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The Toronto Star notes Toronto receives, on average, 72 millimetres of rainfall every May. This past May, Environment Canada figures show, saw 142.6 mm of rain soak Toronto.

Jet streams, which carry air and moisture from west to east like a conveyor belt, “are driven by temperature differentials,” explains Steve Brown, Ontario president of the Canadian Water Resources Association. That means if the cold part becomes warmer, moisture moves through the jet stream more slowly.

It explains why, across the province, we’ve seen “well above average,” precipitation. Because the stream has been “stuck,” with low pressure systems for about two months, moist air has been lingering over eastern North America, leading to more persistent rainfall, said University of Toronto geography professor Joseph Dislodges.

Because the Arctic has been heating up incredibly quickly, Pomeroy said, jet stream pressure has dropped. That’s slowed average wind speeds and, he said, resulted in storms lingering longer.

“That’s exactly why the moisture hung around and filled all the lakes and all the storages and anywhere it could possibly flow,” he said.

With record high lake temperatures pumping more moisture into an atmospheric cycle, slowed by low pressure jet streams, Pomeroy expects another massive rainfall is to come.

With lakes already full, Pomeroy said, “it doesn’t take much to cause a flood.”

Lake Ontario is especially vulnerable because it’s the smallest of the Great Lakes and the rest all feed into it, Brown said.

Pomeroy said with about 40 per cent of Toronto Island under water, it would make more sense to pay residents and others, especially those in the country’s various flood plains, to move out rather than to rush in with relief when disaster strikes. This is important because, Brown said, the disasters are getting more frequent and more severe.

“The extreme weather we’ve seen is the new normal. More is on its way,” said Natalia Moudrak, director of the Intact Centre at the University of Waterloo.