What's a derecho and why is it so destructive? The science behind this powerful storm

When Canadian tornado expert David Sills studied the forecast on Saturday morning, he never expected the line of storms headed for Windsor, Ont., would soon strengthen into Canada's first derecho in decades, wreaking havoc across southern Ontario and Quebec.  

Sills, who is the executive director of the Northern Tornadoes Project at Western University, was outside doing yardwork at his London, Ont., home when he heard a rumble in the distance; he couldn't believe the line of storms was still so active. 

"I'm thinking, 'What? Why is this thing still going?'"

He went back inside to study the forecast, and that's when the storm arrived at his doorstep.

"All of a sudden it hits and it's just like a hurricane," Sills said. "It's just getting stronger and stronger … I watched as a tree came down on my neighbour's roof across the street."

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

An ominous wall of wind and rain

A derecho, pronounced deh-REY-cho, is a long-lived, fast-moving thunderstorm that causes widespread wind damage. This particular storm system was fed 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 started to cause damage. 

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

Unlike the rotating winds in a hurricane or a tornado, a derecho's winds are straight. That doesn't mean it's any less damaging; its winds can topple trees and lift up roofs. Another feature of a derecho is that unlike the slow building of a supercell thunderstorm, the business end of a derecho is at the front. 

That's why when you witness a derecho, Sills said, it often looks like an ominous wall of wind and rain. 

"When it hits, usually the worst of it is within a couple minutes of it hitting," he said. 

Making that destructive wall of wind even worse, is that it can sometimes produce tornadoes as well.

"Really, it's just a spectrum of wind that affects a long area," Sills said. 

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

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

Sills said he expects there could be even more.

Even if that's the case, "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 120 km/h winds.

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

The last string of significant derechos that hit Canada were in the 1990s, including one in 1999. According to the U.S. National Oceanic and Atmospheric Administration, that storm cut a path through Thunder Bay and sparsely populated areas of northern Ontario before crossing into Quebec, where it killed one person, toppled trees, damaged buildings and overturned boats in the Montreal area.

"It is the widespread nature of a derecho that can really cause havoc in a city," Sills said. 

What made Saturday's storm especially unlucky was that several urban centres were directly in its path.

"This was an unusual event because it affected the most populated part of Canada," Kimbell said.

Environment and Climate Change Canada issued a broadcast 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.

"That's the first time they've done that, and it probably saved lives," Sills said.

Still, the storm left a path of destruction in its wake, killing 10 people and leaving roughly 900,000 homes and businesses without power in Ontario and Quebec at its peak. It continued all the way to Maine, where there were also reports of damages.

Climate change could bring more derechos

Pinning down whether or not the rare event could be linked to climate change is difficult. Because derechos are so infrequent in Canada, Sills said it's impossible to say whether they're increasing or not. 

But, he said, the ingredients necessary to form a derecho "may come together more often" as a result of the effects of climate change.

A derecho happens when there's a lot of heat and moisture available and they are often tied to heat domes. Sills said climate projections point to a warmer atmosphere that will creep northward, which means this is the kind of storm Canadians can expect to see more of in the future. 

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

"There wasn't much in the way of any indication in the models of this big derecho coming through," he said.

"The computer models we rely on to give us a heads up for these types of events, they've got a long way to go."