So far this fall the Prairies have already seen several large early-winter-like storm systems. Southern parts of Alberta received a significant snowstorm from Sept. 28 to 30 while southern Manitoba experienced a significant rain and snowstorm from Oct. 10 to 12. Most of our big autumn or winter snowfalls come from areas of low pressure that develop to the lee of the Rocky Mountains. One area of development is over Alberta, producing what’s affectionally known as an “Alberta clipper,” while the second area is over Colorado, which produces the infamous “Colorado low.” This issue I thought it might be a good idea to re-examine this topic and discuss why so many lows seem to develop to our west on the eastern side of the Rocky Mountains.
In the past we have discussed how jet streams, when they follow a curvy pattern, can slow down and speed up. This slowing down and speeding up can then result in regions of descending air (high pressure) and ascending air (low pressure). But this doesn’t explain why lee-side lows develop. When trying to understand what causes these lee-side lows to form, the leading theory talks a lot about absolute vorticity, relative vorticity and the Earth’s vorticity. In case you are wondering, vorticity means the spin that a package of air will have. While thinking about the best way to try and discuss this topic, I decided to try and avoid explaining all the details that go along with the different types of vorticity, as it not only confuses me but would make for rather eye-glazing reading!
I will try to simplify things as best I can. Firstly, the further you move toward the equator, the lower the Earth’s vorticity. Relative vorticity is basically how much spin a moving parcel of air has. If it is spinning counter-clockwise (cyclonically) it gains positive vorticity, and if it spins clockwise (anticyclonic) it also gains relative vorticity, but in a negative direction.
OK, now, here is the important part: the Earth’s vorticity and the relative vorticity equal a constant absolute vorticity. This means that if air moves southward toward the equator and the Earth’s vorticity decreases, to compensate for this and to keep the absolute vorticity the same, the relative vorticity of the parcel of air has to increase. The same thing holds true if the air is moving toward the poles. The Earth’s vorticity will increase and therefore, the relative vorticity of the air has to decrease to keep the absolute vorticity constant. Now, remember: increasing vorticity leads to cyclonic motion (low pressure) and decreasing or negative vorticity leads to anticyclonic motion (high pressure).
Under the tropopause
Let’s now apply this understanding to the development of lee-side lows. As air flows in from the Pacific Ocean it is forced upward by the Rocky Mountains. At the same time the tropopause (the top of the troposphere) acts as a lid, causing the air to become squeezed and shrink vertically. To compensate for this the air must spread out horizontally. Because the depth of the air has decreased, the absolute vorticity also decreases (don’t ask me why — it just does – trust me!). To make up for this, the relative vorticity of the air must also decrease, since the Earth’s vorticity hasn’t changed. So, now the air starts to curve anticyclonically, or toward the southeast.
As this air moves to the southeast it moves into increasingly lower and lower Earth vorticity, so the relative vorticity must increase to compensate. This means the air gains a cyclonic spin and turns toward the northeast. We have now created a trough of low pressure, but why does this sometimes result in Alberta clippers or Colorado lows?
There are a couple of answers to this question. First, these regions have some of the highest mountains. This greatly shrinks the column of air as it is forced between the mountains and the tropopause, and the greater the shrinking the more the vorticity must change, resulting in the development of stronger troughing. Strong Alberta clippers form when this troughing is able to tap into very cold arctic air sliding down along the mountains, creating a big temperature contrast with milder air being pulled up from the south.
As for the Colorado low’s impressive strength, it has to do with the low’s southern location. As the low develops, it begins to draw air in from the south. With Colorado’s location, warm moisture-laden air from the Gulf of Mexico is often available. If you remember back to our discussions about thunderstorms, moisture in the air equals energy, especially when it is forced to rise and condense, releasing all of its heat. This extra energy can really get these storm systems going, resulting in those big bad Colorado lows.
For Alberta’s late-September storm and Manitoba’s early-October storm, we saw these strong troughs of low pressure form to the lee of the Rockies. Moisture-laden air was then drawn up from the south and pulled up over cool air that had dropped in from the north, resulting in some extremely large snowfall and rainfall totals.