Just weeks after Alberta experienced a record shattering rain event, a new storm began to form over Toronto, Canada. This storm brought with it a train of powerful thunderstorms that, one after the other, dumped torrential rain that would accumulate to nearly 5 inches in some areas and which would break the city’s all time record for rainfall within a 24 hour period.
As unusual as this event is, it is particularly abnormal for it to follow such a short time-span after the deluge that inundated Calgary and its surrounding regions. Human-caused climate change has created the conditions that make extreme weather of this kind increasingly more likely. Added heat energy creates instability in the atmosphere. The added heat and moisture increases the strength of the elemental forces that drive and enhance storms. And receding sea ice and land snow cover (during summer) have brought about dangerous changes to the Jet Stream that make weather patterns over a given region more likely to persist for long periods of time and that are more likely to link weather systems from north to south, creating the conditions for exceptionally strong storms.
It is in this context that record rainfall occurred over Toronto on July 8th. The set-up for this event involved the confluence of two weather systems — one from the north, and another from the south. These two streams of stormy weather and moisture came together in a flow over Toronto that was then ignited into a long strand of thunderstorms by a boiling atmospheric instability as northward streaming warm air collided with cooler Arctic air.
(Precursor to Toronto Floods, July 5. Image source: Lance-Modis)
In the above image we can see the Jet Stream, which had sagged for so long over the Eastern US and toward the Gulf of Mexico, now rushing along across southern Canada. It is indirectly visible via an accompanying band of clouds rushing across the top of this image. To the south, you can see a broad river of moisture flooding up from the Gulf of Mexico, behind the Appalachians, and up toward the Great Lakes. It is this river of moisture that dumped many inches of rain over the US Gulf Coast from July 3-6. This same river of moisture would rush northward to meet with a storm system brought along by a dip in the Jet.
(Toronto Flood, July 7th. Image source: Lance-Modis)
In this second image, we see the moisture flow from the Gulf of Mexico surging north and forming a head near the Great Lakes, just south of Toronto. From the north, a second large bank of cloud and moisture begins to move south as the Jet commences another dive toward the Eastern US. The stage is now set for the historic storm that drenched Toronto.
(Toronto Flood, July 8th. Image source: Lance-Modis)
In the last image we can now see the two flows of moisture combined into an immense cloud over the Toronto region. Note the elongated east-west orientation of the cloud flow as storm after storm sets up in a train-like pattern to drench Toronto. The meeting of two air flows — one from the Gulf of Mexico, another from the Arctic — combined to make this storm so powerful. And an anomalous orientation of the Jet Stream enabled such a storm to form.
In the end, the Toronto storms shut down the power grid to over 300,000 people, turned roads into rivers for a period of up to ten hours, flooded out rail lines and crippled the largest city in Canada. It also left many neighborhoods there looking like this:
(Image source: CTV)
All the result of a 100 year rain event following just weeks after a 100+ year rain event striking the Alberta region of Canada. The Alberta flood was the third most costly weather disaster in Canadian history at $3 billion dollars. Though not likely to rival the cost of the 2013 Calgary floods, yesterday’s Toronto flooding adds substantial damage to an already battered country.