Toronto Area Extreme Rainfall Trends - Comparing Engineering Climate Datasets with Future Weather & Climate Study Predicted Trends

Environment and Climate Change Canada's Engineering Climate Datasets summarize observed annual maximum rainfall over various durations from 5 minutes to 24 hours.  Theses series are used to derive IDF tables and charts that describe the intensity, duration and frequency (i.e., return period) of extreme rainfall.  IDF tables are used to support engineering design of storm drainage and wastewater systems, and are used to define rainfall patterns used in hydrologic modelling.

The City of Toronto commissioned Toronto's Future Weather & Climate Driver Study - the 2012 results indicate projected changes in extreme rainfall for a few durations and return periods.  Results of the Outcomes Report are here  The baseline period for the study is 2000-2009 and statistics are predicted out to 2040-2049.

The Engineering Climate Datasets have been updated in early 2019, including for two Toronto-area climate stations with long records called "Toronto City" and "Toronto International Airport".  The following tables compares the predicted increase in extreme rainfall in the 2012 study with trends in the same statistics from 1990 to 2017 at these two Toronto-area stations.

A key take away is that the Future Weather & Climate Driver Study does not agree with the direction and magnitude of changes in the actual statistics, which are based on real observations (not modelling predictions).  Some actual statistics have been decreasing since 1990, not increasing as predicted int eh study.  When a statistic is increasing, it is at a significantly lower rate that what is predicted in the study.

The following chart compares the past 100 year daily data to the study predictions - the Toronto study seems to have a hockey stick shape, jumping significantly upward by the 2040's which does not match the past trends.

The next chart shows changes in 10 year hourly rainfall. The Toronto study significantly understates the value today, suggesting it will double by the 2040's - the predicted future value has already been in place since the 1990's however.

It is questionable whether the City of Toronto should consider any changes to design criteria for municipal infrastructure considering these future predictions - best to follow ASCE's approach and incorporate flexibility in future design and wait and see with the 'observational method'? - if observations show that there is no change in the statistics, there should be no significant driver in changing design criteria, especially based on models that do not match the magnitude or trend in actual extreme rainfall statistics.

John Robson of Climate Discussion Nexus Share Causes of Urban Flooding - Highlights CBC Ombudsman Findings on 100-Year Storms

This blog has examined causes of urban flooding and non-causes of flooding too. A new video by the Climate Discussion Nexus examines causes of urban flooding and references the recent CBC Ombudsman review of 100-year storm trends (i.e., no changes in extreme rainfall).

The video notes that we have always had flooding which is correct - that is something we have noted as well, like in this presentation to the WEAO / OWWA joint climate change committee:

The presentation noted how GO Train flooding, Toronto Island flooding and Toronto basement flooding area note new phenomena as shown in these images:

A review of the inquiry for Premier Davis on Toronto Don River flooding noted flooding since the 1800's as noted in this post: - an Environmental Assessment in 1983 noted this:

This Acres Consulting report noted the influence of hydrologic changes on peak flows and flood damages. We have described these changes as well such as in this JWMM paper:

GTA watershed urbanization changes were summarized as follows:

Here is a wider perspective:
And here are urbanization trends in other southern Ontario municipalities:

We have assessed the trends in extreme rainfall which supported the CBC Ombudsman decision - here are the english-version findings:

Even Minister McKenna has reinforced comments made in Canada's Changing Climate Report stating in a June 2019 letter "the observational record has not yet shown evidence of consistent changes in short-duration precipitation extremes across the country":

Other CBC story corrections about extreme rainfall are summarized here:

As the Climate Discussion Nexus video notes, the insurance industry has claimed that there is an increase in extreme rainfall caused by climate change - this has been reiterated by senior executives as in this op-ed in the Globe and Mail by Charles Brindamour and Dean Connor "Climate resilience must be part of every government’s agenda" (The Globe and Mail. September 25, 2018., or as in this op-ed in the Financial Post by Craig Stewart "Counterpoint: Insurance claim costs are rising because severe weather is making flooding worse" (Financial Post. February 7, 2019.

