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Geography influencing thunderstorms

Breezes off lakes Winnipeg and Manitoba can help build thunderstorms when they wouldn’t develop otherwise

Last week I asked for questions and ideas for different weather articles and I did get several good questions. Just remember this weather page is for you, so once again, feel free to email me any questions or topic ideas that you may have, at [email protected]

The one question that really stood out for me is one of those weather enigma questions — that is, people ask about this topic, everyone seems to have an opinion about it, and even weather experts tend to disagree about it. No, I’m not talking about tomorrow’s forecast! This has to do with severe weather, and in particular, tornadoes. Not just tornadoes in general, but rather, does local topography (fancy word for the local geography or terrain) influence the development and movement of tornadoes? I decided to broaden this topic a little bit by including the movement of thunderstorms in general.

Now the first thing I need to point out is some of the information I will share with you is personal insight from some meteorologist I know. I won’t name names, but if they speak in public you will not hear this type of information. Oh, it’s not that juicy, so don’t get too excited!

What we have to realize with most tornadoes is that they affect a very small area. Typically a tornado on the Canadian Prairies is around 50 to 100 metres across, with the largest, rarest tornadoes, reaching one to two kilometres in diameter. The average length or path for a tornado is only around five km, with most of them much shorter than this. I couldn’t find the longest tornado path in Canadian history, but the very destructive tornado that hit Edmonton on July 31, 1987 was reported to be about 40 km in length. That said, let’s look at the facts: tornadoes are relatively small and only move over very short lengths. A 100-m-wide tornado travelling for 2.5 km will only cover 0.252 km of ground area, or about 60 acres.

So tornadoes don’t typically cover much surface area, but so what? Well, we need to combine this with the fact that on average, only two to five tornadoes are expected to occur across any region of the Prairies each summer. Combine this with the small size of most tornadoes and the amount of open land we have, and it’s not surprising most of us have not seen a tornado, at least not up close and personal.

No immunity

What has this to do with tornado development and topography? Well, if you read most of the information about tornadoes you will find it states that topography (at least in non-mountainous areas) does not have a direct impact on tornado development. For the most part I believe this to be true. If a thunderstorm has developed that’s capable and going to produce a tornado, then a tornado will form, regardless of the topography. There have been a number of reports and stories about how different towns and cities claimed to be “immune” to tornadoes and were then badly damaged or wiped out by one. Again, it comes down to their small size. You can have hundreds of tornadoes go across Manitoba with only minor damage, but one tornado in the wrong place can be devastating.

Now we come to the gist of the question and the juicy stuff. If tornadoes don’t seem to care about the topography, why does it seem that some areas see or experience more of them? Well, I think it has to do with how topography can influence the development of thunderstorms, which, after all, form the tornadoes. It does appear there are certain areas, or circumstances, that either help to trigger or strengthen thunderstorms, or in some circumstances seem to cause thunderstorms to move in a certain direction.

For example, lake breezes off Lake Winnipeg and Lake Manitoba (and some other major lakes) can help to trigger the development of thunderstorms when otherwise thunderstorms wouldn’t develop. The Manitoba Escarpment can also have this effect. Once thunderstorms have developed and start to move they tend to move with the prevailing upper level winds, but here is where the controversy comes in. While really large storms don’t seem to care what is in their way, smaller but no less intense storms, just as capable of producing tornadoes, can sometimes seem to be influenced by the topography. I say “seems” a lot because it is not scientifically shown or proven — it is merely hearsay. Most of us who have lived in a location for a long time have probably seen this and I have talked to people in the know that have also mentioned this. It seems like there are areas that appear to influence storm movement. One area I know of is the south basin of Lake Winnipeg. Here thunderstorms seem to move to the southern end of the lake, or across the northern end near the narrows — but again, not all the time. Another example comes from a meteorologist I talked to years ago: he told me he has often seen storms split into two as they enter Winnipeg, with the south and north end getting hit the hardest. Don’t believe me? Talk to someone who lives in these parts of Winnipeg!

What I’d like to know is, do you have any local topography that you think influences thunderstorms?

About the author

Co-operator contributor

Daniel Bezte

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

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