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Paroisse Saint-Hughes Church in Sarsfield, Ont., on May 22.Spencer Colby/The Globe and Mail

A deadly storm that swept across Southern Ontario and Quebec on the weekend, resulting in the deaths of at least nine people, has caused massive damage to large swaths of the electrical grid, which could leave hundreds of thousands of customers without power for days.

Ontario utility Hydro One said the storm, which started around noon on Saturday with winds reaching more than 130 kilometres an hour, toppled large electrical transmission towers in the Ottawa area, and more than 600 hydro poles across the province.

“The distinction with this storm is the severity of damage we’re seeing,” said Hydro One spokeswoman Tiziana Baccega Rosa.

“Steel transmission structures aren’t supposed to come down … it’s very very extreme damage.”

The storm, known as a “derecho” – a widespread line of thunderstorms that can cause as much damage as a hurricane or tornado – triggered several emergency alerts. It was the first time Environment and Climate Change Canada (ECCC) used its broadcast-interrupting weather alert system for a thunderstorm.

Across Ontario, utility companies were dealing with hundreds of power outages affecting more than 400,000 customers – about 170,000 of them in the Ottawa area.

Over the provincial boundary, Hydro-Québec said the storm knocked out power for 550,000 customers from Gatineau to Quebec City at its peak. As of Sunday afternoon, there were close to 350,000 customers still cut off.

Hydro Ottawa systems director Joseph Muglia said about 5,000 customers had their power restored on Sunday, but many others would have to wait a bit longer. “It will be, more than likely, a three- to four-day event,” he said. “All our resources are on it.”

Elsewhere in the province, more than 270,000 Hydro One customers were still without power as of late Sunday.

Meanwhile, the Ontario towns of Uxbridge, north of Toronto, and Clarence-Rockland, east of Ottawa, declared states of emergency after numerous buildings were damaged and people were displaced. “The largest pressure is actually the lack of power and infrastructure,” said Uxbridge Mayor Dave Barton.

“At the moment, we don’t know what we don’t know. Because most phone lines are down, we don’t know who needs assistance and who doesn’t.”

Police in Ontario reported that seven people were killed by falling trees in locations across the province during the storm Saturday, and an eighth died when hit by a falling tree branch in the storm’s aftermath on Sunday.

Among the deaths, a person was killed when a tree hit their camping trailer in a conservation area south of Waterloo, Ont., and a 59-year-old man died on an Ottawa golf course.

In Quebec, a woman was killed when her boat capsized on the Ottawa River.

Some people felt the emergency alerts could have been issued earlier. In Brampton, Ont., Ryan Weichel said he only got an alert about the storm after it was already in full force. One of the fatalities occurred in his city – a woman was hit by a tree while she was out on a walk.

“It was sent far too late,” said Mr. Weichel, whose work as a private-security contractor involves weather risk assessment. He said authorities should have known about the severity of the approaching storm since it had hit Waterloo earlier in the afternoon.

“Had that emergency warning been enacted earlier, there might’ve been the opportunity for people outdoors to get to safety.”

Colin Ballantyne said he was camping at Bronte Creek Provincial Park near Oakville, Ont., with his daughter on Saturday when he received the emergency alert warning of extreme weather. It came “literally a minute after the rain had started” – catching many campers off guard, he said.

“The dust on the roads was whipping up a sand storm and the family across from us, their tent was blown flat and all their stuff was soaked,” Mr. Ballantyne said. “The park was sold out this weekend and now there are empty sites everywhere.”

Environment and Climate Change Canada said its alert system for high winds is triggered when they reach more than 130 kilometres an hour. Its first such reading came in at 12:23 p.m. in Waterloo, and an alert was sent out to that area at 12:30 p.m. Another alert was sent out to cellphones in Toronto at 12:45, and more alerts were sent later in the day as the storm moved further east.

ECCC meteorologist Gerald Cheng said part of the difficulty of forecasting thunderstorm events is deciphering whether destructive winds at higher altitudes will actually reach ground level.

“We need to study these events and if there was anything on the radar that could have given us hints,” said Mr. Cheng.

While alerts are an important part of safety during natural disasters, Mr. Cheng reminded people to stay vigilant when inclement weather is forecasted.

“Alerts are just one method of staying informed when nature gives us signals. When dark clouds roll in, it’s time to seek shelter,” he said.

When asked about the timing of the alerts, Ottawa Mayor Jim Watson said he thought the system worked well.

“I think these things sometimes don’t have enough lead time because of weather patterns, but I can only imagine they tried to get it out as quickly as possible.”

Tim Trytten, who was formerly the Alberta government’s team leader on emergency alerts, said his province would have followed a similar protocol of progressively alerting communities only as the storm moves closer to them.

“You have to keep in mind these are rapidly developing natural phenomena,” said Mr. Trytten.

“There’s always a lot of emotion attached to an event, especially one as severe as this, and you have to look at what did people know, when did they know it and what did they do about it.”

Prime Minister Justin Trudeau thanked the emergency crews working to restore power in a tweet Sunday afternoon, and said the federal government stood ready to offer support if needed.

A tornado watch was issued in the Eastern Townships area of Quebec on Sunday afternoon and another in New Brunswick, which was still in effect Sunday evening.

Mr. Cheng said that the watches were due to the same weather system that caused Saturday’s derecho and a deadly tornado in Gaylord, Mich., on Friday, where two people were killed and dozens were injured.

With reports from The Canadian Press

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