As we work our way into summer, an ugly weather term has popped up: upper-level low. The coolish, unsettled weather we’ve seen in early July has largely been the result of an upper-level low that formed over Hudson Bay.
Let’s look at upper-level lows and why they become so annoying, especially in summer.
Before we dig into this topic, I want to point out a weather record that was unofficially broken on July 3 and again July 4. According to the University of Maine’s Climate Change Institute, Earth’s average temperature hit 17.18 C, breaking the previous record of 16.92 C set on July 24, 2022, and Aug. 14, 2016.
Read Also
June brings drought relief to western Prairies
Farmers on the Canadian Prairies saw more rain in June than they did earlier in the 2025 growing season
With El Nino getting well established, I would not be surprised to see this record broken several more times this summer.
Now, about those annoying upper-level lows. The topic seems to pop up every three to four years, usually in spring. So far this year we have not had to deal with the typical upper-level lows that usually form over Hudson Bay and then meander, pumping cold unsettled air southward into Manitoba.
The worst year for this was 2004, when this type of upper low kept forming and reforming all summer, resulting in one of the coldest summers on record.
Whenever I hear mention of an upper-level low or a cut-off low, I cringe. Unless you are in a drought and praying for rain, upper-level lows are never a good thing. They are usually tough to forecast because they tend to move very slowly, and because of that, they can stick around for days or even weeks.
It’s difficult to determine how long they will last and their slow movement can result in plenty of chances for rain. Depending on the nature of the upper-level low, we will sometimes see widespread rains, while other times the rain will come in the form of scattered showers or even thundershowers.
The current upper low is meandering well to our north, so we will most likely see scattered showers, maybe the odd thundershower, and cooler weather.
Unsteady eddies
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. These can form for a number of reasons, but most of them form along the boundary between two different air masses and are associated with the jet stream.
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, they can break away and when they do, they tend to meander until they either slowly weaken or get caught up in the main current once again and quickly move away.
Usually, the jet stream flows in a low amplitude or gentle wavy pattern. Sometimes we see extreme loops develop in the jet stream, creating strong ridges of highs and troughs of low pressure.
As these ridges and troughs strengthen, cold air in the upper atmosphere is pulled into the low, creating a pool of cold air above the low. This reduces the height of the atmosphere over that region, which lowers the pressure in the upper atmosphere, creating an upper low. Upper lows are not that unusual but it is unusual if they break away and become a cut-off low.
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. Once this happens, the upper low does not have any strong steering currents so it wanders around.
In the summer, we tend to see a lot of showers and thunderstorms develop. During the day, the sun warms the surface area under these lows. The warming air then starts to rise and as it does, it finds a cold atmosphere around it. The air continues to rise, creating showers and thunderstorms, usually by mid- to late afternoon.
These showers and storms tend to weaken overnight, only for the whole cycle to begin again the next day. So, with upper-level lows we will usually see a nice, bright, sunny start to the day. Then by late morning clouds develop, followed by showers or thundershowers in the afternoon. These stick around until early evening and we begin to see the skies clear.
To sum it all up, upper-level lows on their own are not necessarily a bad thing. Things get bad when they break away from the jet stream or are cut off from it, forming a cut-off low.
These lows move very slowly and bring prolonged periods of cool, unsettled weather. Also, because of their slow movement, we can see large amounts of rain and forecasting becomes difficult.
