Every year around this time I get a lot of email asking when we should expect the first snowfall, or when do I expect winter will begin? Some regions of agricultural Manitoba have already seen their first snowfall, but the warm weather we’ve been experiencing has long since removed any traces of that. So, while I have written about this several times over the last six years, I thought it was time to revisit the question of when we can expect the first snow to fall and when we should, “normally” expect winter to arrive.
As we have learned 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. The tough part about trying to figure out when winter starts 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, or should we simply use the astronomical date of Dec. 21, or
Earliest start Oct. 27, 1972 Oct. 6, 1959 Oct. 6, 1959
Average Nov. 14 Nov. 14 Nov. 10
maybe just stick to the meteorological date of Dec. 1?
I think most people would agree that 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 poses some problems. What if, for example, it snows five centimetres on Oct. 22 and 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? For me, 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 in Table 1.
From this table we can see that 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 as 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 in which we should expect winter to begin. If winter begins before or after these dates, it is 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? Looking 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 of occurrence 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 kind of event only occurs once every 30 or so years.
One thing I need to point out is that 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 is so hard to predict whether winters will be wet or dry; often it only takes one storm to make a wet winter!
A second question that comes up with the talk of snow is just when will the cold temperatures 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.
TABLE 1. FIRST SNOWS
Dec. 16, 1939
Nov. 3-26 Dec. 15, 1974
Dec. 14, 1981
Oct. 29-Nov. 22