Tropical Storm Hermine formed Wednesday in the Gulf of Mexico, threatening to flood and cause storm surges in the US states of Florida and Georgia, forecasters said.

A hurricane watch was in effect for northern Florida, where Governor Rick Scott declared a state of emergency in 42 countries to free up resources for the storm.

"It is crucial that every Floridian has a plan in place to ensure their families, homes and businesses are fully prepared," Scott said in a statement.

The storm, which formed in the middle of the Gulf, was located 415 miles (665 kilometers) west-southwest of Tampa.

It was moving northward at a leisurely pace of two miles per hour, the Miami-based National Hurricane Center said. It had top winds of 40 miles per hour.

"The combination of a dangerous storm surge and the tide will cause normally dry areas near the coast to be flooded by rising waters moving inland from the shoreline," the NHC said, warning that the storm was due to reach Florida's northwestern coastline by late Thursday.

"There is a danger of life-threatening inundation within the next 36 to 48 hours along the Gulf coast of Florida from Aripeka to Indian Pass."

Up to 20 inches (51 centimeters) of rain could fall over western Cuba on Wednesday.

By Friday, up to 15 inches could fall over portions of central and northern Florida, and up to 10 inches through early Saturday in coastal areas of Georgia and North and South Carolina.

"These rains may cause flooding and flash flooding," the NHC warned.

The Atlantic hurricane season runs June 1 to November 30, but this year's first hurricane, Alex, formed in January in an unusual weather event.

Earlier this month, the season's second hurricane, Earl, left more than 45 people dead in Mexico.

The 2015 season was less active than average, with 11 tropical storms in the Atlantic, of which four became hurricanes and two major hurricanes.