How a retrograding upper low is keeping us cool

Cut-off lows tend to bring prolonged periods of unsettled weather

We all knew it was probably going to happen, after having one of the earliest and nicest starts to spring on record — snow and cold air moved in, reminding us it is still early spring across the Prairies. Weather models had been having trouble with the medium-range forecasts for the last several weeks, with my forecasts often mentioning a lot of uncertainty. When the weather models have trouble, it usually means a change in the weather pattern is going to happen. While it was a decent spring snowstorm, it was not that unusual or that intense and not a lot of really cold air moved in behind the system. What was unusual was how the system evolved.

Up until a few days before the two-day storm system, the weather models only showed a broad area of light snow over our region, as an area of low pressure moved through Saskatchewan and weakened as it moved through North Dakota. That’s exactly what happened, but at the same time, an upper-level low, or cut-off low, was strengthening over the Great Lakes. The Saskatchewan low ended up merging with the Great Lakes low, which helped to pull that low back to the west. I will have to admit, whenever I hear mention of an upper-level low or a cut-off low I kind of cringe. Unless you are in a drought and praying for rain (or snow), upper-level lows are never a good thing. They are tough to forecast, they tend to move very slowly, and because of that they can stick around for days or even weeks! This slow movement then results in plenty of chances for rain or snow, which, for us at this time, was certainly not a bad thing.

It seems like every couple of years I must touch on the topic of upper-level lows and retrograde motion. In fact, looking back, most of the times I have written about this topic are in May. With spring beginning about a month earlier than usual this year, the timing of this topic is strangely similar.

This spring’s upper-level low was unusual, as upper-level lows go. Usually in spring we see upper-level lows form over Hudson Bay and simply sit there spinning away, pumping cold, unstable air into Manitoba. As I mentioned earlier, this upper low was over the Great Lakes and slowly retrograded back into Minnesota over a three-day period. It was actually very interesting to watch. All our precipitation from this system came from the east and at the height of the storm the westward-spreading precipitation shield stretched from the Great Lakes all the way to the extreme-southeast corner of Alberta. From the radar and satellite loops of the storm system, it looks like moisture from Lake Superior was streaming westward and falling as snow across our region. The one positive that comes from this setup, besides some much-needed moisture, is that the low only stayed around for about four days before moving back off to the east.

Between two masses

What are upper-level lows and cut-off lows and what causes them? Upper-level lows are often associated, at least at first, with strong surface lows. Surface lows can form for a number of reasons, but a vast majority of them form along the boundary between two different air masses and are associated with the jet stream. If you remember back to some of the articles I’ve written about the jet stream and waves, you might remember areas of low pressure tend to form in the turbulent flow along the edges of the jet stream. This is kind of like watching eddies form in the water when two different currents meet. These eddies or lows, if they stay along the edge of the two different currents or jet stream, tend to move along fairly quickly. Occasionally, these features can break away from this region and when they do, they tend to meander around until they either slowly weaken or get caught up in the main current once again, then quickly move away.

Upper lows in themselves are not that unusual, but having them break away and become a cut-off low is. If the jet stream is strong enough, or the curving around the area of low pressure becomes very exaggerated, the upper low can break away from the main flow of the jet stream. Thus the upper low doesn’t have any strong steering currents and tends to just meander or wander around. In the summer, because these upper lows are pools of cold air in the upper atmosphere, we tend to see a lot of showers and thunderstorms develop. During the day, the sun tries to warm the surface area under these lows. The warming air begins to rise as it finds a cold atmosphere around it. This allows the air to continue to rise, creating showers and thunderstorms, usually by mid- to late afternoon. The showers and storms tend to weaken overnight, only for the whole cycle to begin again the next day. In the spring these systems tend to bring us exactly what we saw last week: cool, wet weather.

To sum it all up, upper-level lows on their own are not necessarily a bad thing. Things get bad when these lows break away from the jet stream or become cut off from it, forming a cut-off low. These meandering lows bring prolonged periods of unsettled weather and because of their slow movement, we often see large amounts of rain or snow, if it is cold enough. Typically, we only see one or two of these cut-off lows impact our region, so hopefully the latest will be the last one we see this spring and summer, but hopefully it will not be the last time we see significant precipitation.

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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