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Just why do weather patterns get stuck?

If you remember back to the last article we started a discussion that was aimed at trying to explain why weather can sometimes get “stuck” in a particular pattern. How sometimes we see weather changes very rapidly and other times it seems that no matter what, the weather just stays the same. After an extremely brief discussion outlining how long waves form and how they are the main controlling factors for the weather in this part of the world, we then touched on the first atmospheric conditions that can cause our weather to become “stuck” — that was the Omega Block. This week we’ll look at other blocking weather patterns or systems that can cause our weather to become stuck in a particular pattern.

Now, as I tried to point out in last week’s article, when we talk about blocking systems causing the weather to get stuck, we are not necessarily saying that these systems result in the exact same weather day in and day out. While this occasionally does happen, as with some cold snaps in the winter and heat waves in the summer, these blocking systems tend to set up stable patterns in the jet stream which favour one overall type of weather pattern over another. Early this summer, a blocking pattern developed over Alberta that produced rain and record flooding. It wasn’t raining for days on end all day long, but rather, the conditions remained in place that allowed for wet, rainy weather to remain over a large area for much longer than is usual. These blocking patterns will often break down for short periods of time and then re-establish themselves. This is what can make weather so difficult to forecast.

Now, on to looking at the other types of blocking systems we can experience here in our part of the world. Another type of block that is similar to the Omega, but does not occur as often, is the Rex Block (named after the person who first studied it back in the 1950s). With the Rex Block, instead of having two areas of low pressure with a ridge of high pressure in between, there is a strong area of low pressure located south of a strong ridge of high pressure. The jet stream flows over the ridge of high pressure and then dives southward then west as it curves around the area of low pressure. This creates a “backward S” shape to the jet stream and effectively blocks any systems from moving eastward. This type of system usually forms in the late winter and springtime off of the west and northeast coasts of North America.

The next two types of systems that we can refer to as blocking systems are the cut-off low and high. Again, if you remember back to our discussion last week about how the cold air at the top of our planet sags southward in large, curving blobs, sometimes these blobs of cold air will break off to form a circle or pool of cold air (similar to how an oxbow lake can form from a meander in a river). Once this pool of air breaks away from the long wave, it loses its connection to the jet stream. This means that it no longer has strong steering currents to move it around, so the pool of cold air and the associated area of low pressure will either just sit where it is and slowly weaken, or it will slowly meander around. Either way, the area that finds itself under one of these cut-off lows can end up seeing cool, unsettled weather for an extended period of time.

The cut-off high works in much the same way as the cut-off low, but this time an area of high pressure will become separated or pinched off from the jet stream, and just like the cut-off low, the cut-off high will either just sit where it is or move very slowly. If you are stuck under a cut-off high you will see an extended period of warm, dry conditions.

The final blocking system is the split flow. This refers to the flow of the jet stream and as the name implies, the jet stream splits, usually into two separate streams. This occurs when the jet stream encounters an area of high or low pressure, but instead of going over or under it, the flow splits and some of the energy goes over it and some goes under. While the areas under the split flow will still see rapidly changing weather, the area in between the flows will usually become fairly stagnant, experiencing stable conditions with temperatures usually slowly warming as long as they remain between the two jet streams.

I hope this helps (at least a little) to explain why we can get “stuck” in certain weather patterns.

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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