So far this spring, a good portion of Alberta and Saskatchewan has not seen any heavy rainfall events. If you check out the map above you’ll see the large red and amber areas that represent rainfall amounts that are 60 per cent of average or less.
Hopefully, my writing about heavy rainfall won’t put a jinx on the weather, but we have to remember that we are moving into the time of year when heavy rainfall events occur more frequently.
In my last article about heavy rainfall we looked at conditions that bring short-term intense rainfall events. In this article, we are going to look at long-term heavy rainfall events. Just to review, a short-term rainfall warning is given when 50 millimetres or more rain is expected to fall within a one-hour period. These types of events are almost always associated with severe thunderstorms. Long-term rainfall warnings are issued when 50 millimetres of rain are expected to fall within 24 hours or 75 millimetres within 48 hours.
Unlike short-term rainfall warnings, these long-term warnings may or may not include thunderstorms. Often, when we see these heavy one- or two-day rainfall events, they are associated with one of two things, both being areas of low pressure. Either we see a slow-moving surface low move through, bringing multiple rounds of rain and thunderstorms, or we see a slow-moving upper low that will typically bring a long period of steady rain. The one extra ‘ingredient’ that is brought into the mix, especially over western Alberta, is the upslope winds that can really enhance what would typically not be a heavy rain event.
A classic example of a system that brings heavy rainfall was the storm system that brought the record flooding to parts of southern Alberta back in 2013. There was high pressure sitting off the southeast coast of the United States, and the clockwise circulation around this high was pulling up plenty of moisture from the Gulf of Mexico. At the same time, an upper low was moving into the southwestern U.S., and as these lows cross the Rocky Mountains they typically strengthen. The third ingredient was an area of high pressure over Eastern Canada that was blocking the usual eastward movement of weather systems. These features all worked together to produce a large area of low pressure that stretched from Colorado all the way up into Alberta.
This low, or series of lows, were slow moving or even stationary, and were able to tap into large amounts of moisture, both of which would have brought large amounts of rain on their own. Add the location of the low, which created a strong easterly flow of air that rose up along the mountains, and some regions ended up seeing truly epic amounts of rain and resultant flooding.
Luckily, these perfect conditions don’t come together very often but Alberta has, over the years, seen some big one-day dumps of rain. To try and put things into perspective I looked up some of the one-day rainfall records from across Alberta.
Here’s what I found. (I only looked at stations that had more than 50 years of rainfall data and included stations that recorded more than 100 millimetres of rain in one calendar day.)
Next time we’ll take a look back at the spring of 2015, and then look ahead to see what kind of weather we might expect in Alberta this July and August.