(Updating position, winds)

By Tom Brown

MIAMI, June 1 (Reuters) - A tropical storm bearing down on Florida formed in the Gulf of Mexico on Friday, just in time for the official June 1 start of the Atlantic hurricane season.

The Miami-based National Hurricane Center said Tropical Storm Barry was 195 miles (313 km) west of Key West, Florida, at about 11 p.m. (0300 GMT on Saturday).

Barry had maximum sustained winds of about 50 mph (85 kph) and was heading north at about 9 mph (15 kph).

The storm was expected to weaken gradually but still could bring 3 to 6 inches (7.6 to 15.2 cm) of rain to Florida and parts of Georgia and North and South Carolina, the hurricane center said. Up to 10 inches (25 cm) were possible in some areas.

Barry was expected to make a gradual turn to the north-northeast and increase in forward speed during the next 24 hours.

A tropical storm warning was issued for the west coast of Florida, from Bonita Beach northward to Keaton Beach, signaling that tropical storm conditions were possible there within the next 24 hours. Tropical storms have maximum sustained winds ranging from 39 to 73 mph (63-118 kph).

Dave Roberts, a forecaster at the National Hurricane Center, said Barry was unlikely to strengthen into a hurricane.

"Based on the environment that this system is moving beneath, that would be a stretch at this time," Roberts told Reuters. He said it threatened, however, to pack a wallop with its rainfall and gusty winds.

The 2007 Atlantic hurricane season, which forecasters have predicted will be more active than normal, got off to an early start on May 9 with the formation of Subtropical Storm Andrea off the U.S. coast. But Andrea lacked the warm core and organized thunderstorm activity of tropical systems.

The United States was struck by devastating hurricanes in 2005, including Katrina, which flooded New Orleans and parts of the Gulf coast and killed about 1,500 people. Florida was also raked by a series of hurricanes in 2004.

Forecasters had also warned that 2006 would be an active Atlantic hurricane season, but it turned out to be subdued, with 10 storms and no hurricanes hitting the United States.

The Colorado State University team of experts under forecast pioneer William Gray renewed its call on Thursday for an above-average 17 storms in the 2007 hurricane season, saying nine would grow to hurricane strength.

Of those, five will grow into major hurricanes of Category 3 or higher with winds over 110 mph (177 kph), the team said in its forecast, which was unchanged from April 3.

The height of the six-month hurricane season is usually not until August and September.

The Colorado State researchers also said there was a 74 percent chance at least one major hurricane would hit the U.S. coastline.

Other private and government weather researchers have also predicted an active season. So far, none has forecast anything like a repeat of record-breaking 2005, when 28 tropical storms spawned 15 hurricanes.

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