As the rain pounds down in Calgary and residents look wistfully at the grey-hued skies, one might ask themselves: Why do we get so much rain in June?
With up to 100 millimetres forecast for the Calgary region and nearly double that possible for the foothills, plus having it on the 12th anniversary of the disastrous 2013 southern Alberta floods, it’s a reasonable question to ask.
There is an atmospheric answer, too.
While June is traditionally a month with the most historical precipitation, it doesn’t mean that other months won’t get significant rain. Still, the same atmospheric changes are in action, and it often happens in the month of June.
Since 2019, June has had the most precipitation in four of those five years, and will likely be five of the past six, given the intense rains the city is experiencing now.
Christy Climenhaga, a scientist with Environment and Climate Change Canada, said that two primary factors make June a robust time for rain. First, she said the polar jet stream tends to orient itself over Western Canada, steering more storms in this direction.
“You still have a pretty good temperature gradient too, between warm and cold, where you can get these stronger systems to build and develop,” she told LWC.
The polar jet stream, typically found between the 30th and 60th parallels, is situated in the troposphere, or the area where most of the weather patterns are formed. The jet stream forms where colder polar air meets warmer, mid-latitude air, creating a wide temperature band and greater wind speed.
Here’s how NASA describes it: “Meandering around the planet like a rollicking roller coaster in the sky, the Northern Hemisphere’s polar jet stream is a fast-moving belt of westerly winds that traverses the lower layers of the atmosphere. The jet is created by the convergence of cold air masses descending from the Arctic and rising warm air from the tropics. Deep troughs and steep ridges emerge as the denser cold air sinks and deflects warm air regions north, giving the jet stream its wavy appearance.”
Climenhaga said that the polar jet stream moves further south during the winter and further north during the summer.
“As we get in between, it will align a different way. It generally travels in that direction from what we call zonal, so west to east,” she said.
But of course, it often gets these waves throughout it that can feed moisture in from the Pacific, or they can carry smoke in a wildfire situation. It can bring warmer than normal weather. It really does govern a lot of the atmosphere.”
Moisture begets moisture
Climenhaga said that when we get to this point in June, the temperature is typically consistently warmer.
“Which means that there’s starting to be more moisture available for these storms,” she said.
“The air can hold more of that moisture, and it has that ability to feed any rainfall. So, it doesn’t mean that every June we’re going to get this sort of a setup, but it does kind of prime things to have these, really potent systems.”
Climenhaga said that every year is going to be different. Though, she said Calgary has had some of its largest, short-term rain events in the month of June.
Of course, the 2013 Calgary flood was brought on by 200+ millimetres of rain over two days. In June 2005, Calgary was hit with 247.6 mm of rain spread out over three significant storms.
Climenhaga noted a 2015 June system that brought 60 mm over three days, and another in 2022 that dropped 80 mm in early June.
“I mean, it all depends on the system. It all depends on not only the system itself, but the pre-existing conditions,” she said.
“This type of system likes June. It tends to kind of pull in that moisture at this time of year and really feed it into the Rockies.”
In this particular case, Climenhaga said they’re seeing moisture-laden air coming in from more of an easterly direction, hitting the Eastern Slopes of the Rocky Mountains and being driven upward.
“That’s why we’re seeing the highest amounts expected in the foothills, in the elevation as that, what we call upslope flow kicks in and just really drops that rainfall in that area,” she said.
“That’s kind of what we saw in some of those other June rainfall events that we were talking about, where a lot of moisture piles up in the in the basins in the foothills.”
The City of Calgary has said that it is well-prepared to withstand the substantial rainfall, with preparations made in advance of the weekend storm.
