One day I want to do an in-depth post on gradually changing attitudes among climate scientists re attributing individual events to global warming. This is not that post. Instead I've just ripped some text from Dr. Jeff Masters at Wunderground that talks about how unusual patterns in the jet stream brought Alberta its great flood:
...June 18 - 22, when a ridge of high pressure over Alaska broke all-time heat records in the state, with unofficial readings as high as 98°F. A low pressure system became trapped over Alberta, Canada, bringing the city of Calgary a $3 billion flood disaster. This was the most expensive flood in Canadian history, and third most expensive natural disaster of any kind for the country. The only more expensive disasters were a 1989 wildfire ($4.2 billion) and a 1977 drought ($3 billion.)
When the jet stream goes into one of these extreme configurations, it freezes in its tracks for weeks, resulting in an extended period of extreme heat or flooding, depending upon where the high-amplitude part of the jet stream lies. The scientists found that because human-caused global warming is causing the Arctic to heat up more than twice as rapidly as the rest of the planet, a unique resonance pattern capable of causing this behavior was resulting. According to an email I received from German climate scientist Stefan Rahmstorf, one of the co-authors of the study, unusually extreme jet stream amplitudes likely played a role in the May - June Central European flooding event.
So there you have it. For now.