Toronto Extreme Rainfall Trends - 100-Year Daily Rainfall in Engineering Climate Datasets

Previous posts have reviewed trends in extreme rainfall across Canada, in various regions including southern Ontario, and in the Greater Toronto Area (GTA), including Toronto and Mississauga where long-term climate data is available for review.

Projections of future extreme rainfall increases are commonly made as part of climate change studies. A review of past trends in extreme rainfall was made in the 2021 National Research Council flooding cost benefit guidelines, as summarized in a previous post. The following chart was included in those guidelines and shows the trends in 100-year daily rainfall at two GTA climate stations in downtown Toronto and at Pearson International Airport in the adjacent municipality of Mississauga.

The chart shows the 100-year rainfall depth using data records up to 1990 and then adding more recent data up to 2017. The chart shows that the 100-year rainfall at Pearson Airport/Mississauga has been decreasing slightly when recent data is added after 1990. Meanwhile the Toronto rainfall has been increasing slightly (see dotted and dashed black lines on the chart above for the trends).

Several climate studies have projected that the 100-year daily rainfall would increase over coming decades as shown on the chart. The Toronto's Future Weather & Climate Driver Study by SENES projected a doubling of this rainfall statistic by 2040-2049, relative to a 2000-2009 baseline value (see the orange dashed line on the chart above, where the 2000-2009 value is shown at 2005 and the 2040-2049 value is shown at 2045).

Some additional data has been analyzed by Environment and Climate Change Canada for the Toronto climate station, now including data up to 2021. This allows the 100-year daily rainfall statistic to be updated with a few more years of data. The chart below shows the additional Toronto data point circled in yellow at 2021.

While the Toronto rainfall statistic up to 2017 was 97.5 mm, the value up to 2021 decreased slightly to 97.3 mm. The value up to 2017 reflected the prior July 8, 2013 extreme event, creating a jump after 2007 when the value was slightly lower at 94.7 mm. As more data is observed below the 2013 extreme, the statistic should continue to decrease as more data is added and analyzed.

The take-away? Observational data, including data up to 2021, does not support the projected significant increases in 100-year daily rain in climate studies. The Toronto data is available over the period of 1940 to 2021.

How far off are the projected increases in extreme rainfall? The Toronto Future Weather & Climate study projected a theoretical 31mm/decade increase over 40 years - that was for Pearson Airport climate station. Actual data at Pearson Airport shows an observed increase of only 3.1mm/decade.  This considers a value of 115.1 mm in the middle of the 1950-2003 period and a value of 125.5 mm for 2003-2017 - that later value is estimated to generate the current value of 117.3 mm by using a weighted average across all years from 1950 to 2017. For Toronto the actual increase is only 2.0 mm/decade.

On average the GTA (Toronto and Pearson/Mississauga) increase is about 2.5 mm/decade, or less than a tenth of almost 31mm/decade projected in the SENES climate/future weather study.

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Further reading in previous posts on extreme rainfall trends:

1) Rainfall intensity trends in Canada:

a) 226 long term climate stations in the Engineering Climate Dataset are used to show actual trends between rain intensity statistics up to 2007 and then up to 2017: https://www.cityfloodmap.com/2020/12/design-rainfall-trends-in-canada.html

b) more of the above plus observed annual maxima rainfall trends as reported in the 2021 National Research Council of Canada (NRC) "National Guidelines on Undertaking a Comprehensive Analysis of Benefits, Costs and Uncertainties of Storm Drainage and Flood Control Infrastructure in a Changing Climate": https://www.cityfloodmap.com/2022/02/nrc-national-guidelines-on-flood.html

2) Rainfall intensity trends in Southern Ontario:

a) ECCC's Engineering Climate Dataset Intensity Duration Frequency (IDF) trends for long-term southern Ontario climate stations, comparing statistics up to 1990 and current values (v3.3 datasets with some station data up to 2021): https://www.cityfloodmap.com/2023/05/southern-ontario-extreme-rainfall.html

3) Rainfall extreme reporting (?) in the media (including Toronto, Mississauga trend review):

a) Thinking Fast and Slow About Extreme Weather and Climate Change, inspired by the late Daniel Kahneman (RIP good sir), exploring the cognitive biases in extreme rainfall reporting in the media: https://www.cityfloodmap.com/2015/11/thinking-fast-and-slow-about-extreme.html

b) my paper with "Thinking Fast and Slow" themes published in the Journal of Water Management Modelling with the title "Evidence Based Policy Gaps in Water Resources: Thinking Fast and Slow on Floods and Flow": https://www.chijournal.org/C449

4) Local studies that observed no increases in design rainfall when updating IDF values:

