A satellite image provided by the National Oceanic and Atmospheric Administration shows Hurricane Bill in the Atlantic Ocean early Wednesday morning. (NOAA/Associated Press)

Weather forecasters in the Maritimes are keeping a close eye on Hurricane Bill as it churns north in the Atlantic Ocean.

Bill strengthened to a dangerous Category 4 storm Wednesday morning as it neared the Leeward Islands, in the Caribbean. It's expected to head north and reach Atlantic Canada as a much weakened storm by Sunday.

CBC meteorologist Peter Coade said based on Wednesday's charts, the centre of Bill will move due south of Halifax on Sunday afternoon, pass between Guysborough County and Sable Island in the evening, and Glace Bay in Cape Breton around midnight.

Coade said the storm will lose much of its power as it travels north through cooler waters off the U.S. coast.

"In any event, the computers are pretty much in consensus that it is going to have an effect on us, and that will be in the form of heavy rain and likely strong northeast wind," he said.

Peter Bowyer, with the Canadian Hurricane Centre in Dartmouth, N.S., said a lot could change with Bill between now and Sunday. However, he said, it's prudent to prepare.

"It seems to be, for all intents and purposes, heading in our direction," said Bowyer. "So this is a really good time for folks to use this week and leading into the weekend to get ready for hurricane season."

Riding a scary wave

Surfers are preparing for big waves. Lesley Choyce, a veteran surfer in Lawrencetown, N.S., said it may be a big thrill for some, but he worries about less experienced surfers.

"I watch new guys who are just learning, and it's a whole lot more dangerous for them and they make a heck of a lot of bad decisions surfing in conditions that they're really not prepared for," said Choyce.

Hurricanes are categorized on the Saffir-Simpson scale from one to five, indicating the storm's intensity at a given time.

At 11 p.m. AT Wednesday, Bill was an "extremely dangerous" Category 4 hurricane, according to the U.S. National Hurricane Center.

Its eye was about 505 kilometres northeast of the Leeward Islands and 1,415 kilometres south-southeast of Bermuda and the storm was moving northwest at about 29 km/h.

An aircraft sent out to the storm found it had sustained winds of 215 km/h with higher velocity gusts, and hurricane-force winds extending up to 140 kilometres from the eye, the centre said. Further strengthening was possible over the next 24 hours.

Wind shear and warm waters are providing the necessary factors to maintain a hurricane, said Todd Kimberlain, a forecaster with the centre.

"Those are two essential ingredients not just for the formation but also the maintenance of hurricanes," he said.

The hurricane centre warned the Leeward Islands and Bermuda to be on guard as the first hurricane of the Atlantic season roared by them.

'Keeping an eye on it'

Bermuda is expected to take the worst hit from the storm, Kimberlain said.

"We are keeping an eye on it for sure," said Nick Camizzi, a forecaster with Bermuda's weather service.

But the exact path of the hurricane is still unclear and it may pass between the islands and the United States without making landfall, the hurricane centre said.

Residents on islands in the northeast Caribbean and along the eastern coast of the southern United States should also expect wave swells and rip currents through at least Saturday, the centre said.

The two other named storms of the Atlantic hurricane season, Ana and Claudette, have had little effect on any mainland communities.

With files from The Associated Press