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Bad weather forecasts? Blame the leaky bucket

Spring and fall are always the most difficult seasons to forecast weather in as things change quickly

I think I need to begin this article with an apology for the fairly dismal forecasts that I’ve been making this past couple of weeks. It is at times like this that I really appreciate the fact that I don’t have to create a forecast every day — especially at this time of the year!

Now, I could cop out by saying that it is really the weather models that have been having a dismal time with forecasting lately, since my forecasts are simply an interpretation of these models, but that would be the easy way out. Part of modern-day weather forecasting is not just trusting what the weather models are saying, but also deciding when you should believe the weather models and when you should trust your own instincts.

Lately, the weather models have been having some trouble and have been leaning towards a warmer-than-average pattern and I have simply bought into it, even when my gut instinct was telling me something different. Just like a lot of things in life, once you start second guessing yourself things can often go from bad to worse. That said, the bad weather forecasts lately are not just due to the struggling weather models and me second guessing myself, they also have to do with the time of year.

Forecasting can be difficult at the best of times, but fall and spring tend to be some of the toughest times — why?

A good part of the answer lies with the Earth’s energy budget… and we’re not talking oil here. Understanding the Earth’s energy budget, or balance, is a fairly big and somewhat complicated subject. Distilling it down to a key point — Earth receives energy from the sun and loses that energy back out into space. Depending on where you are on the Earth and what time of the year it is, you are either receiving more or less energy than you are losing.

In agricultural Manitoba, we receive excess energy from the sun in the summertime — this is why we have nice warm temperatures. During the winter we lose much more energy than we receive — making us cold. Overall, our region of the world loses more energy than it receives, which helps explain why our winters are long and cold, while our summers are short and warm.

One way you can help picture the energy budget or balance is to picture a big bucket with some holes on the bottom. No matter where you are on Earth you would have the same bucket. Now, let’s start filling up the bucket with water, which will represent energy coming in from the sun. During the winter, the flow of energy into our bucket will be very small. During the brief period when the sun is in the sky we might gain a little bit in our bucket, but once the sun goes down the bucket will quickly empty. During the summer, our bucket is receiving a lot of energy and it quickly fills up and starts to overflow, during a summer night the level drops a little, but this amount is quickly replaced the next day.

In the spring and fall things are different. In the spring we begin to receive more energy in our bucket during the day and the level starts going up quickly, but at night we still lose a fair bit, and the level drops down quite a lot. The same thing happens in the fall. During both the winter and summer we see very little change to the level of energy in our buckets – in the winter it stays low and in the summer it stays high. In the spring and fall the levels of energy in our buckets are changing rapidly, as we go from an empty winter bucket to a full summer bucket, or from a full summer bucket to an empty winter one.

We need to remember that the weather, in its simplest explanation, is the atmosphere’s attempt to equal out the buckets, or distribute this energy. In the summer there is a lot of energy around, but everyone around us also has a lot of energy – everyone’s bucket is full. This is why we rarely see really strong areas of low pressure in our area. We do see thunderstorms due to the high energy levels, but big organized lows don’t happen that often.

In the winter we don’t have much energy, thus very few thunderstorms, but our buckets are empty, so the fuller buckets to our south will try to fill ours up (nice of them isn’t it?). This movement of energy is what fuels big strong areas of low pressure. During both of these periods, day-to-day forecasting is still difficult, but overall, the atmosphere behaves in a relatively predictable pattern as energy levels remain rather constant.

During the spring and fall, energy levels are changing rapidly. Different areas lose energy quicker than others, depending on local conditions. Some buckets remain a little fuller longer, while others empty out quickly. Trying to figure this out is difficult, as it does not necessarily follow a set pattern. If we get a big difference in the bucket levels we could see some strong lows, if the difference remains small, weak lows. Along with determining the strength of the low-pressure systems, their track can also be difficult to predict. Lows will tend to follow the edge of warm and cold air. This edge is determined partly by our energy balance. As the energy budget is changing rapidly at this time, so can this boundary.

So, the next time our forecast or Environment Canada’s forecast is off, just think back to your leaky bucket.

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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