Many studies: https://www.cityfloodmap.com/2018/03/extreme-rainfall-and-climate-change-in.html

While media and the insurance industry has repeated that climate change has been responsible for increased flood damages and insurance claims over past decades, the lack of increases in extreme rainfall means that other factors are at play. These include fundamental changes in hydrology in urbanized communities, e.g., increased watershed development and intensification. See previous posts for some examples of expanding urbanization in Ontario communities over previous decades: https://www.cityfloodmap.com/2016/08/land-use-change-drives-urban-flood-risk.html

When fact checkers look into media statements regarding extreme rainfall trends, including the CBC and Radio Canada Ombudsmen offices, data shows no overall increase in extreme rain across Canada. This post shares corrections made by the CBC over recent years: https://www.cityfloodmap.com/2019/06/cbc-correcting-claims-on-extreme.html

Can We Use Daily Rainfall Models To Predict Short Duration Trends? Not Always - Observed Daily and Short Duration Trends Can Diverge

One can assess trends in rainfall intensities over various durations and return periods using Environment Canada's Engineering Climate Datasets.  National trends based on updating 226 station IDF curves were shown in an earlier post.

What are the trends in regions of Canada that have experienced significant flooding in the past?  And do the trends projected by models for long durations (1 day precipitation) match observed data trends?  No - some 24-hour trends are decreasing despite models estimating they will go up (or have gone up because of increasing temperatures).

Also, what is happening with observed short duration intensities, the ones responsible for flooding in urban areas, compared to the observed 1-day trends?

The data show short duration and long duration trends diverge. Therefore relying on models of 1 day precipitation to estimate what is happening with short duration, sudden, extreme rainfall should be done with caution.

A couple charts help illustrate these observed data trends and show what is wrong with relying directly on models to project local extreme rainfall.

This is the trend in observed rainfall for southern Ontario climate stations, using median changes in IDF statistics:

Southern Ontario Extreme Rainfall Trends

Long duration intensities are decreasing and short duration intensities are decreasing even more.  The extreme intensities (red dots = 100 year, orange dots = 50 year) decrease more than the small frequent storm intensities (green dots = 2 year).  Observed data diverges from Environment Canada models that suggest intensities are going up due to a warmer climate (see recent CBC article).

These are the trends for Alberta observed rainfall when new data are added and are reflected in the most current v3.10 datasets:

Alberta Extreme Rainfall Trends

In Alberta, long duration intensities decrease significantly (100 year is down by 4% on average).  Meanwhile the short duration intensities increase.  The long duration decrease is contrary to Environment Canada's simulation models that estimate 1 day rainfall at a sub-continental scale.

In northern Ontario, trends are different than in southern Ontario as shown below:

Northern Ontario Extreme Rainfall Trends

In northern Ontario the long duration intensities have increased but short duration intensities have decreased on average.  So we see short and long duration rainfall trends are diverging when we consider new data.

Climate modellers may suggest that simulated 1 day precipitation can guide what happens during short durations too.  Observed data suggest otherwise.  Trends actually diverge.

In brief, for this sample of regions shown above, we see these trends:

Location                 Short Duration Trend         Long Duration Trend

Southern Ontario      Larger Decrease                        Decrease
Northern Ontario            Decrease                              Increase
Alberta                            Increase                               Decrease

Remember "All models are wrong, some are useful".  Climate models do not accurately project changes in extreme rainfall in Canada based on observed data.  Furthermore, simulated 1 day precipitation trends from models cannot be used to assume short duration trends related to flooding in urban areas - short and long duration rainfall trends are observed to change in opposite directions in sample regions across Canada.

When using 1 day rainfall trends to estimate short duration trends, given the actual observed data trends above, it may be appropriate to conduct sensitivity analysis on potential shorter duration trends, especially if those shorter durations influence system behaviour (e.g., 'flashy' urban drainage systems).  Those short duration trends trends may be in an opposite direction or magnitude than the 1 day trends. For example, in Northern Ontario the 1 day 100-year intensities have increased 2% as a result of the most recent IDF data updates, however the intensities for durations of 2 hours or less have mostly decreased.

The following chart compares the 30 minute, 1 hour and 2 hour 100-year intensity trends with the 24 hour 100-year trends at 226 climate stations across Canada.

The correlation of short duration trends with 24 hour trends is weak with R-squared value of 0.12 for 2 hour trends, 0.06 for 1 hour trends and 0.006 for 30 minute trends.  This suggests that short duration trends are not correlated with 24 hour trends.   

