MIAMI (Reuters) - Tropical Storm Bertha, moving briskly across open ocean waters, may strengthen into the first hurricane of the 2008 Atlantic storm season sometime during the next 48 hours, U.S. weather forecasters said on Sunday.

Bertha, which formed on Thursday near the Cape Verde islands off Africa, had maximum sustained winds approaching 50 miles per hour (85 km per hour) at 11 a.m. EDT and was heading west at 21 mph (33 kph), the U.S. National Hurricane Center in Miami said.

"Gradual strengthening is expected during the next 48 hours, and Bertha could become a hurricane during that time," the forecasters said in an advisory.

Storms are judged to be hurricanes when their wind speed reaches at least 74 mph (119 kph).

Bertha is nowhere near land, and the hurricane center's tracking forecasts have the storm staying at sea and taking a general west-northwest route over the next five days. The storm's speed was expected to ease during the next two days.

"It is much too early to determine if Bertha will eventually affect any land areas," the forecasters said.

The storm's center was near latitude 17.4 north and longitude 45.1 west, or about 1,185 miles east of the northern Leeward Islands.

Bertha is the second tropical storm of what has been predicted to be an above-average storm season in the Atlantic and Caribbean.

Energy markets closely monitor tropical storms in the Atlantic because of the potential for them to threaten oil installations in the Gulf of Mexico. The 2004 and 2005 hurricane seasons, which included Katrina, the hurricane that swamped New Orleans, left dozens of oil rigs toppled or damaged and sent crude prices to what were then record highs.

The Atlantic-Caribbean hurricane season runs from June 1 to the end of November, with August and September usually the busiest months.

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