Tornadoes and Climate Change in Canada

Introduction

Tornadoes are amongst the most unpredictable and violent weather phenomena in Canada and pose significant economic and public safety risks. Tornadoes primarily originate from supercell thunderstorms. In Canada, these types of storms are more typically observed across the Prairies, southern Ontario and Quebec, but they can occur elsewhere if atmospheric conditions are favourable to their development. With climate change, the atmospheric ingredients that combine to form these types of severe, rotating thunderstorms are changing.[1] These climatic changes could alter both the frequency and intensity of tornadoes and expand their geographical extent.

This article builds upon our previous article on Thunderstorms and Climate Change in Canada. It is recommended you read the Thunderstorm article first.

What are tornadoes and how do they form?

Tornadoes are narrow vortices of air that extend from a thunderstorm cloud to the ground. They often occur within larger storm systems, particularly severe thunderstorms known as supercells, which feature a deep, persistently rotating updraft called a mesocyclone.[2] Other types of vortices exist, including cold-core funnels, waterspouts, and even pyrotornadoes (tornadoes generated by fire; Box 1). However, this article will focus on supercell tornadoes, as these vortices cause the most destruction and thus pose the greatest risk to human safety and infrastructure.

Tornado formation, also called tornadogenesis, is an active area of research. Tornado formation requires highly specific atmospheric conditions. This is demonstrated by the fact that, although most violent tornadoes originate from supercell thunderstorms, fewer than 20% of supercell storms result in a tornado.[3] Tornado researchers do, however, know the basic building blocks of tornadoes:

  1. Atmospheric Instability: The atmosphere’s change in temperature with height determines whether the atmosphere will be stable or not. An unstable atmosphere occurs when temperatures drop off quickly with height. This causes parcels of relatively warm air to rise, which can lead to cloud formation and storms. Conversely, if the atmosphere cools slowly or if its temperature increases with height, the atmosphere is stable, preventing upward movement of air. Atmospheric instability is enhanced when cold, dry air overlies hot, humid surface air. Such conditions are often found during the summer in the lee of the Rocky Mountains, where air, having been forced to rise, cool, and dry over the mountains, moves over the relatively hot, humid air of the central American Plains and Canadian Prairies. A highly unstable atmosphere promotes the development of thunderstorms. To learn more about atmospheric instability and its role in creating thunderstorms, read our previous article on Thunderstorms and Climate Change in Canada.
  2. Moisture: The more moisture available, the more intense a thunderstorm can become. This is because a large amount of energy is released when water vapour condenses into liquid droplets. This energy, in turn, keeps a parcel of air warmer than it would otherwise be, enhancing its buoyancy, which in turn, increases atmospheric instability.
  3. Wind Shear: This term refers to the change in wind speed or direction as you go higher in the atmosphere. When wind speed increases with altitude, it can create rolling motions in the air, like a rolling pin moving along a surface. If the wind direction changes with height, it can introduce horizontal rotation (Figure 1). When these rotating winds interact with a storm’s updraft, they can cause the entire storm to rotate. If this rotation continues and strengthens, it can lead to the formation of a long-lasting supercell thunderstorm.

Predicting tornado formation is difficult because weather models lack the fine-scale resolution needed to represent the complex movement of air within convective storms. However, advancements in data assimilation techniques and high-resolution modeling are helping to overcome these limitations. That said, it is still difficult to predict more than a few hours in advance exactly when and where a tornado will develop, how strong it will be, or the precise path it will follow.

Box 1: In August 2023, researchers from the Northern Tornadoes Project (NTP) confirmed the first fire-generated tornado case in Canada, occurring near Gun Lake in British Columbia. This rare phenomenon, referred to as a pyrotornado, formed from a vortex over or near a wildfire. The NTP at Western University investigates the impacts of climate change on tornadoes in Canada.

Click here to watch a video of this event captured by firefighters.

Tornadoes in Canada

Canada experiences approximately 80 tornadoes per year, making it one of the most tornado-prone countries in the world.[4] Most tornadoes are observed in the Prairies, southern Ontario, and southern Quebec. These regions experience a tornado season that extends from April through September, with the highest activity in the summer months, corresponding to peak severe thunderstorm activity. While most Canadian tornadoes are small and short-lived, some are very powerful and destructive (Box 2).

Box 2: Tornadoes are classified using the Fujita (F) scale and the Enhanced Fujita (EF) scale, which are based on estimated wind speeds and the resulting damage to structures and vegetation. The original Fujita scale, ranging from F0 to F5, was developed in 1971 and categorized tornadoes based on observed damage. In 2007, the Enhanced Fujita scale was introduced to provide a more accurate assessment of tornado strength by incorporating more detailed damage indicators. The EF scale also ranges from EF0 to EF5, with EF0 indicating the weakest tornadoes and EF5 representing the most destructive.

