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Expect more heavy summer rainfalls

When we think about severe summer weather we almost always think of damaging winds and hail. While these two features can result in severe damage, the kind of damage is often highly localized. One thing about severe summer weather that we tend not to think about, until it creeps up on us, is heavy or extreme rainfall. When you look at the impact of heavy rain and the resultant flooding, it by far outweighs most of the other summer severe weather events.

Just look at the weather over the last couple of weeks across the Prairies. After I pointed out that it looked as if we had finally switched into a summer-like pattern, with weaker weather systems and more thunderstorms, we saw another unusually strong upper low form over the Prairies that brought copious amounts of rain to western Alberta and parts of southern Manitoba.

Before we take a look at this unique weather event, I thought we should first look at just what Environment Canada uses as criteria for rainfall watches and warnings.

First of all, according to Environment Canada’s website, it does not issue rainfall watches, so we only need to look at warning situations. If it is going to be a short-duration event, such as a thunderstorm, you need to expect upward of 50 mm of rain in one hour before a rainfall warning will be issued, at least across the Prairies. It is actually lower over the East and West Coasts. While this might not make sense at first, if you think about it, they rarely get the intense thunderstorms that inland areas of Canada receive.

If the rainfall event is expected to be a longer-term event, the criteria for a warning is when 50 mm of rain are expected within 24 hours, or 75 mm of rain are expected within 48 hours. Sometimes, due to the nature of summer storms, you can have both types of warnings going on at the same time.

Now a lot of talk recently has been about the heavy rains we’ve seen across different parts of the Prairies over the last several years. The heavy rainfalls across western Alberta resulted in flooding that came near, or even far exceeded, the record flooding of 2005 in these areas. The 2005 flood was a one-in-100-year flood, and now only a few years later the region is seeing another one-in-100-year flood. In fact, at the Elbow River near Bragg Creek, the peak flow during the 2005 flood was 308 cubic metres per second. During this year’s flood the flow rate hit an astonishing 513 cubic metres per second before it stopped reporting.

Here in southern Manitoba, the same system that brought all of the rain to Alberta brought two rounds of heavy rain over the weekend. Slow-moving thunderstorms on Friday night into early Saturday morning brought upward of 75 to 150 mm of rain in the Reston area, according to Environment Canada. Unofficially, some localized heavy downpours were experienced, with amounts as high as 212 mm reported near Portage la Prairie. Check out the table for some of the larger rainfall totals for the weekend across southern and central Manitoba.

So why all the heavy rain? Well, it is summer, and we’ve seen and will continue to see heavy rainfall events. It’s the nature of our summer weather. Why do we seem to be seeing so many heavy rainfall events lately? It could simply be the general pattern or cycle we are in, but personally, I believe it is tied indirectly to global warming. A warmer atmosphere can hold more moisture. That moisture will eventually have to fall out of the atmosphere. When it does, it will fall out in large amounts.

People will argue against this by saying we have seen rainfall events as big, if not bigger, in the past when there was no global warming, and they are right. However, when these events occurred they tended to be isolated and very infrequent in nature. What we see now are much more widespread events occurring much more frequently.

I think simply chalking these events up to regular weather and continuing on as usual is the wrong way to go. To be successful going forward we need to start realizing that not only will we have to cope with dry periods, but we will also have to deal with heavy rainfall events. Just how we can deal with both of these successfully is the $1-million, or rather, $1-billion question.

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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