Storm closes parts of I-10 in NC

* Storm finally moving after stalling in Gulf of Mexico

* Torrential downpours threaten already flooded areas

* Storm seen crossing Florida into Atlantic by Thursday

MIAMI, June 26 (Reuters) - Tropical Storm Debby drifted slowly eastward over Florida's Gulf Coast on Tuesday, threatening to dump more rain on areas already beset by flooding.

After stalling in the Gulf of Mexico, the storm was finally moving but was expected to take two more days to finish its wet slog across Florida.

Nearly 20 inches (51 cm) of rain has fallen in two days on coastal Wakulla County, which is famed for its natural springs. Roads were under water in many parts of the surrounding "Big Bend" area where the Florida Panhandle meets the peninsula.

Forecasters at the National Hurricane Center said Debby could bring another 4 to 8 inches (10 to 20 cm) of rain and possibly tornadoes to north Florida in the next two days.

Debby had top winds of 45 miles per hour (75 km per hour). It was expected to weaken on Thursday as the center crossed over land, but could strengthen back into a tropical storm as it moves into the Atlantic Ocean, the forecasters said.

The center of circulation was still in the Gulf of Mexico, about 85 miles (140 km) west of Cedar Key, Florida. But Debby was a large and ragged storm, with most of the thunderstorms and rain north and east of the center, over Florida.

Tropical storm warnings were in effect for much of Florida's Gulf coast and could be extended inland as the storm moves slowly east over the state in the next few days.

Governor Rick Scott declared a state of emergency on Monday and ordered all state agencies, including the Florida National Guard, to provide any necessary assistance requested by local governments.

"Because of the broad impact of Tropical Storm Debby, virtually every county in Florida could be affected," Scott said. "Some communities are already grappling with flooding, wind damage and electrical outages."

Debby was the first tropical storm of 2012 to form in the Gulf of Mexico. Over the weekend, the storm idled about a quarter of U.S. offshore oil and natural gas production in the Gulf of Mexico, based on figures issued by U.S. offshore regulators.