September’s warmth in Montreal, Que., may have been unexpected in some ways, but it’s also a clear signal of a growing trend in the province over the past three decades.
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A phenomenon is disrupting autumn in Quebec. Read on below to find out why.
Heavy trend
The warming trend in autumn in Quebec is confirmed by closely observing certain data.
By comparing the average temperature in Montreal between two 30-year periods, we observe a very clear increase. For the metropolis, September stands out with the second-highest increase after December. If this trend is confirmed, it has accelerated in recent years.
Heat surges
We can do the same exercise of comparison with heat waves with the same periods. During much of the fall, encompassing time from Sept. 1 to Nov. 30, they increase in number. That data was compiled for Montreal.
“We are seeing an increase in the frequency of heat waves in the fall,” says MeteoMedia meteorologist Patrick Duplessis. “It is defined by a sequence of at least three consecutive days with temperatures at least five or seven degrees above normal. Regardless of the threshold used, the sequences are more numerous in recent years than in previous decades.”
The trend is accelerating
Since heat waves have been longer and more frequent over the past 30 years, this trend appears to be accelerating. Here, our expert noted a piece of evidence for Montreal: Almost all of the long heat waves have been recorded since 2017. It was in 2017 that two of them occurred, including a heat wave at the end of September.
“Autumn heat waves are not only more frequent, but have tended, especially since 2017, to be particularly long,” says Duplessis. “Of the six longest sequences since 1941, five of them have occurred in the last seven years. This may be a new trend that is taking hold.”
Gentle warmth
The 20-degree threshold is an indicator of the heat that dominates in the fall. The same trend is observed in the number of days with maximums that exceed this threshold. It is not only in Montreal, but throughout the province that this number has increased during the period from 1991 to 2020, when compared to the previous three decades.
“The budding of summer heat in the fall is not just a Montreal reality,” explains Duplessis. “We are seeing an increase in the number of days with maximum temperatures above 20 C after Sept. 1 throughout Quebec and the Maritimes. The main culprit is the month of September, which has warmed up considerably in recent decades.”
WATCH: Montreal spends 26 days of September above 20 C
Text of the original story was translated to English by The Weather Network.
With files from Patrick Duplessis, a meteorologist at MeteoMedia.
