Newfoundland's Avalon, Burin and Bonavista peninsulas — which cover much of the eastern half of the island and the majority of its population — have been put on a hurricane watch for Tuesday.
Leslie, which had been slowly churning in the Caribbean for days, is expected to lash into Newfoundland on Tuesday morning, and drop as much as 150 millimetres of rain over a 36-hour period.
Winds could gust as high as 140 km/h in exposed areas.
Leslie's reach will be extensive. Environment Canada has put Newfoundland's south coast and other areas of Newfoundland on a tropical storm watch, with warnings for winds.
A trough of low pressure that has been stalled around Newfoundland's west coast could help push the storm into the island, meteorologists believe.
Environment Canada meteorologist Devon Telford said the storm will combine with a cold front, which will mean a lot of rain in some areas.
"The rain's going to be very heavy and it's going to be falling in a very short amount of time, so the total rainfall for most of the island, except for the Avalon, you're looking for anywhere from 70 to 110 millimetres when it's all said and done," Telford said.
Leslie is a large storm, covering about 800 square kilometres on Monday.
Expected to make landfall Tuesday
CBC meteorologist Ryan Snoddon said Leslie will push much of its moisture to its western track, while in the east, the storm will largely be about its fierce winds. He said residents of St. John's, for instance, will likely see a fraction of the rain that will come to the west.
Leslie comes almost two years after Hurricane Igor, a powerful storm that ripped apart roads, destroyed bridges and washed away infrastructure. The storm killed one man and knocked out power in many parts of eastern Newfoundland.
"This trough will accelerate and steer Leslie toward southeastern Newfoundland where it is expected to make landfall as a marginal hurricane or strong tropical storm later Tuesday morning," Environment Canada said in a statement Monday morning.
"As Leslie interacts with the trough it will enhance the heavy rainfall already occurring with the trough as well as strengthen the winds behind it."