Eastern Canada is bracing for what forecasters said could be a once-in-a-lifetime storm.

Hurricane Fiona is set to make landfall there Saturday as it barrels up the Atlantic Ocean, where it has left behind a trail of destruction in parts of the northern Caribbean.

Fiona, which will arrive in eastern Nova Scotia around 9 a.m. local time Saturday, could bring floods, heavy rain and dangerous winds across Eastern Canada, forecasters said.

“This could be one of the worst, if not the worst, hurricane they’ve ever experienced in their lives,” said Dan Kottlowski, a senior forecaster and lead hurricane forecaster at AccuWeather Inc.

In a rare occurrence, Fiona was forecast to arrive in Eastern Canada even as it was still intensifying, Mr. Kottlowski said. Hurricanes frequently make landfall in the area, he said, but the storms usually weaken by the time they reach land.

He said Fiona was set to be just as strong as Hurricane Juan, a Category 2 storm that made landfall in Nova Scotia in 2003. Juan was blamed for eight deaths and caused about $200 million in damage, officials said.

Canadian Prime Minister Justin Trudeau said on Twitter Friday that the country was mobilizing resources to support provinces as needed. “Please take proper precautions and listen to local authorities,” he said.

Forecasters predicted that Fiona would cause significant damage, with high winds that could knock down structures and lead to power outages. Parts of Newfoundland and Nova Scotia’s coastlines could see waves as high as 65 feet, Mr. Kottlowski said.

“This really just looks like a really terrible situation,” he said.

The storm Friday morning was about 600 miles south of Halifax, Nova Scotia, and generating winds up to 130 miles an hour, according to the National Hurricane Center in the U.S.

The Atlantic hurricane season started off slow this year but has intensified this month. North America was set to see at least two hurricanes in the coming days: Fiona and a storm now known as Tropical Depression Nine, which forecasters said could arrive in Florida next week as a major hurricane.

Bob Robichaud, a meteorologist from Canada’s environment and climate change department, warned Canadians at a news briefing that they needed to finish their storm preparations by Friday night. Fiona was so large that storm conditions were expected to begin on Friday even though the storm wouldn’t make landfall until hours later.

“The impacts are going to be felt way beyond where the center of the storm goes,” Mr. Robichaud said.

While Nova Scotia and Prince Edward Island were forecast to get the brunt of the storm, eastern Quebec, New Brunswick and Newfoundland and Labrador are also expected to be affected, Mr. Robichaud and the Canadian Hurricane Centre said.

Mr. Robichaud said Fiona was set to be downgraded from a hurricane to a posttropical storm by the time it makes landfall in Canada.

“That does not mean a weaker storm,” he said at a news briefing Thursday, adding that Eastern Canada could get more than 7 inches of rain by Sunday morning.

He said Fiona would be extremely strong and dangerous because it would combine with a ribbon of pressure moving east across Canada.

Fiona wasn’t expected to breach the mainland U.S. Still, Jamie Rhome, the acting director of the National Hurricane Center, said Friday the storm would churn up enough waters to send dangerous rip currents to the U.S. East Coast.

Fiona has been churning up the Atlantic since Sunday, when it made landfall in Puerto Rico. The storm was upgraded Wednesday to Category 4, the second-highest hurricane designation. Fiona was blamed for the deaths of at least seven people—four in Puerto Rico, two in the Dominican Republic and one in Guadeloupe.

Roughly 800,000 residents of Puerto Rico were still without power Friday, according to poweroutage.us, after Fiona caused an islandwide blackout.

Hundreds of Federal Emergency Management Agency employees were already on the ground in Puerto Rico when Fiona struck, and more resources have since been deployed by the U.S. government.

President Biden, speaking at a FEMA briefing in New York this week, promised continued support to the storm-battered island. “We’re all in this together,” he said. “We’re with you. We’re not going to walk away.”

On Friday, the White House said Mr. Biden granted 100% federal funding to remove trees and debris still blocking flooded roadways, which slowed progress of power and water crews looking to restore services.