The National Hurricane Center is monitoring two areas in the Atlantic Ocean for potential tropical development behind Hurricane Erin, with the closest area to watch expected to to track near Puerto Rico and the Caribbean.

A tropical storm warning has been issued for North Carolina as Hurricane Erin churns up the east coast of the U.S. as a Category 2 storm.

The first area has been under the eyes of the National Hurricane Center for a few days as it emerged off Africa’s west coast.

With thousands of miles of ocean water above the 80-degree Fahrenheit temperature needed to fuel hurricanes, Cape Verde storms are some of the most dangerous that threaten North America. About 85% of all major hurricanes — Category 3 and higher — start out there, according to the National Hurricane Center.

Hurricane Erin continues to churn in the Atlantic waters hundreds of miles off the U.S., prompting officials to close beaches along the East Coast from the mid-Atlantic to the Northeast.

The National Hurricane Center expects the storm to strengthen over the next several days and says it could become a hurricane by late Thursday.

The National Hurricane Center is tracking two tropical waves in the eastern and central Atlantic Ocean, far from the U.S. coast.

Forecasters are tracking two tropical disturbances behind Hurricane Erin, but neither shows signs of becoming an immediate threat to land. One system east of the Lesser Antilles has only a medium chance of development,