The Canadian Hurricane Centre in Halifax staged a news conference Friday, saying meteorologists wanted to issue early warnings because the Category 1 hurricane is so huge.
"Everyone in the Maritimes, certainly everyone is southern Quebec and eastern and southern Ontario should be monitoring this storm," said spokesman Bob Robichaud.
"I know it's a big area but it speaks to the size of this storm. The storm will be a very huge storm by the time it gets close to here ... It's getting a lot of attention and deservedly so."
Robichaud said it was too early to predict the strength of the winds or rainfall amounts for Canadian territory, but he said the latest computer models suggest powerful gusts and heavy downpours are on the way for Monday and Tuesday.
He said southwestern Nova Scotia was expected to see the worst of the storm, but that prediction could change in the days ahead, particularly if Sandy heads farther south.
Typically, large hurricanes like Sandy have been known to race up the coast and clip the edges of the Maritimes and Newfoundland, causing sporadic damage.
But this storm is different.
A large, high-pressure system over the Maritimes is expected to block Sandy's advance, pushing it into the mid-Atlantic states on late Monday or early Tuesday. The storm is also being fed by a trough of low pressure in the U.S. Midwest.
Robichaud warned the complex interaction of these three weather systems has produced an unusually scattered set of track forecasts. That means the predicting the progress of the storm has been more difficult that usual.
The storm, which has been blamed for causing more than two dozen deaths in Haiti and Cuba, is expected to remain a blustery mess until Wednesday — Halloween. That's partly why it's being called a Frankenstorm.
"Every now and then when forecasters write their technical discussions they like to add some catchy nickname to a storm," said Robichaud. "But make no mistake. This is going to be a very dangerous storm all along the east coast of the U.S."
The storm is comparable in size to the "Perfect Storm" of 1991, which remained off the coast of New England, pushing huge waves and causing $200 million in damage.
This time, the storm is headed for a densely populated zone, somewhere around New York or New Jersey, American forecasters say.
"The ("Perfect Storm") was particularly large, and this one is certainly on par with the size of that particular storm," said Robichaud. "The only difference is this storm is actually going to affect more people on land."
Forecasters south of the border say the rare merging of three big weather systems could cause serious damage.
"It's going to be a pretty dangerous event for that part of the U.S.," Robichaud said. "We're going to be dealing with this for most of next week."
Also on HuffPost