Authorities have issued hurricane warnings for Nova Scotia, Prince Edward Island and Newfoundland.
Meteorologists in the US assess that Fiona will have time to develop into a "large and powerful post-tropical cyclone with hurricane force winds".
Amanda McDougall, mayor of the Cape Breton area, says it's getting "very scary."
Residents there have begun to stock up on supplies and storm-proof their properties.
In Prince Edward Island, residents have received text messages from authorities warning that the storm may cause flooding.
Trudeau: "Hold on"
Canada has historically been relatively spared from storms of this type, as they usually lose their strength as they move in over cooler ocean waters.
Prime Minister Justin Trudeau has postponed a trip to Japan and announced that the storm looks set to hit the country hard.
- Of course we hope that there will not be much to do, but we suspect that there probably will be, he says, and continues with an invitation:
- Heed the instructions of the local authorities and persevere for the next 24 hours.
See how the Canadians prepare for the storm in the clip above.