Tropical Storm Erin formed in the eastern Atlantic Ocean on Monday, forecasters said. The storm could strengthen to become the Atlantic's first hurricane of the season.

The storm formed just west of the Cabo Verde islands off the western coast of Africa, the U.S. National Hurricane Center in Miami said. Erin was expected to move west, but it wasn't immediately clear if it would pose any threat to the U.S.

The National Weather Service predicted an above-normal season for the Atlantic basin this year with an expected number of named storms between 13 and 18, five to nine of which could become hurricanes.

A tropical cyclone becomes a tropical storm when its maximum sustained wind speeds reach at least 39 mph. Hurricanes are more powerful with sustained winds of at least 74 mph. Hurricanes are rated on a scale ranging from Category 1 to Category 5, which is the most severe rating. A storm is considered to be a major hurricane when it reaches Category 3 strength with sustained winds of at least 111 mph.

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