(Montreal) The torrential rains, violent thunderstorms and even tornadoes that struck Thursday are not in themselves an uncommon phenomenon, but the extent of the area covered by the giant storm cell is surprising.
Posted yesterday at 5:55pm.
Pierre Saint-Arnaud The Canadian Press
The cell became prone to tornado formation south of Ottawa, where a tornado did occur, and the tornado warnings then shifted to Sainte-Anne-de-la-Pérade, on the middle road between Trois-Rivières and Quebec.
In other words, the atmospheric steamroller covered a distance of about 450 kilometers.
“Very rare”
“It is actually a very large area. “It’s very rare that we have such a large storm coverage that is so well organized,” acknowledges Environment Canada meteorologist Simon Legault.
“It was the same tumultuous mass that sustained itself that sustained the risks through radar. We could see the rotation in the cloud, but from radar you never know if it hits the ground. »
A cocktail with precise ingredients
Obtaining such a cell requires a combination of several meteorological phenomena, explains his colleague Antoine Petit, also a meteorologist at the federal agency.
“The main ingredient is wind shear. It is a wind that changes direction and strength at different altitudes.
“Sometimes the intensity is too great, sometimes it’s not enough. Sometimes the change of direction isn’t enough, sometimes it’s too abrupt,” he explains. However, when the shear conditions are optimal, a so-called 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 hit everyone and came in from the US Midwest that crossed the Great Lakes yesterday afternoon and was quite strong. »
The table was set
The ground had been particularly well prepared by a light morning rain and then the sun came out which destabilized the air mass by heating it. With optimal shear and the arrival of the cold front, all the conditions for a perfect storm were in place.
It may come as a surprise that all municipalities were caught off guard, but it is difficult to prepare, argues Simon Legault. “We can’t predict that far in advance, but at least we know that the month of July is favorable for 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, “at the moment we are talking about funnel clouds. So these are embryos of what could have become tornadoes,” says Simon Legault. Experts will determine if they have touched the ground, a mandatory criterion for confirming 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 be a few more days, maybe not until next week,” says Mr Legault.
Contrary to what was announced earlier in the day, this isn’t the first time Montreal has been under the influence of a tornado warning, confirms Simon Legault. “According to our statistics, this is the fifth time a tornado warning has been issued for Montreal. The last time was May 25, 2012. Before that it was 2009 and 1999. It’s very rare and I don’t think a tornado landed during that time. »
Strong winds and torrential rains
Since there was no tornado, there was a lot of rain and strong winds throughout the affected corridor.
PHOTO JOSIE DESMARAIS, THE PRESS
A few inches of rain collected on Rue De Rouen in Montreal.
At least 50 millimeters of rain were recorded almost everywhere, with values reaching and exceeding 80 millimeters in Montreal and Nicolet in particular. “If we’re talking about getting 80mm in two hours, the average in Montreal in July is 90mm,” says Simon Legault. And even if the average in Quebec is closer to 100mm, we can say that in a short time, in a few hours, it is equivalent to almost a month of rain. There’s a reason the systems can’t handle so much rain. »