As the so-called Frankenstorm arrives on the doorstep of the U.S. eastern seaboard, the Canadian Hurricane Centre said Sunday that Sandy could bring winter weather to parts of Ontario.
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.
But 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," Mr. Robichaud said from Halifax during a media teleconference on Sunday.
The centre said parts of southern Ontario should brace for 100 kph winds that could cause a bit of a mess, especially along western Lake Ontario.
Mr. 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, he said.
"One of the impressive things about the storm is its sheer size," Mr. Robichaud said. "We're looking at a fairly large storm."
It's comparable to 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, Mr. Robichaud said.
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."
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. ET.
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.