ST. PETERSBURG, Fla. — The first named storm of the Atlantic hurricane season, Subtropical Storm Alberto, gained strength as it approached the northern Gulf Coast, emptying out beaches in Florida ahead of Memorial Day.The storm disrupted long holiday weekend plans from Pensacola in the Florida 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 amid high surf and dangerous conditions.It also triggered mandatory evacuations of some small, sparsely populated Gulf Coast barrier islands in one Florida county. The Florida Division of Emergency Management said in a statement Sunday that a mandatory evacuation has been issued in Franklin County for all barrier islands there and those in the county living directly on the coast in mobile homes or in recreation vehicle parks.Alberto got an early jump on the 2018 hurricane season, which doesn’t officially start until June 1. The storm prompted Florida, Alabama and Mississippi to launch emergency preparations over the weekend amid expectations Alberto would reach land sometime Monday. 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,” according to National Hurricane Center in Miami.Gusty showers were to begin lashing parts of Florida on Sunday, and authorities were warning of the possibility of flash flooding.At 7:30 p.m. EDT Sunday, Alberto was centered about 195 miles west of Tampa and had maximum sustained winds of 65 mph — up from 50 mph earlier.Forecasters said Alberto has most recently taken a north-northwest track that would bring it over the northern Gulf of Mexico during the night and make landfall on or in the vicinity of the Florida Panhandle on Monday.