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Monitoring thunderstorm movement

Storms can strengthen or weaken as they move. Use online and visual tools to track their movements

In the last issue we continued our look at thunderstorms and severe summer weather by examining the direction thunderstorms move. In this issue we’ll learn how to determine the strength of a thunderstorm.

Pretty much any storm can produce severe weather. If there are severe thunderstorm watches or warnings out, and you see a storm approaching, then you need to assume that it will more than likely contain severe weather. Putting that aside, how can the average weather enthusiast determine how severe a thunderstorm might be?

In this day and age, where a vast majority of us have access to the Internet seven days a week, 24 hours a day, there are some really good online tools that can help. By far the most useful tool is the radar imagery from Environment Canada ( Environment Canada makes available near-real-time radar images from its full network of radar stations. While the images are delayed by about 10 minutes, for the most part, it makes it fairly easy to see just how intense a thunderstorm is.

According to Environment Canada’s website, when you look at a radar image you are looking at a picture of precipitation distribution (called “echoes”) and its intensity. The radar echoes are represented by a series of coloured pixels, and by looking at the scale to the right of the radar image you can determine the intensity. This intensity scale is broken into two columns, the one on the right represents what is called “reflectivity in dBZ,” and the scale on the left is the corresponding rate of fall. A good rule of thumb, according to EC, is that the higher the reflectivity value on a radar image, the heavier the rate of precipitation it is detecting.

Another good feature of these radar images is that they can be animated, which allows you to see the movement and growth of thunderstorms over a one- to three-hour period. I find by using this feature you can get a good feel for how a storm is moving and developing as it comes toward you. You will be amazed at how fast storms can either strengthen or weaken as they move.

Another good source of thunderstorm information is to look at real-time lightning data. While there are government sites available they tend to have a long delay period, making the data difficult to use for real-time weather prediction. There are private lightning detectors around and if you want to have access to all of these sites you can go to I have to admit, I am one of the weather geeks with my own lightning detector. You can check it out at Severe thunderstorms are often accompanied by intense lightning and while these private lightning detectors might not be the best at pinpointing the exact location of thunderstorms, they are good at recording the number of lightning strokes.

Along the horizon

OK, enough with the technology. What if you’re not connected and you see a storm approaching? What are some things you can look for that will help you determine the strength of the storm? First, you need to look at the colour of the storm. The darker the clouds, the more intense the storm usually is. Keep in mind that bright sunshine will make a storm look much darker than it usually is. Along with the darkness of the storm, look to see if the clouds are of a uniform darkness or if there are lighter and darker spots. Severe storms tend to be fairly uniform in colour, at least from a distance. You should also look at the base of the storm, or the region along the horizon. If you can see light coming through this area that means there is not a lot of rain falling from the storm and that the storm is not very big — yet.

As the storm gets closer, if you see a greenish tint to the clouds, then you should take shelter immediately. Green thunderstorms are usually very intense and will almost always contain very heavy rain and occasionally large hail and even tornadoes. If you see a line of billowing clouds or what appears to be rolling clouds racing toward you, just ahead of what looks to be the main dark area of thunderstorm clouds, then once again, you should take cover. These clouds are usually associated with a strong gust front pushing out ahead of the storm. These are strong straight-line winds that can approach and sometimes exceed 100 kilometres per hour.

Another thing to watch out for is intense or continuous lightning as the storm approaches. This indicates that the storm has very intense up-and-down drafts, which in turn, translate into a strong storm. Finally, watch to see if there are any clouds that appear to be either rotating and/or are lower than the main area of clouds. These features can often be associated with the development of tornadoes.

Next week we’ll continue our look at severe summer weather by exploring one of the two most destructive parts of a thunderstorm: 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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