It’s that time of year when severe summer weather and tornadoes can form

Tornadoes have occurred in nearly all regions of Canada — 
here is what to look for when a severe storm is approaching

This map shows the total amount of precipitation that fell across the Prairies compared to average during the 30-day period ending on June 9. It is evident that this has been a fairly active period, with a large portion of all three Prairie provinces showing near- to above-average amounts of rainfall. The wettest regions were found in Alberta, stretching from Grande Prairie southeastwards towards Coronation and western Saskatchewan and southeastern Saskatchewan to southwestern Manitoba. Interestingly, north-central Saskatchewan was very dry during this period.
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As we enter the peak season for severe weather across the Canadian Prairies, I figured now would be a good time to continue our look at severe weather and tornadoes.

While Alberta doesn’t see the same number of tornadoes relative to areas to the east and south, the province is still subject to these events. Before we begin our look at tornadoes let’s step back a little bit and discuss how you can know if a storm is capable of producing severe weather.

The answer is actually not that easy. The first thing is to listen to Environment Canada for watches and warnings. If a watch is issued, it means that the potential exists for severe thunderstorms, but they have not yet developed in your area. When you hear that there is a watch you should watch the sky for any development, and if any storms do develop they have the potential to become severe. If Environment Canada issues a warning, this means that a thunderstorm with some or all of the characteristics of a severe storm has developed and has been confirmed by eyewitness or radar. This means you should take precautions immediately.

If you are out in the field without access to a radio what can you watch out for?

First of all, recognize the condition: How warm and humid is the air? Remember, a moist atmosphere means there is a lot of energy available. Look for a dark or threatening sky — and then look closely at the area between the storm and the ground. If you can see through it, the storm is likely not severe yet.

Lots of lightning or nearly continuous thunder is a good indication of a severe storm. As the storm approaches, keep an eye out for things like a green sky and mammatus clouds (clouds that look like bag-like sacks that hang beneath a cloud). These conditions usually indicate that the storm contains huge amounts of water and has very strong up- and downdrafts. Finally, watch out for any kind of rotation within the storm. This means the storm has become very strong and has the capability of producing a tornado.

Worldwide, Canada is second only to the United States in the number of tornadoes occurring each year, with an average of about 70 reported. Southern Ontario experiences the highest number of tornadoes, followed by southern Manitoba, Saskatchewan, and then central Alberta. While these areas report most of Canada’s tornadoes, they have occurred in nearly all regions of Canada and there is evidence to suggest that tornadoes are underreported in the lesser populated areas of Canada.

Tornadoes can strike at any time of the year, but in Canada, tornado season runs from April to October, with the peak months being June, July and August. This differs from the U.S., where tornadoes peak in April and May. This is due to the amount of cold air that is available for severe storm development. In the spring, the southern and central U.S. have become quite hot, but cold air is still closely available to help develop thunderstorms. By midsummer, most of the cold air has retreated well into Canada, putting our region into warm conditions, however, we still have cold air fairly close by to our north.

As to how tornadoes form, the real answer is, we just don’t know. Tornadoes usually develop from supercell thunderstorms, which are difficult to predict. Even if we were able to accurately predict where and when these thunderstorms would develop, the intense part of the thunderstorm usually only covers an area of a few hundred square kilometres. Within this few hundred square kilometres, the really severe weather may only occur in a small area of maybe 10 to 20 square kilometres.

Now, if we look at the size of a tornado, we would find that they range from as small as about 40 metres to as large as two kilometres across, with the average width being around 100 to 200 metres. This means that, as far as weather phenomena are concerned, tornadoes are very small, which makes them very hard to study first hand.

The best guesses on how they form is that wind shear within the thunderstorm, which is wind blowing at different speeds and/or directions at different heights, creates a horizontal column of rotating air. Then the strong updraft within the supercell thunderstorm pulls and elongates this rotating air causing it to speed up and become vertical. If the rotation becomes strong, and lasts long enough it will eventually elongate to the ground forming a tornado.

Most tornadoes are relatively weak, with about 75 per cent falling within the F0 to F1 range of the F0 to F5 scale of tornado classification. But we do see the occasional strong tornado. In fact, the Edmonton tornado of 1987 was classified as an F4 tornado and at times was estimated to be 1.0 kilometre wide.

While I do enjoy a good thunderstorm now and then, here’s hoping that we don’t see any really severe weather this summer.

About the author

AF Contributor

Daniel Bezte

Daniel Bezte is a teacher by profession with a BA (Hon.) in geography, specializing in climatology, from the University of Winnipeg. He operates a computerized weather station near Birds Hill Park, Manitoba.

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