The 26th named storm of the record-breaking hurricane season had top sustained winds near 85 mph, despite earlier predictions that it would weaken, according to the National Hurricane Center in Miami. Epsilon had sustained winds of 70 mph earlier in the day; hurricanes have winds of at least 74 mph.
"There are no clear reasons and I am not going to make one up to explain the recent strengthening of Epsilon," said hurricane specialist Lixion Avila.
But he said cooler waters, higher wind shear and drier air should cause Epsilon to gradually weaken later in the day. Epsilon first reached hurricane strength on Friday and is the 14th hurricane of the season.
At 10 a.m. EST, Epsilon was centered about 725 miles west-southwest of the Azores and moving east near 12 mph.
The Atlantic hurricane season began June 1 and officially ended on Wednesday.
Epsilon was only the fifth hurricane to form in December in more than 150 years of records, hurricane specialist Stacy Stewart said. The latest that a hurricane has formed in the Caribbean was Dec. 30, in 1954.