How rare is it for a tornado to touch down in Alberta? With the right conditions, it turns out the development of tornadoes in the province is fairly common during the summer months.

Alberta receives an average of 10 tornadoes each year, however only 3 major tornadoes have ever been recorded; Edmonton in 1987, Holden in 1993 and Pine Lake in 2000.
 

  • The Edmonton tornado lasted for just over an hour, killed 27 people, injuring more than 250 and causing more than $330 M in damages.
  • On July 29, 1993 a tornado hit a sparsely populated area near Holden, Alberta. Nobody was killed, but the resulting winds did $3 million in damage.
  • The Pine Lake tornado tore through the Green Acres campground on July 14, 2000. 12 people were killed, more than 100 were injured. Approximately $13 million of property was damaged.

Tornado intensity is defined on the Fujita Tornado Intensity Scale with a range of F0 to F6. The tornado that struck Edmonton was categorized as an F4 tornado, a devastating tornado with winds of 333-418 kph. The tornadoes in Pine Lake and Holden were categorized as F3 tornadoes.

 

How does a tornado form?

 

There are a lot of things we still don't understand about tornado formation.  However, according to CTV Meteorologist Josh Classen,here's the general theory:

Warm, humid air rises into colder, drier air and creates a strong updraft within a thunderstorm.
If there's enough of a wind speed increase with height, AND a clockwise directional shift to the wind with height (ex: SE wind at lower levels, W wind aloft), a funnel cloud can be created.  

"This still doesn't guarantee a tornado," says Classen, "That funnel cloud needs to gain enough strength to touch down & become a tornado."   

Researchers continue to study the factors that allow some funnel clouds to remain aloft, while others touch down.

 

Watches vs. Warnings

 

When conditions look favorable for tornado-producing storms, a Tornado WATCH will be issued by Environment Canada.
This is usually issued for a large area, most of which will not be affected by a tornado if one touches down.

However, since we can't determine exactly where the tornado will touch down, it's necessary to alert anyone in the general area to the potential for a tornado. Residents in that zone should prepare for the possibility of a tornado in the area.

If a tornado touches down or it becomes apparent a tornado is developing, a Tornado WARNING will be issued by Environment Canada through Alberta's Emergency Public Warning System (developed following the Edmonton Tornado in 1987).

That WARNING is for a smaller area that is in the path of the tornado.  Residents in that WARNING should take immediate safety precautions.

 

Forecasting tornadoes

 

When meteorologists analyze the atmosphere, we look for "clues" to tornado potential," says Classen.

There are a number of severe weather indexes (CAPE, Storm Relative Helicity, Lifted Index, Bulk Richardson Number, SWEAT, EH), that measure how unstable an air mass is.

While the vast majority of tornadoes develop out of Supercell High scores on these indexes, that doesn't necessarily guarantee a tornado. The high scores give meteorologists an early indication of what areas to watch.

Computer models may have come a long way in the past 15 years, but even with all the technology, meteorologists still aren't able to indicate exactly where tornadoes will develop.