First Snowfall

I have to admit that I did not believe the forecasts, so I wasn't prepared for this. I figured that all of my blogging about snow was at least a few weeks away.

But since we've now had our first real snowfall, is October 7th actually early?

We're going to take a quick look at the history of Edmonton's first snowfall of each year, with a couple of caveats:

• This will be "major" snowfalls of at least 1cm. Anything less than that doesn't count.
• I'm going to use September as the starting point. We have actually gotten snow twice in August (August 25, 1900 and August 21, 1992), but I'm going to skip those. There's never been any snow recorded in July, though.
• Even by limiting it to September, a lot of these first snowfalls would have melted off again long before the real winter began.
• This will be a mix of data from Blatchford and the International, because the International only goes back to 1961, and Blatchford's data disappears after 2007

So then, here is the whole recorded history:

This chart is a little hard to interpret - each year is along the bottom, and the date of the first snowfall is along the side. So the higher the line goes the later in the year the snow fell, and the lower the line the earlier the snow.

Blatchford is in Blue from 1882 through 2007, and the International is in Orange from 1961 onwards. The two overlap from 1961 through 2007 and in many cases they match, but there are some years like 1981 where the International's first snow was on October 21 while Blatchford's wasn't until December 10.

Looking at all of the recrods, the breakdown of how often each month gets the first snowfall is:

• September - 25% of the time
• October - 46%
• November - 27%
• December - 2%

Limiting that to just 1995-2016 it shifts a bit:

• September - 14%
• October - 55%
• November - 27%
• December - 5%

So October is definitely when this is most likely to happen. And our likelihood of September snowfalls has actually fallen a bit in the last few years.

Here is the same graph, but with a 5-year "average" added on top:

An "average" date is sortof a strange idea, but this just smooths things out a bit and makes trends a little more visible.

What's interesting is that going way back to the beginning, in the 1880s snow generally fell much later in the year? It's possible that's true, but I think it's also possible that it's pretty hard to measure snow depth, and that our methods have improved over the years.

Beyond that, the "average" date has pretty much stayed in October for the last century.

Looking at 130+ years is a little hard to grasp, so now lets focus on more recent history:

This is the graph of the annual dates again, but this time only going back to 1980. Here we can see just how often the dates for Blatchford and the International match, with only a couple of large exceptions like 1981 and 1985.

So how unusual is snow on October 7th?

On this chart there's a dashed line to help compare 2016 to the other years, and in recent memory:

• 2014 had 4cm on September 8, and 2004 had 5cm on September 9th
• 2012, 2009, and 2002 all had snow right about now.
• Everything else was later.

So we are definitely on the early side of things this year. And if you exclude 2004 and 2014 then this is just about as early as we ever get snow.

Here's one final look at the data, with the 5 Year averages going back to 1980:

Back at the end of the 70s and beginning of the 80s there were several years with really late snow, so the average was pushed to almost mid-November. But then right after that 1983, 1984 and 1985 were all in mid-September, and the average dropped down to the end of September. Since then though it's been pretty solidly between Thanksgiving and the end of October.