Violent thunderstorms, strong winds, significant material damage: the storm that has hit Quebec in recent days has a name. This is a derecho, a “rather rare” weather phenomenon that has not occurred since 1999 in the province.
Posted at 6:00 a.m.
“Sailors call it a squall line. Essentially, a derecho is a very fast-moving violent thunderstorm line that can travel up to 1,000 kilometers in a matter of hours. In Quebec, the last time we saw that was in 1999. And it was more from west to east,” explains Environment Canada meteorologist Samir Al-Alwani.
Little known to the general public, a “derecho” is formed mainly when hot and cold air masses meet. “It’s a bit like having an imaginary front, and on each side, there is hot and humid air, then cool and dry. In good Quebec, it’s a clash of air, ”sums up the expert, who specifies that other elements, such as strong winds at altitude, also favor the formation of this phenomenon.
“Basically, it’s very cold air coming in from the north, with very warm and humid air coming in from the Gulf of Mexico. When the two meet, it makes convection. It’s a bit like a casserole on the stove: it’s bubbling hard, “illustrates Jean-Pierre Blanchet, professor in the department of Earth and atmospheric sciences at the University of Quebec in Montreal (UQAM) .
In the coming years, with climate change, it seems certain that this kind of overflow will occur more often, say the two specialists.
It’s sure that [la fréquence de ces évènements] will increase, because the amounts of colder air and warmer air increase. People do not realize that when we talk about climate change, it is not the warming that counts, but above all the variability, therefore the amplitude between the lows and the highs.
Jean-Pierre Blanchet, professor in the Department of Earth and Atmospheric Sciences at UQAM
“Climate change has a role, for sure, that could play in increasing 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 had occurred in Quebec dates back to 1999, its incidence is much less rare in Ontario, where we see about “one per year”, specifies the meteorologist.
Why was Montreal spared?
If, in several regions, electricity failed and the winds were destructive, the island of Montreal, it felt almost nothing, apart from a little rain and winds a little stronger than at the normal. It is that the storm has “separated on Montreal, as we often see, moreover”, explains Mr. Blanchet.
“In fact, the Montreal region is somewhat protected by the divergence that there is in the low levels. This means concretely that the presence of rivers means that when the air current comes from the southwest, the air leaves Montreal. Ultimately, therefore, when the systems approach Montreal, they will often weaken,” he explains.
Saturday and Sunday, then, is precisely what happened to the derecho storm line. “She broke up. One branch went to the Laurentians, and the other to Montérégie. That’s why in Montreal, we hardly felt anything,” continues the teacher and researcher.
The regions most affected by the blackouts, as of May 23, were indeed the Laurentians, Lanaudière and Outaouais, with each still tens of thousands of Quebecers without electricity at the time of writing these lines. However, the Sûreté du Québec has not reported any major event related to the breakdowns.
“Destructive” gusts
Saturday, late afternoon, strong winds were recorded in several places in Quebec, in particular on Lake Memphremagog with gusts of up to 151 km / h, in Trois-Rivières with peaks at 96 km / h and in Gatineau up to 90 km/h. In Ontario, peaks of 132 km/h were recorded in the Waterloo region.
Figures that are not surprising for Samir Al-Alwani. “The number one characteristic of a derecho is that the wind that allows it to move so quickly blows it straight down to the ground. So it creates really destructive gusts, as we have seen in several sectors, ”he said.
“What you have to do to fight climate change is to stop driving your car. It’s that simple. And it must be done not today, nor yesterday, but the day before yesterday! “, finally slips Professor Blanchet. “If we stopped today, the temperature would continue to rise for another ten years. At some point, you will have to think of another way of living,” he concludes.