Southern Ontario Observed Rainfall Intensities Decreasing - Annual Maximum Values Lower In Environment and Climate Change Canada's Engineering Climate Datasets (Version 3.0)

Ontario extreme rainfall annual maximum design intensity IDF trends climate change
Long term southern Ontario observed maximum rainfall trends,
according to Environment and Climate Change Canada's Version 3.0
Engineering Climate Datasets - decreasing trends in rain intensity and
more significant decreases than increases. 
Good news! Rainfall intensities have been decreasing in Ontario, Canada's most-populated province according to newly-released data. Less intense rain means lower urban flooding risk, contrary to many media reports that have confused future predictions of more extreme weather as a climate change effect with actual observed changes in the past. 

Maximum annual rainfall amounts over short durations at Ontario climate stations are used to derive engineering design intensities used in design of infrastructure such as sewers, culverts, channels, and ponds - the things that help convey rainfall runoff safety away from otherwise vulnerable people and property.

Environment and Climate Change Canada (ECCC) has recently updated its Engineering Climate Datasets that include a statistical analysis of observed trends in maximum values observed each year. The newest data are identified as Version 3.0 and are available as part of the Intensity-Duration-Frequency (IDF) Files on the ECCC website:

The previous Version 2.3 datasets showed decreasing annual maximum values at 21 southern Ontario climate stations with at least 30 years of observations - see previous post.

The updated Version 3.0 datasets continue this decreasing trend, showing that at the same 21 climate stations with an average observation period of 47 years:

  1. There are 42% more decreasing trends than increasing ones across all durations and stations (55.6% decreasing trends vs. 39.2% increasing ones).
  2. There are 75% more statistically significant decreases than increases (7 significant decreases vs. 4 significant increases).
This table shows the station name, ID, trends for each duration of 5 minutes to 24 hours, as well as the length of record and the most recent year in the Version 3.0 dataset.

Ontario Severe Rainfall Trends Climate Change Effects on Extreme Weather
Southern Ontario Observed Maximum Rainfall Trends - Environment and Climate Change Canada
Engineering Climate Datasets - Version 3.0
Trend Direction and Significance for 21 Climate Stations with Long Period Records (Greater than 30 Years)

Other observations:

  1. There are no statistically significant increases for durations less than 6 hours - that means the short duration convective storms burst that can lead to urban flooding related to most infrastructure systems do not show any appreciable increases.
  2. Overall downward trends are contrary to insurance industry statements, particularly the disproved "Telling the Weather Story" claim that there has been a one standard deviation increase in the probability of extreme rainfall according to Environment Canada data (the "Story" was only a theory/concept incorrectly cited and widely misreported as real data).
  3. Overall downward trends are contrary to many media reports citing a new normal of wild weather. Fortunately, some media, lead by the the Financial Post's Terence Corcoran are engaged in a critical review of urban flood drivers including extreme rainfall and the means to mitigate flood damages:
  4. CBC staff and the CBC Radio Canada Ombudsman have helped focus on facts Environment and Climate Change Canada data and corrected many stories on increasing storm frequency or intensity as noted here:
  5. Analysis by the School of Engineering at the University of Guelph, published in the International Journal of Environmental Research in 2015, looked at monthly trends and suggested that "The decrease in August extremes seems to have a significant impact on the annual extremes in the southwest and southeast regions":

Urban flooding is certainly an important issue to be addressed. And there are many factors that affect today's flood risks as explored in a previous post. While the insurance industry has suggested a link between increasing flood damages to increasing rain extremes due to climate change, given the wealth of evidence pointing to other quantifiable factors like increasing hydrologic and hydraulic stresses - and no change in rainfall extremes! - means that there is not even a correlation much less a causation relationship between flood damage and rain extreme trends (i.e., damages are up but rain intensities are down). This was pointed out in my Financial Post OpEd

Effective flood mitigation strategies must recognize the intrinsic capacity limitations in the vast amount of legacy infrastructure built over 30 years ago, and focus on reducing risks by addressing any level of service gaps through adaptation. Cost-effective and timely methods can include increasing the conveyance capacity of grey infrastructure, as opposed to mitigating rain/weather stresses that have not appeared to change, based on official, national engineering datasets. While such infrastructure investments should consider potential future climate effects, and we have many examples of analyzing stormwater and wastewater systems for such effects, past trends do not point to an increase to date in rainfall extremes. As a result, derived intensity duration frequency values for the stations reviewed above, based on values in the Version 3.0 datasets, shows an overall decrease in design intensities for small frequent and large rare storms across southern Ontario - those results were presented in a previous post, as shown below:

Ontario extreme rain IDF trends
Ontario Intensity Duration Frequency (IDF) Trends - 2 Year to 100 Year for all Durations
Environment and Climate Change Canada Engineering Climate Datasets - Version 3.0

Recognizing trends in observed rainfall maximum values and the derived design intensities will support data-driven, evidence-based policies and programs for achieving flood resilience through strategic infrastructure investments.


The following table explores annual maximum values at Ontario climate stations with over 50 years of record:
Ontario Extreme Rainfall Severe Weather Storm Trends
Ontario Observed Maximum Rainfall Trends - Environment and Climate Change Canada
Engineering Climate Datasets - Version 3.0
Trend Direction and Significance for 11 Climate Stations with Long Period Records (Greater than 50 Years)

The table expands into higher latitude eastern Ontario communities including Kingston and Ottawa as well as to northern Ontario. The eastern Ontario climate stations show an overall consistent trend in decreasing observed rainfall maxima over the shortest durations. Another eastern Ontario station, the Ottawa Airport also shows decreasing trends over short durations, including several statistically significant decreases (i.e., lower observed rainfall intensities) for durations of 10 minutes, 15 minutes and 1 hour.

Previous analysis of the Version 2.3 datasets showed the differences in southern and northern Ontario trends. Increases in intensities in the north, beyond Ontario's largest urban centres, could reflect a shift toward more rainfall events instead of snowfall as a result of warming temperatures. 


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