(Adds Alabama to states declaring emergency, updates storm details throughout)

By Gina Cherelus

May 26 (Reuters) - Florida, Alabama and Mississippi declared states of emergency on Saturday as Subtropical Storm Alberto drove north toward the U.S. Gulf Coast, threatening to bring heavy rainfall and flooding to the coastal states by Monday.

The first named Atlantic storm of 2018 is expected to intensify and bring wind speeds of up to 65 miles per hour (40 km/h) to the northern Gulf Coast when it approaches on the Memorial Day holiday on Monday, the National Weather Service (NWS) said in its latest advisory.

A tropical storm warning has been issued for the Mississippi/Alabama border to the Aucilla River in Florida. Rainfall totals of between 5 and 10 inches (13-25 cm), with up to 15 inches, are possible from eastern Louisiana to Mississippi, Alabama, western Tennessee and the western Florida panhandle, the NWS said.

All 67 Florida counties were issued the emergency notice to give state and local governments enough time and resources to prepare, Florida Governor Rick Scott said in a statement.

"As we continue to monitor Subtropical Storm Alberto's northward path toward Florida, it is critically important that all Florida counties have every available resource to keep families safe and prepare for the torrential rain and severe flooding this storm will bring," Scott said.

His Mississippi and Alabama counterparts also declared states of emergency, citing the threat of coastal and inland flooding from storm rains. Alabama Governor Kay Ivey issued a state of emergency covering 40 counties beginning at 6 a.m. on Sunday.

"Whether you're a resident of this state or just visiting, you need to stay updated on this evolving tropical system," Mississippi Governor Phil Bryant said in a statement. "I ask everyone to please make final preparations to your family emergency plan, especially those that live in mobile homes and low-lying areas." On Saturday evening, the storm was located about 95 miles (153 km) north of the western tip of Cuba and 275 miles (440 km) southwest of the Dry Tortugas, which is almost 70 miles (113 km) west of Key West, Florida, according to the U.S. National Hurricane Center (NHC).

Alberto, which spun up days before the formal start of the 2018 hurricane season, was moving north at about 13 mph (20 km/h) with maximum sustained winds near 40 mph, and higher gusts, on Saturday, the NWS said. Gradual strengthening is forecast until the system reaches the northern Gulf Coast.

Alberto's projected storm track has shifted eastward since Friday, lessening its threat to the active oil production areas in the Gulf of Mexico. Royal Dutch Shell plc and Exxon Mobil evacuated some personnel from offshore Gulf oil facilities on Friday. (Reporting by Gina Cherelus in New York Additional reporting by Gary McWilliams in Houston Editing by Rosalba O'Brien, Marguerita Choy and Paul Tait)