Storm Lee has left 80,000 properties without power and is expected to make landfall around Nova Scotia at 15:00 local time (18:00 GMT) on Saturday.
The storm has become a post-tropical cyclone but is producing hurricane-force winds of 80 mph (130 km/h), the National Hurricane Center (NHC) said.
Flights and ferry crossings have been cancelled and millions in New England and Canada are under storm warnings.
Lee will bring strong winds, rain and coastal flooding, according to the NHC.
Because of the storm's widening size, people will feel its impacts well beyond where the centre makes landfall.
"These conditions are likely to lead to downed trees and potential power outages," the NHC said.
Lee is currently 220 miles south of Eastport Maine and 230 miles south of Halifax, Nova Scotia, and is moving at around 25 mph, NHC said.