As heat continues to build across the Prairies, the chances for thunderstorms have also increased. We began our look at thunderstorm fundamentals that lead to garden-variety thunderstorms, but while these types of thunderstorms are fun to watch, we all know how quickly they can turn into severe thunderstorms. A couple of weeks ago we talked about what it takes to form thunderstorms: heat, humidity, lift and some way to vent the air at the top of the storm. This week we’ll take a look at what it takes to create a severe thunderstorm, turning it into a thunderstorm to truly remember.
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 worse?
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 took a rising parcel of air and pushed on it from the south when it was at the surface. Then, as it rose up a couple of thousand feet, the wind switched direction and now blew from the east. Then, a few thousand feet farther up, it changed again and was 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 counterclockwise, we have an area of low pressure. Air flows inward in a counterclockwise rotation and is then forced to move upward. One thing we can get, if we 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 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 a tornado. While we still do not understand exactly how tornadoes are formed, we do know that rotating thunderstorms can produce tornadoes. 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 storm may form, or sometimes a key ingredient is missing, yet we still get a really severe storm; this is what makes weather so interesting.
Downdraft versus updraft
Of course, not every thunderstorm that develops becomes severe; in fact, much of our summer rainfall comes from your garden-variety thunderstorm, or what we call an air mass thunderstorm. 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, a number of the key ingredients for severe storms are missing.
Usually, in the middle of an air mass, temperature will not decrease very rapidly with height. The wind 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 to 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 will eventually fall back down as a downdraft and this will wipe 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 when we need it during the summer.
I hope you now know just a little bit more about the nature of severe thunderstorms. Next issue we’ll take a look at the latest long-range summer and fall forecasts, to see if they are still predicting warmer- and drier-than-average conditions, or if they are starting to swing toward a cooler and wetter pattern.