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Given recent flooding in British Columbia, it is worthwhile looking at trends in design rainfall intensities in BC. This chart shows that extreme rainfall intensities (red dots with 100-year return period, orange dots with 50-year return period) have not increased for most durations - the 12 hour duration intensities are up slightly on average (less than 0.5% increase), while other duration intensities have decreased by more than 2 % on average (5-minute 100-year intensities).

For the longest duration of 24 hours, intensities have decreased on average - typical 2-year intensities are unchanged on average while the moderate and extreme intensities (5-years, 10-year, 25-year, 50-year and 100-year) have decreased on average.

The following tables shows trends in observed annual maximum rainfall over various durations at BC climate stations with long-term records. These are called the Annual Maximum Series (AMS). Trends in derived design rainfall intensities above (e.g., 2-year to 100-year rainfall rates) follow these trends in AMS.

Southern Ontario Extreme Rainfall Trends - Environment Canada Engineering Climate Datasets IDF Tables

Environment Canada's version 3.10 update to Canada's rainfall IDF tables and curves shows more increases than decreases and slightly more significant increases than decreases - overall trends were shown in a recent post: https://www.cityfloodmap.com/2020/05/annual-maximum-rainfall-trends-in.html.

Regional trends may be up or down and warrant further review.  In Southern Ontario the IDF intensities for long term stations have been reviewed and compared with pre-version 1.00 statistics up to 1990 (similar to version 3.00 and version 2.30 comparisons shared in earlier posts).  The following table shows average changes since 1990, and the surrounding arrows suggest how these changes may influence infrastructure design, if at all.

Southern Ontario IDF Rainfall Intensity Trend Table - Environment and Climate Change Canada's Engineering Climate Datasets, Pre-Version 1.00 (up to 1990) to Version 3.10 (up to  2017)

The 21 stations assessed include:  Sarnia Airport, Chatham WPCP, Delhi CS, Port Colborne, Ridgetown RCS, St Catharines Airport, St Thomas WPCP, Windsor Airport, Brantford MOE, Fergus Shand Dam, Guelph Turfgras CS, London CS, Mount Forest (Aut), Stratford WWTP, Waterloo Wellington Airport, Bowmanville Mostert, Hamilton Airport, Hamilton RBG CS, Oshawa WPCP, Toronto City, Toronto International Airport (Pearson).

The above changes in design rainfall intensities since 1990 do not suggest any overall shift that would affect how municipal drainage infrastructure would be designed considering current weather conditions.  That is, overall intensities have decreased by 0.2% which in negligible.  The 5-minute, 2-hour and 6-hour intensities decreased consistently across all return periods.  The change however is also negligible.  On average frequent intensities, e.g., 2-year intensities expected every couple of years, and 5-year intensities, used for storm sewer design showed overall decreases.  Again the changes are negligible.  The 100-year intensities increased by 0.1% overall which is also negligible, especially considering the confidence limits with such statistics and the uncertainty in curve fitting (Gumbel distributions are used, other distributions would provide shifts in results).  One would expect rare intensities to increase over time for skewed distributions given sampling bias with short records (i.e., limited observations of extreme events are expected to lead to underestimates of 100-year statistics).

Of course considerations must be made to account for future changes and uncertainties.  Some cities and regions have incorporated allowance for climate change effects.  In Quebec a 18% allowance is standardized.  Some cities (e.g., Ottawa) include a 20% stress test to evaluate any unacceptable conditions that warrant design changes to address future potential risks.  Others incorporate stress test hyetographs in the design process - the Windsor/Essex Ontario standards include a stress test event that has 39% greater volume than the standard 100-year design storm (NB - the daily volume is increased to account for that additional volume distributed uniformly across 24 hours, while peak hyetograph intensities are only nominally affected). 

The chart below shows the IDF trends at these long-term record Southern Ontario stations.

Southern Ontario IDF Rainfall Intensity Trend Chart by Duration - Environment and Climate Change Canada's Engineering Climate Datasets, Pre-Version 1.00 (up to 1990) to Version 3.10 (up to  2017)

It is clear that that the short duration intensities (red and orange bars representing 5 and 10 minute durations) have decreased the most as shown in the table above. The chart and table below shows a more simplified version of the above chart indicating the range of changes observed.

Southern Ontario IDF Rainfall Intensity Trend Chart by Duration - Environment and Climate Change Canada's Engineering Climate Datasets, Pre-Version 1.00 (up to 1990) to Version 3.10 (up to  2017)

The above chart shows that 2-year and 5-year IDF rainfall intensities have decreased most consistently among all stations.  Those intensity estimates benefit from many observations each year to determine the statistics.  Rare event intensities from 25-year to 100-year return periods have more equal increases and decreases yet the decreases are greater.  The magnitude of the changes, both increases and decreases are on average negligible.  Even the greatest increase of 1.2% from 1990 to 2017 (27 years) is negligible for the purpose of hydrologic analysis and drainage infrastructure design.

