By the time you are reading this it will be September, but I have to write this almost a week in advance, so you will have to wait another week for the monthly weather roundup, overall summer weather review, who did the best job predicting this summer’s weather and the look ahead to see what fall might have in store for us.
If you did not read the last issue, we took a look at different categories of drought and then looked at the Winnipeg region’s precipitation data to see just how dry it has been. We learned the drought has been slowly building for almost 10 years, with year-to-year precipitation deficits building to the point that the Winnipeg area now has a deficit of over 1,000 mm or one metre — even after we received the rains over the last couple of weeks. I then promised to dig into Brandon and Dauphin’s data to see how those regions compared.
The first station I looked into was Brandon, and to my surprise, that region has no precipitation deficit at all. I started back in 2010 and calculated the running precipitation total, then compared that to the running total if each year saw average precipitation, and no deficit occurred. Overall, annual surpluses ranged from around 100 mm to 320 mm and fell to a low of 33 mm this year, but there are still several months left to push this value up and it is still a surplus and not a deficit. After seeing this, I looked a little closer at the data and it seems pretty much every year Brandon has recorded at least one month with extreme rainfall. The amounts were enough to overcome any dryness that might have been occurring. This highlights a flaw in using yearly precipitation data, as the timing of precipitation can impact some of the types of drought. For example, heavy rainfall in early spring or late August or September would not be highly beneficial if May to July were very dry, at least from an agricultural perspective.
Looking at Dauphin I ran into some problems, as there is a fair bit of missing data. I tried to fill in any missing readings by looking at what was happening at locations around it. For example, if data was missing on July 21, 2015, I would look to see if any other nearby stations reported precipitation on that date. I would also look at the temperatures and temperature trends around that date to see what type of weather pattern might have been in place. Was the daytime high the day before warm, then much cooler after the missing date? This would indicate a frontal passage which is often accompanied by precipitation. I then either put in a value of zero (which was most of the time), or made an estimate based on precipitation amounts that occurred at nearby locations. This method should be OK, though it might have missed the odd thunderstorm.
After doing my best to fix the data, I found the Dauphin region saw a surplus in the running total from 2010 to 2017, ranging from a high of a 350 mm in 2010, dropping down to only two mm by 2017. This downward trend has continued until the present, with the 10-year deficit now sitting at almost 500 mm or half a metre. I might have to dig into the data from a few more southwestern locations to see if the Brandon anomaly is localized or more widespread.
Rain records and relevance
With the rainfall over the last few weeks, a number of weather-related headlines caught my attention. Now I know the purpose of a headline is to make you want to read the article, but the headline that stated something like ‘record rainfall hits drought-ravaged Manitoba locations’ got me thinking. Yes, several locations broke one-day rainfall records on August 20. Winnipeg, for example, received about 65 mm, which broke the previous rainfall record for that specific date, which was 47 mm back in 1983. So, the headline was correct, but it implies that the 65 mm was an extreme event, which it was not.
Rainfall records are not the same as temperature records. For example, if you have 70 years’ worth of data, then for any given day there would be 70 temperatures recorded. During that same time, we would not have 70 rainfall events, as, on any given day, rainfall would only occur about once every three to four years, or about one-third of the time. That is an average; some days of the year will have seen rain more than a third of the time and others less. Rainfall amounts are also quite variable. A specific day of the year may have seen rainfall a third of the time over the 70 years, but each time it rained it was not a heavy rainfall as heavy rainfall events do not occur often. So, did we have record-breaking rainfall? Yes. Was it noteworthy that a record was broken? Not really. Breaking daily rainfall on a specific day is not that notable; breaking the one-day rainfall record for any given day within an entire month is much more noteworthy and is more equivalent to breaking a daily temperature record.
Next week, we’ll look back at the summer of 2021 and take our first glance into the fall and winter forecasts.
For more content related to drought management visit The Dry Times, where you can find a collection of stories from our family of publications as well as links to external resources to support your decisions through these difficult times.