Unbelievably, another Canadian city has been struck by historic flooding, the second in just weeks.
The heart of Toronto was hit on Monday when record rain fell on saturated soil.
Calgary sustained massive flooding June 19-21 when heavy rains compounded problems from snowmelt.
The two cities are more than 2,000 miles apart, with Calgary sitting at the base of the Rocky Mountains and Toronto nestled among the Great Lakes, across the border from Niagara Falls.
But an underlying weather pattern contributed to the flooding of both cities, while their differing geography also played a role, said AccuWeather Inc. senior meteorologist Jack Boston. AccuWeather is The World-Herald's weather consultant.
Boston said an unusually active jet stream is the link between the two Canadian cities, as it is in the flooding that has plagued the Upper Midwest, including the Chicago area.
Here's why the jet stream matters and why it's behaving unusually this year, based on an explanation from Boston:
The jet stream is a river of fast-moving air in the upper atmosphere that divides cold air to the north from warm air to the south. It draws much of its strength from the contrast in temperatures between the two masses of air.
The connection to flooding is that the jet stream is the route most storms hitch a ride along.
By this time of year, the jet stream typically has lost some of its strength. That's because the contrast in temperatures between north and south has lessened with the arrival of summer's warmth.
This year, cooler than normal weather lingered later in the year than normal, so the jet stream has kept its strength well into July.
The Toronto area has received seven inches of rain in four days, Boston said, including its highest 24-hour rainfall on record: 4.96 inches. Records date to 1937.
Most of that rain fell during the Monday evening commute, compounding the dangers from flash-flooding.
The rain inundated streets, transit lines and the lower levels of buildings.
About 300,000 residents lost power, many cars were flooded and about 1,400 passengers were stranded on a commuter train. Rescuers had only two boats to ferry those train passengers to safety, so the rescue took several hours.
Boston said local terrain played a role in both cities' problems.
Toronto's rain was intensified by the atmospheric effects of Lake Ontario, while Calgary's rains were exacerbated by the effects of the Rocky Mountains.
Here's how Lake Ontario affected Toronto: Because the lake is cooler than the land, a breeze blows from the lake onto land. This cooler breeze collides with warmer, humid air over land and the convergence of the two triggers storms.
Here's how the Rocky Mountains affect Calgary and similarly situated cities such as Denver: Winds blowing into western Canada from the southeast bring with them relatively high humidity. As these winds reach the Rocky Mountains, they travel up the slopes, and this lift helps trigger storms.
Here's a story on Canada.com, a major media outlet, that contrasts the two storms:
This report includes material from the Associated Press and Bloomberg News and Canada.com