More threads by David Baxter PhD

David Baxter PhD

Late Founder

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

Canada's last derecho was in 1999, but climate change is shifting conditions

by Jaela Bernstien, CBC News
May 25, 2022


Uxbridge, Ont., resident Allen Harrison cleans up his property on Tuesday, after a tornado produced by a derecho swept through the southern Ontario town. (Evan Mitsui/CBC)

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.


Powerful winds lift up dirt before the storm arrived in Saint-Bernard-de-Michaudville, Que. (Daniel Thomas/Radio-Canada)

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.


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

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.


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

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.


Aerial images shot from a drone show the aftermath of Saturday's storm in Uxbridge, Ont. (Sue Reid/CBC News)
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."

David Baxter PhD

Late Founder

Eight days after devastating derecho, these Ottawans still lack power

by Avanthika Anand, CBC
May 29, 2022

Roughly 10,000 Hydro Ottawa customers still in the dark​


Jai Persaud, left, gestures to his wife Nan as they examine the tree that collapsed onto their neighbour's house after the May 21 storm. Persaud says without power or internet, it's been hard to get the insurance process underway. (Avanthika Anand/CBC)

On Sunday, Anshul Melville took a chainsaw to the trees that fell in the front yard of his Pineglen Annex home during last weekend's fatal storm.

For more than a week, Melville has been clearing up that mess — but his biggest challenge is one he can't fix himself.

"There has been no heat, no warm water," said Melville, one of the roughly 10,000 Hydro Ottawa customers who still don't have power, eight days after the storm blew through.

Melville has been using a rented backup generator just to keep his phone charged, in case of emergencies.

But this temporary solution, even when minimally used, is expensive. Melville says he's spending over $100 a day to keep the generator running — and he's beginning to get tired.

"We desperately, desperately want [our] power back," he said.

'It's been tough'​

In a Sunday update, Hydro Ottawa said it had restored power to 94 per cent of its roughly 180,000 customers affected by the powerful May 21 derecho.

The power utility said it was entering the "last phase of restoration efforts," with crews scattered across the city trying to reconnect "remaining isolated outages."

One of those outages has been at the Merivale home of 95-year-old Mervyn Brown, who hasn't been able to track down a generator.

The past week has been challenging for both him and his wife, he said.

"It's been tough because we're collecting rainwater to flush toilets," he said, adding they don't have drinking water in their home.


Mervyn Brown, 95, says he's been unable to get access to a generator in the days following the devastating May 21 derecho that slammed into Ottawa. Brown says he and his wife have had to throw out about $1,000 worth of food. (Julia Wong/CBC)

Without power for his fridge, Brown says he's had to throw out at least $1,000 worth of food. That's on top of the significant damages from the storm that still need repairing, he said.

Jai Persaud's property was also damaged, and his power outage means he's been unable to get repairs started.

With neither power nor an internet connection, Persaud said it's been hard for him to contact his insurance provider.

"I go to the [nearby] Tim Hortons parking lot to try to use the internet there. It's been very difficult trying to get in touch with people," he said.

Feeling helpless​

Carleton University student Claire Petite lives just off Prince of Wales Drive and said the lack of power and internet has "completely interrupted" her life.

The ongoing outages, she said, are making it hard for her to keep up with her studies.

"The accommodations that the university might be giving students, they're only going to last as long as the majority of students are experiencing them," she said.

"So having to put my hand up and say, no, I need longer accommodations is harder — because you have to make the case that we're still impacted by this, and it's not over yet."

After eight days of cold showers, confusion and complete darkness, Petite said she's beginning to feel helpless.

She said she hopes Hydro Ottawa and the City of Ottawa's cleanup crews haven't forgotten her neighbourhood. So far, she said, she's had no help from either of them.
"We don't know when we're going to get power again," Petite said.

Daniel E.

