So far in our look at severe thunderstorms we’ve looked at the three main severe weather threats: heavy rain, hail, and wind or tornadoes. What we haven’t discussed or looked at is why do some thunderstorms become severe while others do not, and how can you tell if a thunderstorm moving towards you is going to be severe?
So, what needs to happen to take a garden-variety thunderstorm and make it severe? We need to have a hot and humid air mass in place, the air a few thousand feet up needs to be very cold providing for good lift, and we need a strong jet stream overhead providing 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 spectacular?
The first and probably most important “extra” ingredient that can be added to the mix is to have the wind change direction with altitude. 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 several thousand feet, the wind switched direction and is now blowing from the west or 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 upwards. 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 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 their textbook example. Even when all of 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 are referred to as air mass thunderstorms. These storms, as their 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 too 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 will form. Since these storms don’t rotate or have any way to vent the rising air at 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; 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.
Now, how can you recognize if a thunderstorm moving towards you has the potential to be severe? First of all, recognize the conditions — how warm and humid is the air? Remember, a moist atmosphere means there is a lot of energy available. Look for a dark or threatening sky — look closely at the area between the storm and the ground, if you can see through it, the storm is likely not severe yet. Lots of lightning or nearly continuous thunder is a good indication of a severe storm.
As the storm approaches, keep an eye out for things like a green sky, as this usually indicates that the storm contains huge amounts of water and has very strong up and down drafts. Another thing to watch for are roll clouds. These form in front of the storm and are caused by strong winds blowing out of the storm. These clouds will rush by you quickly accompanied by high winds announcing the arrival of the storm. Finally, watch out for any kind of rotation within the storm. This means the storm has become very strong and has the capability of producing a tornado.