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What are the odds for big snowfalls?

As we’ve been learning over the years, certain weather-related questions sound simple enough, but when you actually start to look at the question, it becomes tougher to figure out. Take trying to figure out when winter actually begins: the tough part is how to define just what constitutes the start of winter. Should it be the first significant snowfall? How about when the high temperature consistently stays below 0 C? Should we simply use the astronomical date of Dec. 21, or maybe just stick to the meteorological date of Dec. 1?

I think most people across the Prairies would agree winter doesn’t really arrive until you have snow on the ground, so for us, I use this as our measure of when winter arrives. Even narrowing it down to this still has some problems. What if, for example, it snows five cm on Oct. 22, then by Nov. 8 it has all melted and we don’t receive any more snow until Dec. 4? Did winter start on Oct. 22 or Dec. 4? I call this situation a false start to winter and I would record the winter in this example as starting on Dec. 4. Once I determined this, I went through the snowfall records for Winnipeg, Brandon, and Dauphin, and came up with the results you see here in the table.

From the table we can see all three regions of agricultural Manitoba have seen winter start in October and as late as mid-December. Winnipeg and Brandon both have an average date for snow to stick around of Nov. 14, with Dauphin being four days earlier at Nov. 10. The “usual range” is a measure of the standard deviation around the average. It indicates the range of days within which we should expect winter to begin. If winter begins before or after these dates, it’s a fairly unusual year.

Another question I often get is whether we’re going to get a lot of snow this winter, and are we going to see any major snowstorms? I often have a tough time with this question; as those of you who regularly read my articles have probably already figured out, I really like snowstorms, at least if I don’t have to go anywhere! So, I tend to be a little biased when talking about the probability of snowstorms. If we look back over previous years’ snowfall events in our three different regions, I found that large single-day snowfalls are fairly rare events. When I looked at the number of times Winnipeg, Brandon and Dauphin received more than 10 cm of snow in one day over the last 70 years, I was surprised to find out that on average, all three locations have this occur a little less than twice per winter. When we bump up the single-day snowfall to 15 cm or more, this occurs on average a little less than once per winter. If we increase the single-day snowfall to 20 cm or more, the frequency drops down to around once every five years. Finally, to show how rare really big snowstorms are in our region, if we take a look at the probability of receiving 30 cm or more in one day, we would find that this kind of event only occurs once every 30 or so years.

One thing that I need to point out is, from December to February, agricultural Manitoba, on average, receives about 50 cm of snow. So all it takes is one big storm and we’ll be at average or even above average for the winter. This is why it’s so hard to predict whether winters will be wet or dry: often it only takes one storm to make a wet winter! Another point to make about these types of statistics is that if you get a 30-cm snowfall, it doesn’t mean the chances of seeing another large snowfall that winter are any smaller.

A second question that comes up with the talk of snow is cold temperatures: just when will they move in? The answer to that is almost always tied to when the snow moves in. While we can get some cold temperatures without snow covering the ground, to get extremely cold temperatures and sustained cold temperatures we need to have snow cover.

Snow cover acts in a couple of ways to contribute to colder temperatures. First of all, it insulates the ground, trapping the ground heat and preventing it from warming up the air above it. Secondly, snow has a very high albedo — that is, it reflects a very large proportion of the sun’s energy. So instead of the sun’s energy being absorbed by the ground and then released to warm the air, it gets reflected and we don’t warm up much during the day. Finally, snow, simply put, is cold! We really notice this in the spring, but having snow on the ground at any time of the year acts like a refrigerator to keep temperatures down.

In the next issue we’ll take our annual look at weather-related gift ideas for the upcoming holiday season.

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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