Rain falls in real time,
And rain fell through the night.
No dress rehearsal,
This is our life.

The Tragically Hip

Climate change can feel like an immense and inescapable monster that impacts our lives in no ways and, at the same time, in all ways—especially for those of us in the public safety sector. In the past, flood forecasting meant watching out for “big storms” that we could see coming a few days out and tracking the amount of rain with a few rain gauges. Unfortunately, storms have become more intense and less predictable, often hitting urban areas during the summer months, with a number of recent storms calling our rainfall monitoring networks into question.

Much of the flood control infrastructure in the area, such as our dams and channels, owe their existence to Hurricane Hazel, which was a deadly hurricane that collided with a cold front and then stalled over Toronto in October 1954, resulted in about 280mm (11 inches) of rain in 48 hours and the deaths of 81 people. But the storms that are showing up in southern Ontario today are much different from Hurricane Hazel.

Damage from Hurricane Hazel to houses on the Humber River in Vaughan, Ontario. (Source: Toronto Public Library Special Collections.)

Damage from Hurricane Hazel to the Yonge Street bridge over the Don River in Toronto. (Source: Toronto Public Library Special Collections.)







The Changing Nature of Storms

The first of the major recent storms occurred in Peterborough in July 2004, when a large weather system out of Alberta worked its way to Ontario. The storm brought about 240mm (10 inches) of rain, most of which fell within less than five hours in the early morning, including 78mm (3 inches) in a single hour. When the storm ended, it had resulted in the largest single day rainfall depth ever recorded in southern Ontario, even surpassing Hurricane Hazel. For perspective, the average expected rain fall for the month of August in our area is about 79mm (3 inches), which means that Peterborough experienced an entire month worth of rain in one hour.

Flooding in Peterborough. (Source: Meteorological Service of Canada.)

Flooding in Peterborough. (Source: Meteorological Service of Canada.)










A year later, in August 2005, a line of severe thunderstorms moved across southern Ontario before heading into Toronto and York Region. The storms produced more than 130mm (5 inches) of rain in three hours, with 103mm (4 inches) of rain in one hour in North York, which exceeded the hourly record of both Hurricane Hazel and the Peterborough storm. A section of Finch Avenue collapsed over the Black Creek Crossing and there was damage to a large sanitary sewer in the Highland Creek valley, which resulted in the discharge of untreated sewage.

Nearly a decade later, in July 2013, there was another downpour that struck Mississauga and Toronto when two separate thunderstorms brought upwards of 126mm (5 inches) of rain to the areas around Pearson Airport. The majority of the rain fell within two hours in the evening, just in time for rush hour. About 500,000 households lost power and more than 3,000 reported basement flooding. For the residents that were effected by these storms, the damage to their homes was devastating—not just emotionally but financially. Hurricane Hazel is estimated to have cost, at that time, around $136 million in damages, the Peterborough storm in 2004 cost around $95 million, the Toronto and York Region storm in 2005 cost around $600 million and the Mississauga and Toronto storms in 2013 came in at $1 billion in damage, making it the most expensive storm in Ontario. In total, the major storms events in Ontario from 2005 until now have cost nearly 2 billion dollars in damage.

The Cost of Calamity

The destruction that these storms brought were not limited to water damage either. In 2009, major storms hit the Red Hill Creek system in Hamilton in July and Cooksville Creek in Mississauga in August. In Hamilton, 7,000 basements were flooded and the Red Hill Valley Expressway had to be closed due to flooding, as the area experienced more than 100 mm (4 inches) of rain in a few hours. In Mississauga, most of the damages resulted from sanitary sewer surcharging or “backing up” into basements, as more than 67 mm (2.5 inches) of rain fell in a single hour, deluging the infrastructure. In August 2012, the region experienced a “near miss” when a storm occurred over Lake Ontario, about 15 km offshore from Oakville and Burlington. The rain lasted more than six hours and had peak intensities estimated between 150 and 200 mm (6 or 7 inches) each hour. Had this storm occurred on land, instead of the open water of the lake, there would have been the potential for significant flooding in Halton, Peel or Toronto and the damage would have been disastrous.

Radar scans of the “near miss” that occurred over Lake Ontario. (Source: Risk Sciences International)

It was only a matter of time before a storm of this magnitude would hit Halton and, eventually, it did, in August 2014. The storm—or more accurately, the series of storms that thundered into the area, one after another—arrived in Burlington in the early afternoon. The rain began around 1:30, with at least four distinct downpours, each about 20 or 30 minutes apart, but the bulk of the rain occurred between 3:45 and 6:00 in the late afternoon. About 6,000 properties were flooded and there was severe damage due to flooding of Tuck Creek, Shoreacres Creek and Appleby Creek. (For a more detailed discussion of this event, click here to read our 2015 report.)

