.

Tracey Gasper and her 6-year-old son, Chase, traveled to Biloxi Beach from Donaldsonville, Louisiana, for a day of fun in the sun with a group of friends from Baton Rouge, Louisiana. The weather had scared off the usual crowds expected for the holiday weekend.

"It was a 50-50 chance of whether to come down and we decided to chance it," Gasper said.

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AP writer Rebecca Santana reported from New Orleans.