Spokesman Bob Robichaud said southern and eastern Ontario and western Quebec could see between 50 and 100 millimetres of rain late Monday and early Tuesday.
The storm is expected to interact with a stalled front over Ontario and could cause heavier rainfall — more than 100 millimetres — in areas adjacent to Lake Ontario and northward to Algonquin Park, the centre said.
Forecasters warn that rain could change into snow over central Ontario, although it's too early to predict snowfall amounts.
"Snow is another thing we're going to have to consider with this thing, just because it's drawing in some cold air from the north," said Robichaud from Halifax during a media teleconference on Sunday.
While the storm moved slowly northward over the weekend, its impact was already felt Sunday in a city along Quebec's Saguenay River, which leads into the St. Lawrence.
A cruise ship carrying 5,000 passengers en route to Florida docked in Saguenay, Que. to avoid its path. Vacationers aboard the Emerald Princess were welcomed by Mayor Jean Tremblay.
The cruise ship, which has 15 decks, 14 elevators, four outdoor pools and eight restaurants, is the largest ever to dock at the Saguenay port.
"We're very happy for the visit, but it's a lot of work for us," Tremblay said in an interview. "We are coordinating volunteers, buses."
The hurricane centre said parts of southern Ontario should brace for 100 km/h winds that could cause a bit of a mess, especially along western Lake Ontario.
Robichaud said the Maritimes could see between 50 to 75 millimetres of rain that will linger until Wednesday — Halloween.
It's a storm with far-reaching tropical storm-force winds that extend 800 kilometres from its centre, said Robichaud.
"One of the impressive things about the storm is its sheer size," said Robichaud. "We're looking at a fairly large storm."
It has been compared the so-called "Perfect Storm" of 1991, which remained off the coast of New England, pushing huge waves and causing $200 million in damage.
Sandy's effects are expected to be extensive, and will affect areas well outside the storm's centre, said Robichaud.
He said Sandy's size also means it's likely the messy weather will loiter until next weekend.
"It's going to be a gradually improving thing... To really get rid of this thing, you're looking at very late in the week," said Robichaud.
Six metre waves could lash the Great Lakes, especially over southern Lake Huron, the centre said. Large waves are also expected pound the south shore of Nova Scotia.
Higher than normal water levels are expected on the St. Lawrence River during high tide Monday and Tuesday evenings and could cause coastal flooding in the Quebec City, Que., region.
Sandy was expected to gradually become a massive and intense post-tropical cyclone on Monday.
Forecasters south of the border said the storm was headed for a densely populated zone, likely just south of the New York and New Jersey border, and warn it could cause serious damage.
New York declared a state of emergency Sunday and the New York City transit system was expected to shut down at 7 p.m. on that day.
People along the U.S. eastern seaboard have been bracing for the worst, clearing out grocery store shelves of non-perishable food items and water.
Sandy has so far killed more than 40 people in the Caribbean, wrecked homes and knocked down trees and power lines.
The Canadian government issued a news release Saturday saying it has received no reports of Canadians being killed or injured as a result of Sandy.
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