What is a derecho and why is it so destructive? The science behind this mighty storm


When Canadian tornado expert David Sills studied the forecast on Saturday morning, he didn’t expect the line of storms headed for Windsor, Ont., to strengthen soon to become Canada’s first derecho in decades. , wreaking havoc in southern Ontario and Quebec.

Sills, who is the executive director of the Northern Tornados Project at Western University, he was outside doing yard work at his home in London, Ontario, when he heard a rumbling in the distance; he couldn’t believe the line of storms was still so active.

“I’m like, ‘What? Why is this going on?'”

He went inside to study the forecast, and that’s when the storm came to his door.

“All of a sudden it hits and it’s like a hurricane,” Sills said. “It’s getting louder and louder…I saw a tree fall on my neighbor’s roof across the street.”

That’s when he knew it wasn’t a normal thunderstorm.

Powerful winds lifted the land before the storm arrived in Saint-Bernard-de-Michaudville, Quebec. (Daniel Thomas/Radio Canada)

A disturbing wall of wind and rain

A derecho, pronounced deh-RAY-cho, is a long-lasting, fast-moving thunderstorm that causes widespread wind damage. This particular storm system was powered by a heat dome over the eastern United States.

According to Sills, the system formed south of Chicago on Saturday morning, then crossed the border into the Windsor area, where it began causing damage.

By the time he arrived in Kitchener, Sills said the storm was producing gusts of up to 132 km/h.

Unlike the turning winds in a hurricane or tornado, the winds in a derecho are straight. That doesn’t mean it’s less damaging; its winds can topple trees and lift roofs. Another characteristic of a derecho is that, unlike the slowly building supercell, the business end of a derecho is at the front.

That’s why when you witness a derecho, Stills says, it often looks like an ominous wall of wind and rain.

“When it hits, the worst is usually within a few minutes,” he said.

Part of a utility pole sits on a driveway, along with the roof of a hardware store that was lifted by extreme winds during Saturday’s storm, in the community of Hammond in Clarence-Rockland, Ont. (Justin Tang/The Canadian Press)

What makes this destructive wind wall even worse is that it can sometimes also produce tornadoes.

“Really, it’s just a spectrum of wind that affects a long area,” Sills said.

So far, Northern Tornadoes Project field crews have identified at least one EF2 tornado, which hit Uxbridge, Ontario with winds of up to 195 km/h.

The team is investigating at least four other possible tornadoes in southern Ottawa, London, Ontario, and Rawdon, Quebec.

Sills said he expected there would be even more.

Even if so, “the overwhelming majority of the damage was caused by straight-line derecho winds,” said Environment Canada warning preparedness meteorologist Peter Kimbell.

He said both Ottawa and Toronto airports reported winds of 120 km/h.

A rare event: Canada’s 1st derecho since 1999

The last series of derechos to hit Canada date back to the 1990s, including one in 1999. According to the United States National Oceanic and Atmospheric Administrationthis storm made its way through Thunder Bay and sparsely populated areas of northern Ontario before crossing into Quebec, where it killed one person, knocked down trees, damaged buildings and overturned boats in the region from Montreal.

“It’s the widespread nature of a derecho that can really wreak havoc on a city,” Sills said.

What made Saturday’s storm particularly unlucky was that several urban centers were directly in its path.

“It was an unusual event as it affected the most populated part of Canada,” Kimbell said.

The system formed south of Chicago on Saturday morning and then moved across Ontario, according to tornado expert David Sills. (Environment and Climate Change Canada/CBC News)

Environment and Climate Change Canada issued an alert for a severe thunderstorm, setting off alarms on people’s cellphones in Ontario and Quebec. It was the first time a new feature was tested, allowing the forecaster to trigger an alert for extreme thunderstorms with high winds.

“It’s the first time they’ve done this, and it probably saved lives,” Sills said.

Still, the storm left a path of destruction in its wake, killing 10 people and leaving an estimated 900,000 homes and businesses without power across Ontario and Quebec at its peak. It continued to Maine, where damage was also reported.

Climate change could bring more derechos

It is unclear whether or not the rare event could be related to climate change. Because derechos are so infrequent in Canada, Sills said it’s impossible to say whether or not they’re increasing.

But, he said, the ingredients needed to form a derecho “may come together more often” due to the effects of climate change.

A derecho occurs when there is a lot of heat and humidity available and they are often linked to heated domes. Sills said climate projections point to a warmer atmosphere moving north, meaning this is the kind of storm Canadians can expect to see more of in the future.

Aerial footage taken from a drone shows the aftermath of Saturday’s storm in Uxbridge, Ontario. (Sue Reid/CBC News)

There’s always something to learn from extreme weather events, Sills said, and a key thing for him after this storm is that computer modeling needs to catch up.

“There weren’t many indications in the patterns of this great derecho,” he said.

“The computer models that we rely on to give us an idea of ‚Äč‚Äčthese types of events, they still have a long way to go.”