Canada has experienced several large and damaging tornado events throughout its history, including:

  • Regina Cyclone (1912): Known as the deadliest tornado in Canadian history, this F4 tornado tore through Regina on June 30, 1912, leaving a path of destruction five blocks wide. It resulted in 28 fatalities, 200 injuries, and left approximately 2,500 people homeless, with around 500 buildings destroyed.
  • Edmonton Tornado (1987): On July 31, 1987, Edmonton was struck by a powerful F4 tornado, which became one of the most destructive in Canadian history. This tornado claimed 27 lives and injured about 300 people, causing extensive damage across the region.
  • Elie Tornado (2007): The Elie, Manitoba tornado on June 22, 2007, stands as Canada’s only recorded F5 tornado. While the tornado caused substantial property damage, there were no fatalities or serious injuries reported.
  • Didsbury Tornado (2023): The most significant tornado event in Canada in 2023 was the EF4 Didsbury, AB tornado that occurred on the afternoon of Canada Day (July 1). Documented by the Northern Tornado Project (NTP) field team, this slow-moving supercell emerged from the Rocky Mountain foothills, moving eastward. An extensive survey by the NTP, in collaboration with the ECCC Prairie and Arctic Storm Prediction Centre, concluded an EF4 rating based on the damage to well-built structures, including a residential house that suffered a total collapse. The tornado’s path was 15.3 km long and 620 m at its widest, affecting twelve residences—three destroyed, four uninhabitable, and five damaged.[5]

How will climate change impact tornadoes in Canada?

The influence of climate change on tornado formation and intensification is a complex and actively researched topic. As the climate continues to warm, changes in the key drivers of tornadoes—instability, moisture, and wind shear—are anticipated. These changes could potentially alter tornado frequency, intensity, and geographical distribution, though the extent and nature of these changes remain highly uncertain.

Rising surface temperatures may enhance atmospheric instability, potentially leading to more frequent and severe thunderstorms. Warmer air holds more moisture, resulting in higher humidity levels. This increase in moisture can provide more energy to thunderstorms, possibly leading to more intense storm development under the right conditions. Wind shear patterns are also likely to be affected by climate change. A possible decrease in wind shear due to a reduced temperature gradient between the poles and the equator has been cited as a factor that could mitigate the formation of severe thunderstorms in some regions of North America.

Additionally, changes in jet stream patterns driven by climate change can influence tornado activity, potentially altering the geographical distribution and timing of tornadoes. For example, the tornado season could start earlier than usual.  More northernly parts of Ontario, Quebec, and the Prairies might become more prone to tornado occurrences as weather patterns shift northward along with rising temperatures. That said, the cumulative impact of these changes on tornado formation is still not well understood.

Several other factors, beyond changes in the drivers of severe weather, complicate the determination of how climate change will affect tornado activity, namely:

  1. Limited Data: The scarcity of long-term tornado records makes it difficult to establish clear trends in tornado frequency or intensity (Box 3).
  2. Modeling Limitations: Due to their coarse spatial resolution, climate models do not capture the small-scale and rapidly evolving atmospheric conditions that drive tornado formation.
  3. Complex Atmospheric Interactions: Tornadoes result from the interaction of multiple atmospheric elements. Modelling these interactions remains a significant challenge.

Box 3: Historical tornado climatology. Environment and Climate Change Canada (ECCC) released a tornado catalog covering the years 1980 to 2009, which documents 1,839 tornado events in Canada during this period. Cheng et al. (2013) found that the historical predictions of tornado occurrence in populated areas are reliable with no profound underestimation bias. However, in less populated areas, the study shows that the probability of tornado occurrence is significantly higher than what is represented in the 30-yr data record.[6]

Tornado Map

Figure: Confirmed and probable tornadoes from 1980 to 2009 highlighting prone regions to Tornadoes across Canada. Sills, David, “Tornadoes in Canada: Improving our Understanding” (2013). Environment Canada.

 

Conclusion

In short, climate change is impacting the atmospheric processes that underpin the formation of tornadoes.  Some of these changes could increase the frequency, intensity, and spatial extent of tornado formation and others could decrease the likelihood of tornado formation. Understanding the cumulative impact of projected changes in atmospheric conditions on tornado formation is an active area of research.

[1] Diffenbaugh, N. S., Trapp, R. J., & Brooks, H. (2011). Does global warming influence tornado activity? Eos, Transactions American Geophysical Union, 92(22), 233. https://doi.org/10.1029/2008EO530001

[2] NOAA National Severe Storms Laboratory. Severe Weather 101: Tornado. Retrieved on May 28 from: https://www.nssl.noaa.gov/education/svrwx101/tornadoes/

[3] https://www.nssl.noaa.gov/education/svrwx101/tornadoes/types/

[4] https://www.publicsafety.gc.ca/cnt/mrgnc-mngmnt/ntrl-hzrds/trnd-en.aspx

[5]https://ir.lib.uwo.ca/cgi/viewcontent.cgi?article=1006&context=ntp_reports

[6] Cheng, V. Y. S., Arhonditsis, G. B., Sills, D. M. L., Auld, H., Shephard, M. W., Gough, W. A., & Klaassen, J. (2013). Probability of Tornado Occurrence across Canada. Journal of Climate, 26(23), 9415-9428. https://doi.org/10.1175/JCLI-D-13-00093.1