Severe thunderstorms | The provincial sacrifice of a… derecho

Heavy thunderstorms, strong winds, significant property damage: The storm that has hit Quebec in the past few days has a name. This is a derecho, a “rather rare” weather phenomenon that has not occurred in the province since 1999.

Posted at 6:00 am

Henri Ouellette-Vezina

Henri Ouellette-Vezina
The press

“Sailors call it a thunderstorm line. Essentially, a derecho is a very fast-moving violent line of thunderstorms that can travel up to 1,000 kilometers in a few hours. We last saw this in Quebec in 1999. And it came from west to east,” explains Environment Canada meteorologist Samir Al-Alwani.

Little known to the general public, a “derecho” mainly occurs when hot and cold air masses meet. “It’s a bit like having an imaginary front, and on each side there’s hot and humid air, then cool and dry air. As a good Quebecer, it is a clash of air “, summarizes the expert, who indicates that other elements such as strong winds at altitude also favor the emergence of this phenomenon.

“Basically, it’s very cold air coming in from the north and very warm and humid air coming in from the Gulf of Mexico. When the two meet, convection occurs. It’s a bit like a casserole on the stove: it’s simmering,” exemplifies Jean-Pierre Blanchet, professor in the Department of Earth and Atmospheric Sciences at the University of Quebec at Montreal (UQAM).

In the coming years, with climate change, it seems certain that such floods will become more frequent, say the two specialists.

That’s for sure [la fréquence de ces évènements] will increase as the amounts of colder and warmer air increase. What people don’t know is that when it comes to climate change, it’s not the warming that matters, it’s the variability, that is, the amplitude between the lows and the highs.

Jean-Pierre Blanchet, Professor in the Department of Earth and Atmospheric Sciences at UQAM

“Climate change is certainly playing a role that could help increase the frequency of these events. In any case, we can already see that there are a few more,” adds Mr. Al-Alwani. If the last time a derecho appeared in Quebec dates back to 1999, its incidence is much less common in Ontario, where we see about “one a year”, specifies the meteorologist.

Why was Montreal spared?

When power went out in several regions and the winds were destructive, almost nothing was felt on the island of Montreal, apart from some rain and slightly stronger winds than usual. It is that the storm “separated on Montreal, as we also often do.” see,” explains Mr. Blanchet.

“In fact, the Montreal region is somewhat sheltered by the divergence that exists at the low levels. Concretely, this means that the presence of rivers means that if the airflow is from the Southwest, the air is from Montreal. Ultimately, therefore, systems will often weaken as they approach Montreal,” he explains.

So Saturday and Sunday is exactly what happened with the derecho storm line. “She broke up. One branch went to the Laurentians, the other to Montérégie. That’s why we hardly felt anything in Montreal,” the teacher and researcher continues.

The regions hardest hit by the power outages on May 23 were in fact the Laurentians, Lanaudière and Outaouais, each of which had tens of thousands of Quebecers without power at the time of writing. However, the Sûreté du Québec has not reported any major event related to the mishaps.

“Destructive” gusts

Strong winds were recorded in several places in Quebec on Saturday late afternoon, notably on Lake Memphremagog with gusts of up to 151 km/h, in Trois-Rivières with peaks of 96 km/h and in Gatineau up to 90 km/h H . In Ontario, peak speeds of 132 km/h were measured in the Waterloo region.

Numbers that do not surprise Samir Al-Alwani. “The most important characteristic of a derecho is that the wind that allows it to move so quickly blows it straight to the ground. So it creates really destructive gusts, as we’ve seen in several sectors,” he said.

“What you need to do to fight climate change is stop driving. It’s that simple. And it mustn’t happen today or yesterday, but the day before yesterday! ‘ Professor Blanchet finally slipped out. “If we stopped today, the temperature would keep rising for another ten years. At some point you have to think about a different way of life,” he concludes.

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