A tornado that sliced through a rural area south of Didsbury, AB on July 1 is among the most powerful ever recorded in Canada. It has been rated at EF4 by Western's Northern Tornadoes Project, in collaboration with the Environment and Climate Change Canada (ECCC) Prairie and Arctic Storm Prediction Centre.
The Enhanced Fujita scale or EF scale is used to assign a tornado a 'rating' based on wind speeds estimated through damage analysis.
With numerous witnesses documenting the event through photos, video and personal accounts, NTP and ECCC conducted a thorough ground and drone survey, with one NTP team documenting damage on the evening of the event and two NTP teams working together the next day (July 2).
The on-ground survey teams led by Western wind impacts researcher Connell Miller found that there was enough evidence to rate the tornado damage at EF4, with a maximum wind speed of 275 kilometres per hour. The preliminary path length is 15.3 km and maximum path width 620 m.
Before (top) and after (bottom) high-resolution satellite images from Planet.com. Ground scouring through crops makes the tornado's path easy to see in spots. The red line indicates the the path visible from satellite imagery. The 's' and the 'e' mark the start and end points of the path found by the ground/drone survey team. It is typical for satellite imagery to have somewhat shorter visible paths.
"Thankfully, due in part to timely weather alerts, there was only one minor injury (a cut to a first responder, reportedly)," said David Sills, NTP executive director. "However, climatologically, the Didsbury EF4 tornado enters some rarefied territory among Canadian tornado events."
The Didsbury EF4 tornado is the strongest recorded tornado in Alberta since the Edmonton F4 tornado of 1987 and one of only three tornadoes rated F/EF4 in Alberta (the other hit the Grassy Lake area in 1915). There have been no recorded F/EF5 tornadoes in that province.
It is also only the second tornado in Canada to have damage rated at EF4 on the Enhanced Fujita scale implemented in this country in 2013, with the other being the Alonsa, MB EF4 that also had a maximum wind speed of 275 km/h.
Preliminary map of the Didsbury tornado path with length 15.3 km. (Northern Tornadoes Project image)
Across the country, there have been only 21 'violent' tornadoes rated at F/EF4 or higher. One of these was Canada's only tornado rated at F5: the 2007 Elie, MB. tornado. Twelve of the F/EF4 tornadoes have been recorded in southern Ontario. Saskatchewan and Manitoba have also experienced F4-rated tornadoes (three in SK., all 1920 and earlier; two in MB. from 1977 and 1994).
Though this was a climatologically significant tornado, Didsbury does not feature in the list of Canada's ten 'worst' tornadoes, again due to the single minor injury and limited property damage.
Twelve residences were hit by the tornado: three were destroyed, four were left uninhabitable and a further five were damaged.
When tornado-related damage is surveyed, it is compared to a list of damage indicators (DIs) and degrees of damage (DoD) which help estimate better the range of wind speeds the tornado likely produced. From that, a rating (from EF0 to EF5) is assigned.
One of the houses that was destroyed was found to be well built after an on-site engineering analysis. Using the EF scale tool, the damage indicator (DI) was 'one- or two-family residences (FR12),' and the degree of damage (DoD) was 9 – meaning all walls collapsed. Given the house was well built, NTP could use the 'expected' maximum wind speed for DoD9 of 275 kilometres per hour. The wind speed range of EF4 is 270-310 kilometres per hour. So, the EF rating is low-end EF4.
In terms of nearby corroborating damage, various pieces of farm equipment at the property were flipped and thrown. This includes a combine weighing almost 10,000 kilograms that was tossed at least 50 metres, and then rolled for another 50-100 metres after that.
Combine weighing nearly 10,000 kg was tossed ~50 m and rolled at least another 50 m. (Northern Tornadoes Project photo)
Wind tunnel studies of another combine that was hit by a previous Canadian tornado suggest a wind speed of 230 km/h for just flipping a combine. Tree stubbing or debarking and ground scouring are also consistent with a high-end tornado.
NTP, founded at Western University in 2017 with the support of social impact fund ImpactWX, aims to better detect tornado occurrence throughout Canada, improve severe and extreme weather understanding and prediction, mitigate against harm to people and property, and investigate future implications due to climate change.