Unfortunately, the insurance industry has not ever offered any data on increasing extreme weather trends to counter Minister McKenna's recent statement or the CBC Ombudsman findings since 2015. The insurance industry has in the past mixed up future predicted extreme rain trends with past observations as in the "Telling the Weather Story" report:

As noted in our Financial Post op-ed, the insurance industry has claimed a correlation and causation between extreme weather and flood damages. Unfortunately, there is no rain trend to correlate to making any causation discussion moot. As Dr. Dickinson explains in the video, warmer winters mean lower spring flood potential, and urbanization drives urban flood stresses, not changes in rainfall. More on this University of Guelph analysis is here:

Our rebuttal to the insurance industry's suggested correlation / causation was in the Financial Post:

Well done John Robson and Climate Discussion Nexus for sharing information on this topic.


More reading? - what do engineering studies in southern Ontario say about extreme rainfall trends? typically no past change - see compiled reports / analysis here:

Are More 100 Year Storm Happening? Yes and No. A Proliferation of Rain Gauges Can Now Record More 100 Year Storms, But Fixed Locations Show No Increase

There are many sensational media stories about ghost storms and ninja storms hitting urban areas, and a steady claim that we are experiencing more extreme rainfall, that is, higher intensities for a given probability (called return period), or greater frequency of given design intensities. Often it is stated that we are experiencing more 100 year storms today and that is a "new normal" brought on by a changing climate.

How does the number of climate stations, or rain gauges, that are in operation affect the number of observed extreme events. Well, let's look at Toronto for example.  Several past extreme events were reported in the Staff Report on Impact of July 8, 2013 storm on the City's Sewer and Stormwater Systems dated September 6, 2016: (

During the May 12, 2000 extreme rainfall event, Toronto operated 16 rain gauges as shown on the staff report map below.

Fifteen years later, during the August 19, 2005 storm, the City operated 31 rain gauges as shown below, so almost double the number of rain gauges.  Look at the higher density of gauges in north Toronto where many higher August 19, 2005 rainfall depths were observed.

Then 8 years later, during the July 8, 2013 storm the city operated even more rain gauges, i.e., 35 in total.

And then a few years later, on August 7, 2018, the city operated 43 rain gauges - even more than 2013. I don't have a map but here is a super-cool graph summarizing Toronto Open Data rainfall totals at those gauges over a period of 5 minutes to 24 hours.

And now today as of July 17, 2019, Toronto has 45 active rain gauges as shown in the following map presented to the Ministry of Environment Conservation and Parks' stormwater stakeholder group participating in development of minimum standards for ECA pre-approval.

So let us summarize the trend in the number of rain gauges in the chart below.

Astute blog readers will notice that the number of rain gauges has increased almost 300% since the year 2000. Yes, almost three times the number of rain gauges now. Obviously, more extreme events can be observed and recorded when the number of rain gauges increases dramatically.

The following table shows that in the year 2000, there was a rain gauge every 39.4 square kilometres (16 gauges per 630.2 square kilometres). By 2019, there is a gauge ever 14 square kilometres.


So what is happening at fixed locations where rain intensities are measured? In Toronto and Mississauga, many trends are downward according to the Engineering Climate Datasets:


As a result, design intensities for short durations have been decreasing since 1990:


To recap, many more rain gauges today mean we 'see' more storms - these are typically needed to support basement flooding Municipal Class EA studies (rainfall needed to calibrate hydrologic and hydraulic simulation models), to guide operational activities too.  Many municipalities have installed rain gauges to support inflow and infiltration management programs.

We have a "finer mesh net" to catch these events and add them to our records - we have almost 3 times more rain gauges in Toronto since 2000.

