Bertha has weakened to a tropical storm just a day after becoming a hurricane, and could brush parts of Atlantic Canada late Wednesday as it moves northward.

The storm's maximum sustained winds decreased to near 100 km/h early Tuesday with gradual weakening expected over the next two days.

The U.S. National Hurricane Center forecasts Bertha will pass midway between the American East Coast and Bermuda later in the morning.

The storm is centred about 765 kilometres west of Bermuda and is moving north-northeast near 35 km/h.

The Canadian Hurricane Centre in Halifax said the storm was expected to move well south of Nova Scotia with its northern tip expected to spread clouds over the province.

According to CBC meteorologist Jim Abraham, the storm is expected to pass 400 kilometres south of Nova Scotia on Wednesday before heading into the Grand Banks on Thursday.

"The rain is not expected to affect the Maritimes. The strongest winds will be well offshore, in fact, the winds will be fairly light throughout the Maritimes on Wednesday," said Abraham.

"The passing of the tropical storm may be followed by the development of a low pressure system over Newfoundland late in the week. The biggest effect this will have on the Maritimes is a change to a cooler and less humid airmass on Friday and Saturday."

The Canadian Hurricane Centre said rain from the post-tropical phase of the storm may affect the southeastern portion of Newfoundland.

With files from CBC News