Zeta, the 27th storm of the season, formed Friday about 1,000 miles south-southwest of the Azores islands, according to an advisory posted on the National Hurricane Center's Web site. It posed no immediate threat to land.
The center said it would send out a full advisory later Friday. Tropical storms have winds of at least 39 mph.
It was not immediately known if Dec. 30 was the latest date for the formation of a tropical storm in the Atlantic. But earlier this month, Hurricane Epsilon became only the fifth hurricane to form in December in 154 years of record-keeping. Hurricanes form when their winds exceed 74 mph.
Zeta is the sixth letter of the Greek alphabet, which forecasters turned to after they used up — for the first time — their list of 21 proper names for storms. The record for tropical storms and hurricanes in a season had been 21, set in 1933 before such storms were regularly named.
The 2005 Atlantic storm season, which officially ended Nov. 30, included 14 hurricanes, including Epsilon.
One of the hurricanes, Katrina, destroyed large portions of Louisiana and Mississippi last August in the most costly disaster in U.S. history. Hurricanes Dennis, Rita and Wilma also caused significant damage in the U.S.
Forecasters have said that hurricane seasons are going to be more active than usual for at least another decade — and possibly as long as 50 years.