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.


Bonus: here are a few more predicted 24 Hour 100 Year rainfall values, also going against the observed trends:

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:

The Ombudsman has also recently noted in a new review that "It would have been wrong to state categorically that Canada has already seen an increase in extreme rainfall events" and that its reporting should be unambiguous about the past, present and future, with "distinctions made between observed phenomena and predicted phenomena" (  The Ombudsman notes that CBC reporting describes future changes in extreme weather saying:
  • Canada is warming at twice the global rate, and our north is warming at three times that rate.
  • We can expect more extreme heat, warmer winters, earlier springs and rising sea levels.
  • Precipitation will increase in much of the country.
  • Weather extremes will intensify.

  • The last two bullet points are careful to use the future tense.