For many people the May long weekend marks the unofficial beginning of summer, but it also has the reputation of being something of a disappointment. Does it deserve its reputation?
Victoria Day is still a few weeks away, but I wanted to do this early before the longterm forecasts are available. This is more interesting when we don't know what 2016 has in store for us.
(the long weekend is technically only Saturday-Monday, but here I'll be including the Friday as well.)
Anyone hoping for really "hot" weather is probably going to be disappointed most years. Going back to 1995 there have only been 2 days above 25°C.
Lowering expectations to 20°C helps a bit, but it's by no means a guarantee. In 9 of the last 21 years the temperature didn't reach 20°C on the May long weekend, and in the years when it did, in many cases it was just for a day or two. The only years with all four days above 20°C were 2008 and 2013, while 2011 managed a 3-day streak.
In terms of small victories, you can usually count on the daytime high temperatures to reach above 10°C. The coldest year was 2010 with 3 days below 10°C, and only 5 other years have had days that stayed below 10°C.
There's not much to say about the lows, beyond that fact that even in the cold years the temperature rarely drops below freezing. There have been plenty of lows around 0°C, but it has only dropped below that three times (one of which was last year in 2015)
This is the first time that I've used Environment Canada's precipitation data for anything, and that is partly because it isn't very reliable. There is no recorded data for 2000 or 2014, so in the graph those years show as 0, but that may or may not be correct. The data also doesn't distinguish between rain or snow - based on the temperatures this was probably mostly rain, but with some of the lows around freezing you never know.
So with those caveats we can see that rain-or-maybe-snow on the May long weekend is pretty common. Of the 19 years that have data, only 2004 and 2015 had no precipitation.
I don't have a great sense of what "a lot" or "a little" rain is though, so here is a comparison of how much rain we actually get on a typical rainy day.
On days that it rains in Edmonton, 38% of the time we'll get 1mm or less; 50% of the time it's 2mm or less, and 73% of the time it will be 5mm or less.
That helps put the May long weekend numbers in perspective because it means that the years when the precipitation was just a blip - 1996, 2003, 2013 - probably didn't seem too rainy. But once you get to 2mm in a day - 2002, 2007, 2008 - that is going to feel like a genuinely rainy day for Edmonton. Anything above 4mm or 5mm is going to be quite rainy, and for the days above 10mm - 1999, 2005, 2006 - that's a lot of rain for Edmonton. That's all just a guess though, because rain during the night or from a short thunderstorm isn't necessarily going to ruin a holiday.
Combining it all together then:
Here the summer~ish days (warm, and not rainy) are highlighted in orange.
As usual, this data is all from the Blatchford weather station in central Edmonton. The people who are most disappointed in the May long weekend typically want to go camping, and Edmonton weather won't be a perfect match for that. But looking at a year like 2010, most places within driving distance suffered a similarly cold and rainy few days.
So what to expect from a typical May long weekend in Edmonton?
- It's almost never above 25°C
- It's almost never below 0°C
- It's above 20°C less than half of the time
- 75% of the time there's at least one genuinely rainy day, and 90% of the time there's at least a little rain.
Usually I try to find the silver-lining in our weather, but this one is kind of hit-or-miss.
