EXPLAINER

Mix of 'rare events' led to rapid rise of floodwaters in Montreal's West Island, prof says

The flooding that has forced the evacuation of a number of neighbourhoods in communities in Montreal's West Island and around southern Quebec is the result of a "rare" combination of factors, says McGill University professor Frédéric Fabry.

With snowpack mostly melted in southern Quebec, attention turns to impact of rain in days to come

A 'unusual' combination of rapid snow melt and heavy rains has produced the kind of flooding we're now seeing in areas like Pierrefonds in Montreal's West Island. (Sudha Krishnan/CBC)

The flooding that has forced the evacuation of a number of neighbourhoods in communities in Montreal's West Island and other parts of southern Quebec is the result of a "rare" combination of factors, says McGill University professor Frédéric Fabry.

Warm temperatures in April resulted in a "fairly sudden" melting of the snowpack that was further accelerated by heavy rain in parts of southern Quebec, says Fabry, who researches precipitation physics with McGill's department of atmospheric and oceanic sciences.

That combination of melted snow and rain filled rivers descending from the Laurentians and is now reaching the Montreal area.

"That water is finally beginning to arrive near the St. Lawrence River, following the major tributaries of the St. Lawrence River — the Ottawa River, the Saint-Maurice River and all these others," Fabry told CBC's Daybreak Thursday.

'Conjunction of rare events'

Fabry said the combination of rapid snow melt and heavy rains was unusual, and so is the kind of flooding we're now seeing as a result.

"As a result of this conjunction of rare events, we have an exceptional flooding," he said.

David Phillips, a senior climatologist with Environment Canada, said another factor adding to the problem is the repeated occurrence of storm systems over the last five weeks that didn't just come and go, but stuck around for days.

"They haven't been hit and run," he said. "They've had more time to linger and spread more of their misery. Instead of one day of rain, we're seeing maybe two or three days of rain from the same system."

April had 19 rainy days in total, Phillips said, and many of those days saw heavy amounts of rainfall.

"It was not just a drizzle, an inconvenience — it was a gully washer, like half a month worth of rain in one day," he said.

Good news — and more worry

The good news is the snowpack in southern Quebec is now mostly melted and its influence on water levels is waning, Fabry said.

The question now is what impact the rain forecasted for this weekend will have in the flooded areas, and that has Fabry worried.

Environment Canada is forecasting another system that will linger through Friday, Saturday and Sunday, which could bring more than 30 millimetres of rain to parts of southern Quebec north of the St. Lawrence River.

"The soil is very wet and that means the rainfall will very quickly get into the rivers, and the result, I expect, is that it might actually get worse," Fabry said, referring to the Ottawa River specifically.

Streets bordering the area around the Rivière des Prairies in Pierrefonds were covered by several inches of water. (Simon-Marc Charron/Radio-Canada)

That could mean more trouble for Rigaud, west of Montreal, which borders the Ottawa River and is already experiencing flooding, as well as for Montreal's West Island and Laval.

"Rivière des Prairies and Rivière des Milles-Îles may also start to get affected by it," Fabry said.

Quebec's Public Security Ministry is monitoring the flooding situation across the province. 

Spokesperson Thomas Blanchet said the impact of the rain this weekend on water levels may be limited by the fact it will be spread out over three days.

Blanchet said authorities are keeping a close watch on several large bodies of water, especially those in Laval, Outaouais, Laurentians, Lanaudière, and Mauricie regions that have already overflowed their banks.

With files from CBC's Daybreak