How are tornadoes rated using the EF scale?
Canada adopted the Enhanced Fujita (EF) scale in April of 2013 after using the original Fujita (F) scale for many years. Like the F scale, the EF scale uses observed damage to rate the intensity of a tornado and estimate associated wind speeds. The EF scale however has improved relationships between observed damage and estimated wind speeds, and many damage indicators, each listing degrees of damage from lightest to heaviest.
The EF scale adopted in Canada in 2013 is a slightly modified version of the original that was implemented in the United States in 2007. The main differences are the addition or modification of a number of damage indicators (e.g., farm silos and grain bins - new, trees - modified). It also lists wind speeds in km/h, to the nearest 5 km/h. Additionally, the minimum wind speed for EF0 was lowered to 90 km/h in order to maintain consistency with ECCC warning criteria.
The NTP has made further minor revisions to address issues with certain DIs that made their use for rating problematic, and to allow the use of some in-situ wind speed measurements. This most recent update, made in Spring 2024, is available here.
The EF scale is used in two ways.
First, it is used to rate the intensity of damage to individual damage indicators along the damage path. This allows contouring of damage intensity, particularly useful for strong (EF/F2 and EF/F3) to violent (EF/F4 and EF/F5) tornadoes.
Second, the maximum intensity found along the entire damage path using the EF scale is also used to characterize the strength of the tornado as a whole. EF/F0 and EF/F1 (weak) tornadoes are by far the most frequent in Canada (over 90%), though some of these might have been rated higher if more damage indicators had been present. This is particularly true for some parts of the Prairies.
When only visual evidence of a tornado is available, and no damage is caused to an EF-scale damage indicator, a rating of EF0-Default is assigned. This indicates that a wind speed of at least 90 km/h (the lower bound of EF0 with the Canadian version of the EF scale) is expected to have been associated.
The percentage of Canadian tornadoes that have been rated EF/F4 or higher is less than 0.5%. In fact, there has only been one recorded EF/F5 tornado in Canada, the Elie, MB tornado of 2007.
The F and EF scales have also been used to rate downburst damage in Canada, something done by Fujita himself for US events in various publications. Note that tornadoes and downbursts recorded up to 2012 retain their F-scale rating, and any tornadoes recorded in 2013 or later are rated with the EF scale.
NTP scientists are involved in the creation of an updated version of the EF scale that will eventually become a standard under the American Society of Civil Engineers for the first time. It may be ready in time for storm season 2026. Fingers crossed!
