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 other parts of southern Quebec is the result of a"rare" combination offactors, says McGill University professor Frdric Fabry.

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

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

• LIVEBLOG:The latest on Thursday's flooding situation in Quebec
• TODAY:Quebec municipalities readying for more flooding as rain approaches

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

'Conjunction of rare events'

Fabry said the combination of rapid snow melt and heavy rains wasunusual,and so isthe 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 problemis the repeated occurrence ofstorm systemsover 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 ofone 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 moreworry

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

• Laval warns residents to prepare as more rain expected to cause more flooding

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," Fabrysaid, referring to the Ottawa River specifically.

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

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

• EXCLUSIVE:Pierrefonds-Roxboro workers were ordered to destroy sandbags days before flooding

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

SpokespersonThomasBlanchetsaidthe impact of the rain this weekend on water levels may be limited by the factit will be spread out over three days.

Blanchetsaid authorities are keeping a close watch on several large bodies of water, especially those inLaval,Outaouais, Laurentians,Lanaudire, andMauricieregions that have already overflowedtheir banks.

• IN-DEPTH:Water levels in Quebec are breaking all-time records