Toronto Area Extreme Rainfall Intensity Trends - Environment Canada IDF Curve Updates 2020

Environment and Climate Change Canada's intensity duration frequency (IDF) data describes the rare extreme rainfall intensities used to design drainage infrastructure and to assess river peak flows/flood flows when the big storms hit.

Some climate station IDF analysis have been due for an update for several years.  The new version 3.10 update in 2020 extends analysis to 2016-2017 in many cases.  The trends in short duration intensities can show how flood risks are changing due to changing climate and any more severe weather. Many municipalities and researchers have updated their IDF statistics internally and have reported trends in extreme rainfall intensities (see previous post for Ontario studies: https://www.cityfloodmap.com/2020/05/annual-maximum-rainfall-trends-in.html).

Earlier versions of the IDF datasets are available to characterize annual maximum rainfall, dating back to 1990.  The tables below shows updated trends in 100-year rainfall intensity over a short 5 minute duration at Buttonville Airport in Markham (updated in version 3.10 in 2020) and in Toronto and Mississauga (updated in version 3.10 in 2019).

For short durations, it appears that extreme rainfall intensities are decreasing in the Greater Toronto Area.  The rare 100-year intensities are decreasing from 4-7% at the three locations above.  The more frequent 2-year intensities are decreasing by about 5-8%.

There is higher uncertainty with 100-year storm intensities due the rarity and spatial distribution of events.  However, the more frequent 2-year intensities that rely on many more rainfall observations every year to characterize these average annual peaks are more reliable.  To illustrate this, consider the 95% confidence bands on the Environment Canada IDF 5-minute data for Buttonville Airport, in southern York Region, just north of Toronto:

Extreme Rain 95% Confidence Bands Buttonville Airport, Markham, Ontario - Environment Canada Engineering Climate Datasets v3.10

The 100-year 95% confidence band of 97 mm/hr (2 x 48.5) is 46% of the expected value of 210.8 mm/hr.  In contrast, the 2-year band of 22.6 mm/hr is only 22% of the expected value of 104.7 mm/hr, a relatively tighter band.  The tight band makes the observed decrease in 2-year rain intensities more noteworthy, i.e., the 2-year intensity has decreased 7.6% which is a significant proportion of the uncertainty band.

The longer duration annual maximum rainfall series for Toronto and Mississauga have relatively tighter confidence bands for both 100-year and 2-year intensities:

Extreme Rain 95% Confidence Bands Pearson International Airport, Mississauga, Ontario - Environment Canada Engineering Climate Datasets v3.10

Extreme Rain 95% Confidence Bands Toronto, Ontario - Environment Canada Engineering Climate Datasets v3.10

Given that rainfall intensities are decreasing over most durations in the Greater Toronto Area, based on Environment Canada's annual maximum series and derived 2-year and 100-year IDF design intensities, why does media often report a 'new normal' of more extreme weather?  This may be due to lack of familiarity with data and a tendency to exercise an 'availability bias', i.e., simple listing of recent events that have caused damages and association of those events with increasing rainfall intensities, but without checking the actual rainfall trends or investigating other factors.  For more on cognitive biases in framing and solving complex problems read about Thinking Fast and Slow on Floods and Flow:   https://www.chijournal.org/C449.   Given the proliferation of rainfall gauges across the GTA, many more than the Environment Canada numbers, the observation of many extreme events over a short duration may not be a statistical anomaly at all, as analysis here shows: https://www.cityfloodmap.com/2019/03/are-six-100-year-storms-across-gta-rare.html

The CBC has made several corrections to its stories on changing extreme rainfall trends in the past, sometimes finding violation of its Journalistic Standards and Practices regarding accuracy of reporting - here are some examples: https://www.cityfloodmap.com/2019/06/cbc-correcting-claims-on-extreme.html.  Most recently the CBC Ombudsman in a review entitled "Past, Present, or Future" wrote that "journalists could have been clearer with their choice of tenses" - more on that here: 

https://www.cityfloodmap.com/2020/05/past-present-or-future-cbc-ombudsman.html.

The CBC Ombudsman stated "There are appropriate distinctions made between observed phenomena and predicted phenomena" in its reporting noting this excerpt: 

• Precipitation will increase in much of the country.
• Weather extremes will intensify.

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

The recently updated Environment Canada IDF data in the GTA supports the CBC's perspective that intensified weather extremes are predicted phenomena as opposed to observed ones.