Multi-year drought and severe thunderstorms

Short-lived air mass thunderstorms can’t vent their rising air from the top

Not every thunderstorm that develops becomes severe; much of our summer rainfall comes from garden-variety air mass thunderstorms.

Before we continue our look at thunderstorms, and in particular, severe thunderstorms, I think we need to talk a little bit about the drought conditions that have been slowly deepening across much of southern Manitoba over the last several years.

At first glance, it appears that our current dry conditions began last summer after a wet June across western Manitoba. However, that wet June was just a one-month blip in what has been a multiple-year drought. Looking back at the monthly and yearly precipitation data for Winnipeg, Brandon and Dauphin, you must go back to 2017 to find a year that had near- or above-average precipitation. In fact, if you were to look at the last 36 months over our three different stations, for a total of 108 months’ worth of data, you would find only 10 months reported above-average precipitation. The rest were either near or below average. We’ll dig deeper into these stats over the next few issues.

Rotating

OK, now on to severe thunderstorms! I figure maybe if I write about them, we might actually see some. Right now, I think rainfall in any form would be welcomed.

A couple of weeks ago we talked about what it takes to form severe thunderstorms: heat, humidity, lift, and some way to vent the air at the top of the storm. This week we will look at what takes a severe thunderstorm and turns it into a thunderstorm to truly remember.

So, we have a hot, humid air mass in place, the air a few thousand feet up is very cold, providing for good lift, and we have a strong jet stream overhead providing the venting at the top of the storm. Everything is in place for a severe thunderstorm, but what can Mother Nature add to the mix to make things even more awesome?

The first and probably most important “extra” ingredient that can be added to the mix is to have the wind change direction with altitude. Remember that the atmosphere is three dimensional — that is, air can flow horizontally, but this horizontal direction can change as you move upward. Why would this have an impact on our storm?

To put it in a nutshell, this change of direction can cause the developing storm to rotate. Picture what would happen if you take a rising parcel of air and push on it from the south when it is at the surface. Then, as it rises a couple of thousand feet, the wind switches direction and now blows from the east. Then, a few thousand feet farther up, it is blowing from the northwest. What would happen to our rising parcel of air? It would get twisted; it would start to rotate.

Remember that if we can get air to rotate counter-clockwise, we have an area of low pressure. Air flows inward in a counter-clockwise rotation and is then forced to move upward. One thing we get, if we can get our severe storm rotating, is a small-scale area of low pressure that helps the air to rise even more than it would without the rotation. The second thing a rotating thunderstorm can do is to nicely separate the area of updrafts and downdrafts. This is important, since the downdrafts, even with a severe thunderstorm, will eventually cut the updraft off from its source of warm, moist air. In a rotating thunderstorm, the source of warm, moist air is maintained, giving these storms a long life and a lot of moisture to produce heavy rains.

Another aspect to the storm that a rotating column of air can provide is tornadoes. While we still do not understand how tornadoes are formed, we do know that rotating thunderstorms can produce them. It is believed that rotating columns of air can get squeezed into a narrower shape; as this happens, the wind speeds increase, eventually producing the tornado.

Like most things in nature, thunderstorms rarely behave like a textbook example of a thunderstorm. Even when all the ingredients are there, no storms may form, or sometimes some key ingredient is missing yet we get a really severe storm. This is what makes weather so interesting.

Now, not every thunderstorm that develops becomes severe; in fact, much of our summer rainfall comes from garden-variety thunderstorms, or what we refer to as air mass thunderstorms. These storms, as the name indicates, develop in the middle of a typical warm summer air mass. Because they are in the middle of an air mass, several of the key ingredients for severe storms are missing. Usually in the middle of an air mass, temperature will not decrease that rapidly with height, wind direction will usually remain constant with height, and there will probably not be a jet stream overhead. Nonetheless, we can still have enough heat and humidity for air to rise and thunderstorms will form. Since these storms don’t rotate or have any way to vent the rising air from the top of the storm, they rarely last long. The accumulating air at the top of the storm eventually falls back down as a downdraft which wipes out the updraft, essentially killing the storm. The whole process from the start of the storm to the downdraft killing it can be anywhere from 30 minutes to one hour. While these storms are short lived, they can give brief periods of heavy rain and the odd good gust of wind, especially when the downdraft first hits the ground. These storms often provide us with just the right amount of precipitation just when we need it during the summer.

Next up: severe winds, tornadoes, and hail.

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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