Hurricane Gordon could also pass over the Azores as a weaker storm sometime midweek, so National Hurricane Center forecasters said a tropical storm watch or warning may be required later Monday for the islands in the central Atlantic.
It was too soon to tell whether Helene would hit Bermuda, but the storm with top sustained winds of 120 mph was expected to be near the territory Friday, said Chris Landsea, a meteorologist with the National Hurricane Center in Miami.
Helene strengthened late Sunday into the second major hurricane of the Atlantic season and intensified slightly early Monday. Major hurricanes are those Category 3 and higher.
At 5 a.m. EDT, it was centered about 920 miles east-northeast of the northern Leeward Islands and about 1,160 miles east-southeast of Bermuda. It was moving north-northwest at 7 mph, forecasters said.
Meanwhile, Gordon was in the open Atlantic, centered about 1,245 miles west of the Azores and moving northeast near 20 mph. Gordon, the other storm to reach Category 3 status this year, had top sustained winds near 80 mph Monday.
Gordon was expected to lose its tropical characteristics and weaken before it reached the Azores, but forecasters said watches or warnings could be needed as a precaution. Even if it did become an extratropical storm that gets energy from colliding weather fronts, it could still have tropical storm force winds.
The National Hurricane Center's latest forecast for the Atlantic season expects between seven and nine hurricanes, a slight reduction from earlier predictions.
Scientists have said that weak El Nino conditions had inhibited hurricane development by bringing higher ocean temperatures that increase crosswinds over the Caribbean. The winds can rip storms apart or stop them from forming.
But the National Oceanic and Atmospheric Administration has warned that the El Nino effect on hurricanes has been small so far. And the season, which lasts until Nov. 30, is still at its traditional peak.