Humberto grew to hurricane force far out in the Atlantic on Wednesday, becoming the first hurricane of the Atlantic season, while Bermuda shook off a drenching by tropical storm Gabrielle.

Humberto's maximum sustained winds were near 140 km/h, but the U.S. National Hurricane Center said some weakening was expected over the next two days.

The storm was centered about 630 kilometres northwest of the Cape Verde Islands just before midnight ET and was moving northward at 20 km/h into the open Atlantic.

In Bermuda, Gabrielle caused several power outages overnight, but no major damage was reported on the British territory.

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Tropical storm Humberto strengthened to a hurricane early Wednesday. (NASA/Reuters)

The government announced that all public schools were reopening and there would be sporadic ferry service.

The island's meteorology service warned that strong winds and rain would continue to pelt Bermuda as the storm headed north.

By late Wednesday, Gabrielle had weakened to a tropical depression and was about 220 kilometres west-northwest of Bermuda with sustained winds of 55 km/h. It was moving westward at 9 km/h, and was predicted to pick up forward speed.

It was expected to turn to the north-northeast and remain well off the U.S. East Coast while heading toward a possible weekend brush with Nova Scotia.

It is unusual for this much time to pass in the Atlantic season without a hurricane forming, though not unheard of. Hurricane Gustav was the first of the 2002 season when it formed on Sept. 11 of that year. The record for the latest first-of-the-season hurricane is Oct. 8, 1905, based on records dating to 1851.