The U.S. National Hurricane Center in Miami said at 4 a.m., Bertha was 315 miles west-southwest of the Cape Verde Islands moving toward the west-northwest near 14 mph.
The storm had sustained winds of about 45 mph with higher gusts, and forecasters said an increase in both wind speed and forward motion was likely in the next two days.
To become a Category 1 hurricane, sustained wind speeds must exceed 74 mph.
Meteorologist Dr. Jeff Masters said Thursday's formation of Bertha at 25 degrees west longitude is the farthest east a tropical storm has ever formed in the Atlantic Ocean so early in the season since satellite tracking was established in 1967.
The first named storm this year, Arthur, formed near Belize the day before the season officially started June 1 and drenched the Yucatan Peninsula with rain.