Sometimes it doesn’t pay to jump the gun. Normally I wait until the very end of the month to put out the next month’s long-range forecast. The way March ended I decided to put out the forecast a week earlier than usual and, well, it looks like it’s going to bite me in the butt!
For those of you who didn’t read last week’s long-range forecast, most of the forecasters were calling for average temperatures in April and I went right along with them. At the time it looked like we were going to see a week or so of cool air to start the month and then milder conditions would move back in.
A week later, the medium-range weather models are singing a different tune. It now looks like the first half of April will see well-below-average temperatures, so unless the heat really turns on for the second half of the month we’ll likely see the first below-average month since August 2015 in Winnipeg and May 2015 in Brandon and Dauphin.
If you are reading this then that means you probably follow the weather a little closer than most. So you probably noticed that while southern and central Manitoba were shivering last weekend with temperatures struggling to make it to the freezing mark in most areas, Saskatchewan and Alberta were basking under summer heat. With high temperatures in western Manitoba around the +3 C mark on Saturday and eastern regions staying well below zero for highs, Regina, Saskatoon, Edmonton, and Calgary all saw high temperatures in the low 20s.
High versus low pressure
This sharp contrast in temperatures was the result of two different features working together. The first is the large-scale weather pattern currently dominating most of Canada and the central and northern U.S. There has been a fairly persistent ridge of high pressure to our west and a large trough of low pressure to our east which is associated with the polar vortex rotating around Baffin Island. This allows for mild conditions to develop under the ridge of high pressure, and cold air to work its way south in the trough of low pressure.
We’ve seen our share of the western ridge this winter, which is what brought all the warm weather. What really raised the temperatures to our west last weekend was a series of lows that developed along the boundary between the warm and cold air. The circulation around the lows helped to really bump up the temperatures.
Over the last couple of weeks there has been a strong intensification of the eastern trough of low pressure that has resulted in not only more cold air moving southward, but also a westward push of this colder air. What this setup then creates is what is known as a baroclinic atmosphere across much of Manitoba. For those of you who like to read the twice daily significant weather discussions for the Prairies that is put out by Environment Canada (http://kamala.cod.edu/Canada/lat est.focn45.CWWG.html) you may have noticed this term being used.
So, just what is a baroclinic atmosphere and why should we care? Well, it’s actually a tough topic to try and discuss, visualize, and understand, but I will try to give my best short description.
A typical or textbook atmosphere is known as barotropic. What this means is that in the upper atmosphere the pressure gradient (isobars) and the temperature gradient (isotherms) run parallel to each other. Since the winds at this level follow the pressure gradient line or isobars, the moving air remains at the same temperature since the isotherms are also parallel to the isobars. I have included an image to help show this.
In a baroclinic atmosphere the isobars and isotherms do not run parallel to each other, but rather cross each other. This means that as the winds blow, or the air flows from one area to another, either warmer or colder air is being moved into a region (see image at top).
What does this mean? Well, to put it simply, this helps to create instability in the atmosphere as this difference in temperature causes the mixing of air and forces pockets of air to begin rotating. This then helps to create what we call unsettled conditions, with clouds and spotty precipitation developing in the baroclinic zone. Should an area of low pressure develop and move along the baroclinic zone, the conditions in this area are such that they will typically help to intensify the system.
What can make this type of setup particularly annoying is when the upper pattern is fairly stable, like we see right now, which then allows the baroclinic zone to stay in place for a relatively long period. The zone will move slowly back and forth as stronger areas of low pressure form and slide along the zone and as the upper level ridge and trough push back and forth against each other. So, until this upper level pattern breaks down, we should expect more of this rather annoying weather.