But no. Storm are not becoming more intense. If we see more of them, it is because we are looking harder for them with more extensive monitoring efforts. Given this expanding intensive network of rain gauges today, it is not uncommon, statistically speaking, to observe many 100-year storms over a short time period.  This earlier post explores those statistics in the GTA -

CBC correcting claims on extreme weather trends since 2015 - more should follow their lead and more consistency is needed in CBC reporting

The CBC has done a great job in correcting its reporting of extreme weather frequency claims over the years. Other media organizations like TVO and major newspapers have been given the same feedback on inaccurate reporting as CBC but have not moved to correct exaggerated claims. But while CBC has made may corrections, both voluntarily by its journalists and through the coaxing of its Radio-Canada Ombudsman, it has a tendency to repeat past inaccuracies without benefiting from what it has learned - so there is an opportunity to have the CBC be more consistent in its reporting, even adhering better to its own standards.

What are some examples of CBC's past corrections on extreme weather trends. Here are those that I helped move along.

1) November 2015

The CBC has corrected articles on this topic in the past as well dating back to 2015, confirming that there have been no changes in extreme rainfall. This correction was in response to a statement made by the insurance industry when an insurance broker stated we are having 20 times more storms today. This blog post describes the statement ""A lot of it has to do with the frequency of the storms and I think you could even extrapolate that it's got to do with climate change," ... "we're getting 20 times more storms now than we were 20 years ago." Here is a write-up on that exchange

CBC issued a correction and wrote me a letter dated Nov. 20 2015 (see excerpt in blog post link above and at right) saying "Environment Canada verified that there has been no significant change in rainfall events over several decades". In the article (link: the correction is as follows:

"However, Environment Canada says it has recently looked at the trends in heavy rainfall events and there were "no significant changes" in the Windsor region between 1953 and 2012."

– this finding from 2015 is still valid today - a review of the updated Engineering Climate Datasets v3.0 released in March 2019 shows that across all of southern Ontario in fact observed rainfall intensities have been decreasing as engineering design “IDF” values have been decreasing as a result – this is shown for small frequency storms and large rare storms as well (see previous post on IDF trends, see previous post on decreasing annual maximum rainfall trends, see Stantec's review of Windsor Airport extreme rain trends in the December 2018 Windsor/Essex Region noted in this blog post and in the excerpt to the right - see "Conclusion: Short-term durations events are slightly trending downwards thus no evidence to increasing IDF curves for stormwater design").

2) January 2019

The recent CBC Ombudsman ruling [January 28, 2019] disputes statements made by Dr. Blair Feltmate of the Intact Centre on Climate Adaptation on the frequency of storms linked to flooding (100 Year storms).

See link to decision in English:
Link to the decision in French:

This was in response to a story by CBC's Marc Montgomery that has been corrected:
And corrections to a counterpoint story where I was interviewed by Marc Montgomery and where I had brought up concerns with the accuracy of the initial story:

CBC originally stated “We are experiencing storms of greater magnitude, more volume of rain coming down over short periods of time these days due to climate change. That is causing massive flooding.” However  the CBC Ombudsman concludes that:

"One only had to examine the official Environment Canada data for Ontario as well as for the entire country to acknowledge that the claim made in the article was inaccurate. Such acknowledgement would at the same time have addressed the complainant’s criticism regarding the lack of data to corroborate Dr. Feltmate’s claim about the increased frequency of extreme rainfall events in Canada."

The Ombudsman also found that the CBC did not meet its own standards for accuracy and impartiality stating:

"Review by the Office of the Ombudsman, French Services, CBC/Radio-Canada of two complaints asserting that the articles by journalist Marc Montgomery entitled How to mitigate the effects of flood damage from climate change and Response to a climate change story, posted on September 19 and November 19, 2018, respectively by Radio Canada international (RCI), failed to comply with the CBC/Radio-Canada Journalistic Standards and Practices regarding accuracy and impartiality."

3) April 2019

The CBC corrected this article entitled "Canada warming at twice the global rate, leaked report finds" in April 2019. The article referenced Environment and Climate Change Canada’s (ECCC’s) Canada’s Changing Climate report that reviewed extreme precipitation trends in Canada and stated:

"There do not appear to be detectable trends in short-duration extreme rainfall in Canada ..." and "For Canada as a whole, there is a lack of observational evidence of changes in daily and short-duration extreme precipitation.

The original article linked current flooding to changes in rainfall stating "Although flooding is often the result of many factors, more intense rainfall will increase urban flood risks."