The Burlington storm was devastating and yet the Conservation Halton rain gauges had little to no record of it. Hurricane Hazel affected an area of 30,000 km² was centered over the Humber River watershed. The flooding that occurred in Burlington in August 2014 affected an area of about 200km², which is less than one percent of the affected area of Hurricane Hazel. Due to the narrow width of the storm, it missed all of our rain gauges and the rain gauges of our partner municipalities. One gauge, located at Mainway and Walkers Line, recorded around 124mm (5 inches) of precipitation, and three private gauges closer to the centre of the storms recorded between 160 and 192mm (6 and 7.5 inches). While the area that the Burlington storm impacted was small, the data concluded that it was greater than both Hurricane Hazel and the 100-Year Storm, which means that there was a less than one percent chance of the storm occurring. There is little comfort in this fact for those who suffered damages to their homes and businesses but it gives you an idea of just how extraordinary these recent storm events were.

The infographic above shows the total amount of rain from Hurricane Hazel, compared to the total amount of rain from the short-duration, high-intensity storms.

You may be wondering why many of the recent flooding events in southern Ontario have been the result of thunderstorm activity, rather than remnant hurricane impacts—after all, isn’t a hurricane more powerful than a thunderstorm? Thunderstorms work on convective uplift, which is the upward transport of energy and moisture from the ground and into the atmosphere. Once this moisture has risen far enough, it condenses and forms clouds, and if enough of it congregates together, you get massive cumulonimbus clouds—the ones that look like cauliflower in the sky—which are the harbingers of thunderstorms. However, in southern Ontario we also have the convergence of lake breezes off of the Great Lakes, which gives this process a boost. Whenever there is a difference in temperature between the lake and the surrounding land, known as a temperature gradient, this will cause an airflow pattern from the lake onto the land. Due to southern Ontario being a peninsula between three of the Great Lakes, there will be an area where these breezes converge and, when they do, they encourage convective uplift. If there is any uplift already happening, usually in the summertime, due to warm temperatures and moisture in the air, this process ramps up and increases the chances of a thunderstorm occurring. This zone of convergence also results in southern Ontario being the “tornado alley” of Canada.

The Need for Network Density

When it was first developed, the monitoring network was intended to predict large, slow storms, such as hurricanes, which were the main flooding threats at the time. The rain gauge monitoring network that Conservation Halton depended on was composed of six gauges scattered throughout our jurisdiction—at the Kelso, Scotch Block and Mountsberg Reservoirs and in Millgrove, Aldershot and southern Oakville. In a jurisdiction of about 970 km2, that amounts to one gauge for every 162 km², which means that the storms we are getting today can pass between the rain gauges. Also, all of these gauges were unheated, so they could not accurately capture freezing rain and snow, which complicates our efforts to monitor our changing winter climate.

This map shows the intensities of the storm in Peterborough, the two storms that occurred in the Greater Toronto Area, the “near miss” that occurred over Lake Ontario and the storm in Burlington as well as storms in Elmira and Stoney Creek. (Source: Credit Valley Conservation)

Our rain gauge network was unable to capture the Burlington storm due to the gauges being so spread out but, since then, Conservation Halton has made significant improvements. We have installed brand new rain gauges at Mainway Arena, the Burlington Executive Airport, Burlington Fire Station 1 and the Conservation Halton Administration Office. We have upgraded our existing rain gauges with gauge heater kits so they will be able to track freezing rain and snow. We have also integrated 13 rain gauges operated by the Region of Halton into our monitoring network. In just three years, we have increased our coverage from 6 rain gauges to 25 gauges, with more to be added in the coming years.


New rain gauge at Mainway Arena.

New rain gauge at Burlington Fire Station 1.

New rain gauge at Kelso Dam.


You may be wondering, “Why don’t they just use weather radar?” In short, we do use radar but it isn’t enough on it’s own. Radar gives us a picture of what’s happening up in the sky but what is happening at that altitude often isn’t the same as what eventually makes its way down to the ground. This is why we need rain gauges on the ground to make sure the radar is accurate. Weather radar is an important component of our Flood Forecasting program and we depend on both Environment Canada and the National Weather Service (USA) to track conditions in our jurisdiction but the real meteorological magic happens when you combine weather radar with a network of rain gauges.

You also may be wondering why we don’t depend on Environment Canada’s rain gauges. Environment Canada has been reducing the number of tipping bucket rain gauges in their monitoring network since the late 1980s, so we can’t rely on those stations to fully capture these localized events. (Click here for a graph showing the number of tipping bucket rain gauges in the Environment Canada network from 1953 to 2012.) In fact, the only Environment Canada tipping bucket in our jurisdiction is at Royal Botanical Gardens in Hamilton, so it too would have missed the August 2014 event.

To return to the opening quote, these high-intensity, short-duration storms show us that rain does fall in real-time, which is why we need real-time access to gauges, sometimes through the night, like the storm in Peterborough, and that when a storm does occur, it is definitely not a dress rehearsal. So this is our life here in Halton—watching the forecasts for remnant hurricanes that may be coming our way and having a modern rain gauge network that alerts emergency responders when a storm is capable of causing flooding. With climate change, the chance of these high-intensity, short-duration, storms will only increase and it is our responsibility to predict potential storms, inform emergency responders and support development of  resilient communities.

We would like to thank David Sills from Environment and Climate Change Canada, Simon Eng from Risk Sciences International and Credit Valley Conservation for providing some of the materials consulted in this article.

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Last modified: June 28, 2018

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