MIAMI, July 5 (Reuters) - Tropical Storm Bertha was expected to become the 2008 Atlantic storm season's first hurricane as it steamed toward the United States over open waters between Bermuda and the Caribbean islands, U.S. weather forecasters said on Saturday.

The second tropical storm of what is predicted to be an above-average hurricane season had not strengthened from Friday, when its top sustained winds reached 50 miles per hour (85 km per hour), the U.S. National Hurricane Center said.

But sea surface temperatures and atmospheric conditions were expected to be "more conducive" in about 72 hours for Bertha to reach hurricane strength, with winds of at least 74 mph (119 kph), the Miami-based center said.

The storm's projected path as a hurricane remained uncertain so far out, with the majority of computer models used to predict storm tracks suggesting Bertha would turn to the north-west toward Bermuda. One model, however, predicted the storm would keep on going straight toward the Caribbean.

At 11 a.m. (1500 GMT) Tropical Storm Bertha was around 885 miles (1,420 km) west of the Cape Verde Islands and about 1,705 miles (2,740 km) east of the northern Leeward islands of the Caribbean, the hurricane center said.

It was moving west at a relatively brisk 21 mph (33 kph).

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.

Bertha's formation off the African coast near the Cape Verde islands could be an unwelcome portent of the season ahead.

It is unusual for storms to form so far east so early in the year. And when they do form in that area in June or July, total storm activity for the year tends to be at least average and often above average, according to the National Oceanic and Atmospheric Administration.

(Reporting by Michael Christie; Editing by Sandra Maler)

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