I highlighted sections of the ECCC report stating lack of evidence of changes in rainfall extremes and as a result, this is the correction CBC made in response:

"Corrections, An earlier version of this story said that more intense rainfall contributes to increased urban flooding. In fact, while the report states that precipitation is higher overall, it did not find that episodes of short-duration extreme rainfall have increased or establish a connection between these and increased or exacerbated flooding. Apr 04, 2019 2:23 PM ET"

4) May 2019

In a April 11, 2019 CBC News article by Chris Arsenault entitled "“Canada's building code is getting a climate change rewrite. Is your home ready?” made the statement in the sub-headline “Increased flooding, wildfires and storms mean tough new rules take effect in 2025” which mischaracterizes trends in storms and flooding.

I shared the Ombudsman findings from January 2019, and CBC's earlier corrections on extreme rain trends. I also shared information on key causes of flooding, highlighting urbanization as a key factor per IPCC reporting, ECCC's Canada's changing climate report, local university studies and Ontario case law.

In response to this feedback CBC corrected the April 11, 2019 CBC News article per Paul Hambleton’s email to me on May 16, 2019. In response to the actual data showing no historical trends in extreme rainfall, CBC revised the sub-headline from “Increased flooding, wildfires and storms mean tough new rules take effect in 2025” to “Predicted increase in flooding, wildfires and storms means tough new rules take effect in 2025”.

This is now accurate - there have been no changes to date but there are predicted changes in the future. Canada's Minister of Environment and Climate Change Catherine McKenna has confirmed the lack of observed changes in extreme precipitation in a June 13, 2019 letter to me (see right).

In the letter she reiterates a statement made in Canada's Changing Climate Report stating: "the observational record has not yet shown evidence of consistent changes in short-duration precipitation extremes across the country" - The original report stated more simply (page 117):

"For Canada as a whole, observational evidence of changes in extreme precipitation amounts, accumulated over periods of a day or less, is lacking."

So bravo for CBC for making corrections to its reporting on extreme weather trends! No other media outlet has been receptive to making corrections based on feedback.  CBC's original corrections in 2015 are supported by new local and region data, and the recent corrections are supported by  Minister McKenna's recent statement and Environment and Climate Change Canada reporting. 


Recent CBC coverage seems to go back and repeat the same inaccurate statements that have been made in the past and that have been corrected. Especially for extreme weather trends and their effect to flooding. Perhaps CBC needs to promote what it has already found on the topic of extreme weather trends and that could help it carry forward with more accurate reporting in the future?

R. Muir

Normalized, Inflation and Growth Adjusted Losses for Hydrological Events Like Floods Show Peak Losses in 1990's - Meteorological Event Losses Peaked in 2005.

Flood losses in North America do not seem to be increasing when growth and inflation are considered. That's good news, suggesting newer development is more resilient.

Munich RE's NatCatSERVICE provides information on relevant and catastrophic losses. Insured and uninsured losses are tracked for various events including hydrological events (floods, flash floods, severe storms) and meteorological events (hurricanes, storm surges, floods). Charts showing trends in losses are available from 1980 to 2017 expressed in 2017 $USD including:
  • Nominal Overall Losses - values as they originally occurred
  • Inflation Adjusted Losses - accounting for changes in monetary equivalent
  • Normalized Losses - accounting for growth of values and assets (considering nominal gross domestic product)
Normalized Flood Losses Adjusted for GDP Growth Inflation Adjusted
Normalized Flood Losses - Relevant Hydrological Events in North America 1980-2017 per Munich RE NatCatSERVICE

