(Recasts with system's strengthening into name storm)

MIAMI, Sept 23 (Reuters) - Subtropical Storm Jerry formed over the Atlantic Ocean on Sunday and U.S. forecasters said it was likely to strengthen over the next day or so.

Long-range computer models indicated the storm posed no threat to the United States although it would prove menacing for ships in the open Atlantic.

As of 11 a.m. (1500 GMT), the storm was located about 1,070 miles (1,725 km) west of the Azores with maximum sustained winds near 40 mph (64 kph), the Miami-based National Hurricane Center said.

It said some strengthening was expected over the next 24 hours.

A subtropical storm shares some characteristics with tropical storms and hurricanes but has a cooler core that can slow its development.

Ten named storms have formed in the Atlantic basin so far this year and three of those have become hurricanes. Forecasters had predicted up to 16 storms for the six-month season, which runs from June 1 to Nov. 30.

The 2007 season has also seen, for the first time since records began in 1851, two maximum-strength Category 5 hurricanes making landfall in the same year.

Hurricanes Dean and Felix, which both reached the top rank on the five-step Saffir-Simpson scale of hurricane intensity, hit Mexico's Yucatan Peninsula and Central America in August and September, respectively.

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