Thursday, August 24, 2017

What caused all the damage in N.D.G.?

Power poles were snapped in half in the Montreal borough of N.D.G. after severe thunderstorms on Tuesday, August 22. (Hydro Quebec Photo)

The strong thunderstorm that rolled across the island of Montreal late in the afternoon of Tuesday, August 22nd, downed hundreds of trees and snapped hydro poles in half like toothpicks. Most of the damage was centered around the Montreal borough of N.D.G. Thousands were left without power, with widespread damage to cars and buildings. As mentioned on this blog early Tuesday morning, all the ingredients were coming together for a particularly potent afternoon of thunderstorms activity. Mother Nature did not disappoint.

So what caused all the damage? According to Environment Canada, it was a microburst generated by a severe thunderstorm cell as it swept east-northeast across the city. Thunderstorms by nature are extremely complex entities, with rising and sinking air, as well as tremendous temperature variations from the upper atmosphere to ground level. The National Weather Service definition of a microburst is that cold air from within the storm descends from the middle and upper levels of the atmosphere, often from several thousand feet high. As the cold air reaches the surface of the earth, it rolls out rapidly, usually in the same direction the storm is moving. As the air rolls out, it is rapidly compressed causing the wind speed to increase dramatically. The wind can be enhanced by local geography, topography and buildings. Microbursts are small scale weather phenomena, typically affecting an area only a few square kilometres in direct proximity to the parent storm. This was quite evident in N.D.G., with most of the major damage confined to a small area.

Winds within a microburst can often reach 100km/h or more and in extreme cases, as high as 240km/h. Microbursts are much more common than tornadoes, and responsible for most of the damage generated by thunderstorms here in Canada. In the case of the N.D.G storm, winds were estimated of at least 100mk/h. When examining the damage from N.D.G., it becomes immediately apparent this was caused by a microburst and not a tornado. In a microburst, all wind flows out, with debris lying in straight lines parallel to the outward winds. In a tornado, the wind flows inward, with debris lying helter skelter at all angles.

EF-1 Tornado
In addition to the storms in the Montreal region, Environment Canada has confirmed that an EF-1 tornado, with winds up to 175km/h, touched down in Lachute at 6:10pm Tuesday, August 22. The tornado tore up trees and damaged over 350 homes. Some of the structures had their roofs torn off resulting in 40 people being evacuated.

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