Normalized Flood Losses Adjusted for GDP Growth Inflation Adjusted
Normalized Flood Losses - Catastrophic Hydrological Events in North America 1980-2017 per Munich RE NatCatSERVICE
The normalized, inflation-adjusted losses for hydrological events have peaked in 1990's, e.g., due to the Great Flood of 1993. How about considering the 2017 hurricanes experienced? The losses of Hurricane Harvey, Maria, and Irma are tracked as meteorological events as shown in the following charts:
Normalized Hurricane Losses Adjusted for GDP Growth
Normalized Flood Losses - Relevant Meteorological Events in North America 1980-2017 per Munich RE NatCatSERVICE
Normalized Hurricane Losses Adjusted for GDP Growth
Normalized Flood Losses - Catastrophic Meteorological Events in North America 1980-2017 per Munich RE NatCatSERVICE
Data shows that relevant and catastrophic losses peaked in 2005 due to Hurricane Katrina. Actual, unadjusted losses were higher in 2017 than in 2005, but when adjusted for inflation 2005 losses were near 2017 values - and when GDP growth is considered, 2005 values exceed 2017.

It is common for losses to be reported without adjustments. Making adjustments to normalize losses considering growth in net written premiums (i.e., higher losses are expected with growth in insurance market - more policies, more premiums, more payouts and claims). This was explored in my paper published earlier this year:

Where unadjusted losses, shown at right, suggest a significant increasing loss trend, adjusted losses, shown below, do not indicate a significant increasing trend.

Catastrophic Losses Adjusted for Net Written Premiums
Catastrophic Losses Adjusted for Growth in Net Written Premiums in Canada - per Robert Muir's Thinking Fast and Slow on Floods and Flow.
The Geneva Organization's recent report Understanding and Addressing Global Insurance Protection Gaps illustrates changes in uninsured losses as a share of Gross Domestic Product (GDP). As stated in the report: "Over the past three decades, the share of worldwide uninsured losses in global GDP
has decreased from 0.31 to 0.19 per cent. For high-income countries, the share fell from 0.20 to 0.13 per cent. Upper middle-income countries show a reduction from 0.21 to 0.11 per cent". The significant decreases appear small on the chart, due to the logarithmic scale on the y-axis.


More resources? The initial charts above were from reports prepared using the online NatCatSERVICE.

This is a link to the Munich RE NatCatSERVICE report on North American losses from hydrological events: Hydrological Losses North America 1980-2017

Similarly, this is a link to the report on those losses from meteorological events: Meteorological Losses North America 1980-2017

How about Canadian trends? The following NatCatSERVICE chart was provided by Munich RE specifically for Canada. It shows some increasing trends in losses after accounting for inflation just as the chart above with Canadian catastrophic losses normalized by growth in net written premiums. 

Canadian Flood Damage Trends Insurance Losses

Looking at data from the Insurance Bureau of Canada including loss data and net written premiums, we see both of these values increasing in recent decades as shown below.

The following charts show catastrophic losses vs net written premiums, and catastrophic waster losses vs net written premiums. To smooth out variability in annual losses, a 5-year moving average is used in the charts.

The chart above shows a plateau in water losses vs premiums for the 5-year ranges centred around 2003 to 2010. The 5-year ranges centred around 2011 to 2015 are substantially higher, reflecting the high water losses in 2013 due to the Alberta and Toronto-area floods. The r-squared values suggest a strong correlation between growth in premiums and growth in losses.

Other factors affect risks and losses. Urbanization is a key factor increasing runoff and risks and this has been documented in key case law in Ontario (see previous post with factors). That is, hydrologic stresses increase due to the expansion of urban areas and the intensification of development within urban areas. Other factors include hydraulic constraints in infrastructure systems that can degrade over time - these include natural factors such as blockages of sewer pipes due to build-up of calcite, sediment, roots, FOG (fats, oils, grease), or engineered modifications to collection systems that hold back wastewater flow during wet weather to prevent spills in watercourse - these modifications can in some cases aggravate back-up risks in the wastewater collection system.

Factors such as increasing rain intensity and frequency have been suggested by some however, Environment and Climate Change Canada data (see previous post on version 2.3 Engineering Climate Datasets, and new version 3.0 datasets), numerous regional studies (see compiled engineering and research reports), and the CBC Ombudsman findings (see January 2019 findings on extreme storm reporting), supported by Environment and Climate Change Canada data, all indicate no change in extreme rainfall.


Also see previous post - Catastrophic Losses in Canada - Have Flood Damages Increased Significantly Or Have Changing Data Sources Affected Trends?