Before a Power Outage​

  • Prepare a Basic 72 Hour Emergency Kit. Include:
    • a flashlight and radio – if they are battery powered, ensure you have enough batteries; alternatively, you could purchase crank devices;
    • non-perishable foods that require little to no heat for cooking;
    • a first aid kit; and
    • enough water to sustain you for up to three days.
  • Have at least one land line phone in the home or the capability to charge cell phones during a power outage in order to call emergency services if needed.
  • If you require an uninterrupted supply of power, including for specialized medical equipment, ensure that you have a back-up supply in place or a plan to relocate to a facility that can assist you.
  • If you are a person with a mobility disability living in a building that requires your use of an elevator, be sure to inform the building management you may need assistance when evacuating.
  • Install battery-powered smoke and carbon monoxide detectors, or ensure that hard wired detectors have a back-up power supply. Be sure to check them regularly to ensure they are functioning properly.
  • Have wood-burning fireplace chimneys cleaned every fall to eliminate creosote build-up which could cause a chimney fire. This will help ensure that you are building safe fires during power outages.
  • Ensure that any generator connected to your home's electrical system is done so through an approved transfer panel and switch that has been installed by a qualified electrician. Generators that are incorrectly connected to the power grid could present a safety hazard to Hydro Ottawa employees and a potential technical risk to the distribution network and neighbouring customers.
  • Install a backup power supply for your sump pump. This will help avoid flooding during power outages.
  • If you notice that wires connected to your home are damaged, call a qualified electrical contractor for repair.
For more information on how to prepare for an outage, visit the Government of Canada’s Power Outages brochure.

During a Power Outage​

  • Report power outages and downed power lines using Hydro Ottawa’s 24/7 outage line at 613-738-0188
  • Turn off all lights, unplug appliances and electronics, and turn down heating system thermostats. This will help avoid a power surge when electricity is restored.
  • If your house has a sump pump that does not have a backup power supply, clear valuables from the basement in case flooding occurs.
  • If the doors are kept closed, food in freezers will keep for 24 to 48 hours without power, and food in refrigerators will keep for up to 4 hours. For more information about storing food and water during an emergency, visit the Canadian Food Inspection Agency website.
  • If you use a fireplace for heat, check chimneys for creosote build-up or debris. Do not leave the fire unattended.
  • Close room doors to keep heat in a confined area.
  • Use flashlights. Candles can be a fire hazard. If you must use candles, place them in a non-combustible container away from drapes and carpet and never leave them unattended.
  • Only use camp stoves, and charcoal or gas barbeques outdoors.
  • Only use generators outdoors. Exhaust fumes could lead to carbon monoxide poisoning if they are not properly vented. To prevent exhaust gases from entering the house, operate generators in well-ventilated conditions away from windows and doors, and be sure to follow the manufacturer’s instructions.
  • Plug your appliances directly into the generator using properly rated Canadian Standards Association (CSA) approved cords.

During extended power outages in winter there are extra precautions that should be taken:​

  • Shut off power to the water heater.
  • Shut off water at the main valve (usually found in the basement near the water meter). Use blankets or insulation material to protect the valve, inlet pipe, meter, and pump.
  • Open taps to drain the pipes. Leave the taps open.
  • Flush toilets to empty them.
  • Drain appliances such as dishwashers and washing machines.
  • Stay in your home as long as you are safe, warm and can feed yourself. Emergency Reception Centres will be opened by the City of Ottawa should an emergency be declared. You can go there to get information, to stay warm or keep cool, and for comfort and food. Locations will be broadcast on local emergency radio and TV stations should an emergency situation occur.

After a Power Outage​

  • If wires connected to your home were damaged, call a qualified electrical contractor for repair.
  • Do not enter a flooded basement unless you are sure the power is disconnected.
  • Do not use flooded appliances, electrical outlets, switch boxes, or fuse-breaker panels until they have been checked and cleaned by a qualified electrician.
  • Turn on the main electric switch if it was turned off. Before doing so, ensure that appliances, such as electric heaters, TVs, microwaves and computers, were unplugged. This will help prevent damage from a power surge.
  • Give the electrical system a chance to stabilize before reconnecting tools and appliances. Turn the heating-system thermostats up first, and then wait a few minutes before reconnecting the fridge and freezer. Wait 10 to 15 minutes before reconnecting all other tools and appliances.
  • Open the main water valve if you closed it, and make sure that the hot water heater is filled before turning on the power to it.
  • Check food supplies in refrigerators, freezers and cupboards for signs of spoilage. If you have any doubts that a food item may be spoiled, throw it out.

Daniel E.
Taken by my son with an iPhone 8 just as the storm started...

Great video. Reminds me of why I don't live in Florida anymore :)

When I first moved to semi-rural Arizona, our power was very reliable. If it did go out, it was only for a few minutes. But now, our town has a few outages each year that last at least a few hours, usually in the heat of the summer. And I have no idea why since it's usually not storm related. My guess is that it's partly a labor issue.

But my dog-toting minivan has its own power outlet if I need to charge my laptop or my recently-purchased power station (330W for $150 USD). I am going to be running a fan next time and the TV for my husband -- everything but the watt-hungry Instant Pot :D
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