Storm Alberto churns toward Florida, Alabama and Mississippi

May 27 (Reuters) - Gulf Coast residents should "take this storm seriously," the National Weather Service said on Sunday as Subtropical Storm Alberto drove north through the Gulf of Mexico, threatening heavy rains and winds to the southern coastal states.

Florida, Alabama and Mississippi declared states of emergency on Saturday with up to 15 inches (38 cm) of rain forecast over the Memorial Day weekend, as well as tidal surges and damaging winds, according to Patrick Burke of the weather service's Weather Prediction Center in College Park, Maryland.

"This is jogging more to the east and will hit the Florida panhandle," Burke said. "We're talking eight to 12 inches of rain this weekend, and storm surges on the Gulf Coast."

"It's heavy rainfall on the entire coast," he said.

By midday Sunday, the U.S. Southeast was seeing 50-mile-per-hour (80-km-per-hour) winds and up to 10 inches of rain in some places, Ken Graham, director of the National Oceanic and Atmospheric Administration's National Hurricane Center, said in a video briefing. Downpours caused some street flooding, he said.

"So we need to really be careful with that rainfall," Graham said. The storm was moving north at about 14 mph, he added, "so not real fast."

Alberto, the first named Atlantic storm of 2018 which spun up days before the formal start of the 2018 hurricane season, is expected to intensify and bring wind speeds of up to 65 mph (40 kph) to the Gulf Coast when it approaches over the holiday on Monday, the National Weather Service said.

By midday on Sunday, the storm was about 240 miles (386 km) northwest of Key West, Florida, according to the U.S. National Hurricane Center (NHC).

State officials urged their residents to monitor the storm.

"I ask everyone to please make final preparations to your family emergency plan, especially those that live in mobile homes and low-lying areas," Mississippi Governor Phil Bryant said in a statement.

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, Exxon Mobil Corp and Chevron Corp have evacuated some personnel from offshore Gulf oil facilities. (Reporting by Rich McKay in Atlanta and Suzannah Gonzales in Chicago; Additional reporting by Gary McWilliams in Houston; Editing by Daniel Wallis and Lisa Shumaker)