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Gauging snowfall probabilities

A Winnipeg winter offers about a 90 per cent chance of 30 days in which some snow falls

Back in October, just after the big snowstorm, I discussed fall snowstorms. Well, it seems that the October snowstorm has brought on a flurry of questions (no pun intended!) about the probability of snow. So I figured this week we should take a deeper look into snowfall, a topic we’ve visited a couple of times over the last 15 or so years, but I think it deserves another look.

Years ago — actually, many years ago now — during one of my university courses on water systems, we were looking at stream flows and how to calculate the probability of 25-, 50- and 100-year floods. Once I learned how to do this, I began thinking about snowstorms and how often we should expect big storms to hit us. Using this knowledge, I went through all the data for Winnipeg (a nice long record that tends to be a fairly good representation of southern Manitoba) and applied the same probability techniques I learned for stream flooding, since the statistical theory is equally sound for both things. Well, good thing I kept all that data, so let’s dig into the numbers and see what we come up with.

First of all, I think I have to point out that when we look at snowfall data there are different periods in which you can measure snowfall, or any precipitation for that matter. You can measure the amount of snow that falls in one day, which would be from midnight to midnight. This one is easy enough, since this is the way precipitation amounts are recorded. We all know Mother Nature doesn’t have a very accurate clock, so we can’t always count on a snowfall occurring nicely during this one-day period. Another way to measure snowfall is to calculate how much fell in a given 24-hour period. To calculate this, you need access to hourly data which, unfortunately, is not readily available. We can measure snowfall a third way, which does help us “catch” these 24-hour-period snowfalls, and that is to look at what I call a snow event. A snow event is when we record snowfall amounts on two or more consecutive days.

Let’s begin by examining snowfall days. In Winnipeg, during any given winter, there is a 90 per cent chance that there will be about 30 days when snow falls. About 50 per cent of the time we will see around 45 snowfall days and there is less than one in 100 chance of seeing more than 70 snowfall days in any given winter. Keep in mind that a snowfall day is any day that records snowfall, no matter how small.

Now, when we look at how much snow we can expect to fall during a snowfall day, we find that 90 per cent of the time we can expect 0.2 cm of snow or more, which basically means at least a light dusting of snow. About 50 per cent of the time we can expect more than two cm of snow, which means about half the time it snows we see less than two cm of snow! If we look at how often we should expect five cm or more, we would find the probability has fallen to about 10 per cent. That is, if we get 30 snowfall days during the winter, only about three of them will give us more than five cm of snow. Looking at higher one-day snowfall amounts, we find that 10 cm or more occurs about once in every 25 snowfall days — or once, maybe twice per winter. Getting more than 30 cm of snow in one day is extremely rare. The probability of this occurring is about 0.1 per cent, which means about one in every 1,000 snowfall days, or around once every 20 to 40 years, we could expect to see this much snow in a single day!

As I pointed out earlier, Mother Nature rarely dumps all the snow conveniently during a midnight-to-midnight 24-hour period, so let’s look at snowfall events and see how the numbers shape up.

Probabilities

In any given winter, we typically see about 20 snowfall events during which we record snowfall on two or more consecutive days. About half the time we could see as many as 28 snowfall events, and once in every 100 winters we could experience as many as 40 snowfall events. Just like with the one-day snowfall amounts, the majority of these snowfall events only record small amounts of snow. About 90 per cent of the time we would expect to see 0.5 cm or more snowfall and about 50 per cent of the time we would expect to see more than two cm. Interestingly, these numbers are just about the same as for the one-day snowfall probabilities. If we look at the probability of receiving more than five cm of snow during a snowfall event, we would see that it happens about 30 per cent of the time. Going up to 10 cm or more, our probability drops to about 10 per cent, or around twice per winter. How often should we expect to see big storms with more than 30 cm of snow? Well, according to my probabilities, it would be about one in every 200 snowfall events, or about once every 10 years.

It may appear as though these numbers are a bit off, so let’s take a look at last winter, which was a dry winter, and that showed in the data. There were only 29 days with snowfall, which was close to the 90 per cent chance of seeing about 30 snowfall days. There was only one day in which Winnipeg saw more than five cm of snow, which was well below the statistical average of three times. Finally, there were no snowfall events producing more than 10 cm — events which statistically should happen once or twice a winter.

Next time I think we’ll take a step back and look at global weather from around the world over this last month or two – stay tuned!

About the author

Co-operator contributor

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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