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Although all thunderstorms are inherently dangerous, under some conditions they can form up into long lines that can dump an incredible amount of rain over an area, resulting in flash flooding. Here's how it happens...

Science Behind the Weather: What are 'training thunderstorms'?

Scott Sutherland
Meteorologist/Science Writer

Friday, September 5, 2014, 2:37 PM - With the heat and humidity throughout southwestern Ontario today and a cold front soon passing through, Weather Network forecasters are on the lookout for severe thunderstorm activity, and along with this, the potential for what is known as 'training' storms. So what exactly are 'training' storms?

When a cold front passes through a region that's socked under hot, humid, sticky conditions, like what's happening today in Southern Ontario, it results in thunderstorms lining up along the front as it marches along. That's what's expected today, as this simulated view of the weather radar for Friday evening shows:

In this case, all of these storms will be moving in roughly the same direction, with the entire line marching along at the same pace, as indicated by the arrows in the image above.

However, some thunderstorms - as they march along with the rest of the line - can actually generate new storms directly behind them (upwind). The strong winds flowing down and out of these storms collide with the relatively warmer and wetter air just beyond the storm's boundary, and give that warm/wet air an extra 'kick' upward, sometimes combined with the effects from the local geology (hills, escarpments, etc), producing a new thunderstorm cell. If there's enough heat and humidity and instability, this can continue again and again, with each new storm popping up over the same spot, moving along the same path as its 'parent', and kicking off another storm in its wake, so that all the storm cells become lined up like the cars in a train.

Technically an image of the life cycle of a thunderstorm over time, if taken as a snapshot of a particular moment in time, this also gives an observer's view of how these storm trains develop. Credit: NWS

We saw this happen a few weeks ago, as a train of storms lined up across Burlington, as shown in this Environment Canada radar image from 5:20 pm EDT, August 4, which gives us a view of the train from above.

In this case, each storm popped up over Waterdown, headed off to the east, over Burlington, producing another storm in its wake, and this happened one after the other.

All of that rain falling over such a small area typically results in flash flooding, as rivers, streams, storm sewers and even the streets themselves struggle to deal with that much water inundating the area at once. This was exactly what we witnessed in Burlington on August 4, as the training storms dumped between 100 and 150 mm of rain over the city in just a few hours time. 

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