(Montreal) The onslaught of rain, violent thunderstorms and even tornadoes that occurred on Thursday is not a rare phenomenon in itself, but the extent of the territory covered by the enormous storm cell is surprising.
The cell became prone to tornado formation south of Ottawa, which did experience one, and tornado warnings then moved to Sainte-Anne-de-la-Pérade, mid- road between Trois-Rivières and Quebec.
In other words, the atmospheric steamroller moved a distance of about 450 kilometers.
“Very infrequent”
“It is indeed a very large area. It is very rare that we have such a large storm cover that is so well organized, ”recognizes meteorologist Simon Legault, from Environment Canada.
“It was the same stormy mass that maintained itself, that maintained itself, that perpetuated the risks through the radar. We could see the rotation in the cloud, but you never know if it hits the ground from the radar. »
A cocktail with precise ingredients
It takes a meeting of several meteorological phenomena to obtain this kind of cell, explains his colleague Antoine Petit, also a meteorologist at the federal agency.
“The main ingredient is wind shear. It is a wind that will change direction and strength at different altitude levels.
“Sometimes the intensity is too much, sometimes it’s not enough. Sometimes the change in direction is not enough, sometimes it is too abrupt”, he explains. But when the shear conditions are optimal, this is when what is called a supercell can form.
But that’s not all, says Mr. Petit. “You also need a trigger and yesterday (Thursday) we had it: it was the cold front that swept everyone and arrived from the American Midwest, which crossed the Great Lakes yesterday afternoon and which was quite strong. »
The table was set
The ground had been particularly well prepared by a light rain in the morning and then the sun came out which destabilized the air mass by warming it. With optimal shear and the entry of the cold front, everything was in place for a perfect storm.
One might be surprised that all the municipalities were taken by surprise, but it is difficult to prepare, argues Simon Legault. “We can’t predict that long in advance, but we know at the very least that the month of July is conducive to having heat and humidity and the risk of thunderstorms. »
Two tornadoes confirmed
At least two tornadoes have been confirmed so far, one in Ottawa and one in Mirabel. In the Vaudreuil and Sorel-Tracy sectors, “we are talking about funnel clouds for the moment. These are therefore embryos of what could have become tornadoes, ”says Simon Legault. Experts will determine if they have touched the ground, a mandatory criterion to confirm a tornado.
However, experts from the Northern Tornadoes project are still in Ottawa to analyze the damage and determine the strength of this first tornado. “Then they will come to Quebec. So it may take a few more days, maybe not before next week, ”says Mr. Legault.
Contrary to what had been advanced earlier in the day, this is not the first time that Montreal is under the influence of a tornado alert, affirms Simon Legault. “According to our statistics, this is the fifth time that a tornado warning has been issued for Montreal. The last time was May 25, 2012. Before that, it was in 2009 and 1999. It’s very rare and I don’t think a tornado touched down during this period. »
Powerful winds and torrential rains
In the absence of a tornado, the entire affected corridor received a lot of rain and strong winds.
PHOTO JOSIE DESMARAIS, THE PRESS
A few centimeters of rain accumulated on rue De Rouen, in Montreal.
At least 50 millimeters of rain were recorded almost everywhere, levels that reached and exceeded 80 millimeters in Montreal and Nicolet, in particular. “When we talk about receiving 80 mm in two hours, in Montreal, the average is 90 mm for the month of July,” says Simon Legault. And even if the average is closer to 100 mm in Quebec, we can say that it is almost the equivalent of a month of rain in a short time, in a few hours. There’s a reason the systems can’t handle so much rain. »