JACKSONVILLE, Fla. -

The National Hurricane Center could name a disturbance in the eastern Atlantic as a tropical storm in the next 48 hours, only the third tropical storm to occur in May since 1981.

The storm that has caught the eye of forecasters is 460 miles west-southwest of the Azores in the Atlantic, according to a bulletin from the National Hurricane Center. That system has a 20 percent chance of becoming a named storm in the next two days.

Tropical or subtropical cyclones are rare before June, but they do happen on occasion.

There have been 21 tropical or subtropical storms that have formed in May from 1851 to 2011. The most recent May storms occurred in 2007 and 2008.

In 2007, Subtropical Storm Andrea formed and meandered around the Atlantic off the southeast coast of the U.S. without making landfall. Andrea was the 8th earliest cyclone since 1851 and the first May cyclone since 1981.

In 2008, Tropical Storm Arthur was a short-lived storm that formed over the Northwest Caribbean and made landfall in Belize. The maximum intensity reached was 45 mph.

If the NHC writes advisories on today's system, it would be only the third May cyclone since 1981.