2016/11/07

The First Lasting Snowfall of the Winter

Today we're going to look at when Edmonton typically gets its first lasting snowfall of the winter.

I wasn't sure when to post this - do I wait until we actually have "permanent" snow on the ground? Or do I post it early, and then inevitably it will snow the next day? But with the very warm forecast I don't think we have to worry about snow this week, so now is the perfect time.

Today is also the one-month anniversary of the first snow that we got this winter, back on October 7th. When that happened we'd looked at winter's first snowfall: it's typically in October or November, but there are a few late years that stretch into December, and a few early ones in September (and in recorded history there have even been two in August). But that first snowfall will often melt long before winter really begins, so today we're going to look at when the snow actually sticks around.

This is Part 3 of the ongoing saga of Edmonton snow:

Environment Canada measures the depth of the snow on the ground throughout the year, and today I'm going to be using the data from the International Airport, since it is more complete than Blatchford.

To start with, here are the recorded snow depths during the autumn for the last two decades:

Snow Depth

This shows 1995 through 2016, and how the snowdepth changes for October through December. I haven't included September snowfalls here, because in the past they have always melted-off before the winter starts.

I don't expect the data today to be perfect, for a few reasons. Snowdepth is obviously going to be a tricky thing to measure, and it will depend on the type of snow - fluffy, heavy, that sugary stuff, etc. - and how long it has had to settle. And when Environment Canada measures snowdepth they are doing that in an untouched area, but when snow gets shoveled and piled into windrows it takes a lot longer to melt. There are many times when the Environment Canada data shows 0cm snow, but in the real world there might still be snow piled up along driveways and on boulevards.

Environment Canada also has something that they call "trace" amounts of snow, which means that there is snow, but there is not enough to measure. And the data also has some holes in it - missed readings, or suspicious 0s. It's a little tough to distinguish truly snowfree days from the trace amounts and from the errors. So for the data today I've had to do a little more cleanup and massaging than I normally do.

But even with that, there are some stories here:
  • October snow is pretty common - there's been at least a little bit in 15 of the last 21 years. But October snow doesn't usually stick around - the years with lasting snow were 2001, 2003, 2004 & 2006. 

  • If you zoom way in on this chart, you can see that in years like 2001 and 2004 the late-October snow melted down to the "trace" amounts that I was talking about. And those trace amounts lasted several weeks before the next snow.

  • The big blip of snow that we got in early-October 2016 was unusually large for that time of year, but it disappeared within about a week.

In terms of the first lasting snow, it usually seems to happen in the first or second week of November. And then around the end of November we'll get some big snowfalls that push the snowdepth up above 10 or 20cm.

First Lasting Snow

Here are my best approximations of the dates for the first lasting snow for each year, going back to 1961 when the recordings at the Edmonton International began.

On this chart I've included a line for October 14, which is when we got the second big snowfall for 2016. Early October this year was very cold, and with 9.5cm of snow it seemed like maybe winter had officially started. If that snow had stayed it would have been the earliest lasting snow ever recorded - just beating 2004 and 1984. And in fact, on October 14 there was still snow from our first snowfall on October 7th, so 2016 would have been really, really early. But in the end it did all melt off within about a week.

In this chart there are years like 2001 and 2004 which had October snowfalls that melted down to trace amounts, and which then stayed like that for a several weeks. For those years I'm counting the trace amounts as lasting snow, because it probably looked and felt like winter at the time.

There are also a few years here - 1997, 1999, 2005 - which had 0cm snowdepth recorded at the end of December. So if it all disappeared by New Year's, was there really any "lasting" snow? I'm going to say that yes there was. Those years all had earlier snowfalls that hung around for a few weeks, so in those cases I've used the early dates.

Here is a closer look at a few of those tricky years:


1997, 1999 and 2005 all have zero snowdepth listed on New Year's Day. So for 1997 I'm using the December blip as its lasting snow. For 1999 I'm taking the mid-November snowfall, and for 2005 the early December one. And with 2004 you can see the 3-week stretch that had trace amounts of snow, but right before that there had been a month of snow, so there I've counted the big mid-October snowfall as the lasting snow.

For these years I think it's fair to take the early dates, because there was probably some snow hanging around in windrows or in shaded areas.

Breakdown by Week

Here is a weekly breakdown of when lasting snow actually does show up. The data for all years going back to 1961 is in blue, and the orange bars are for just the last 10 years.

Historically the first week of November was the most likely time for the first lasting snow, happening in 17 years or 31% of the time. The rest of the weeks of November are all at 15%, and then things drop off for both October and December.

In the last 10 years its been a bit different, with the third week of November being the most frequent at 30%. 10 years is a small sample size though, so 30% is just 3 years. But this at least refreshes my memory of recent history.

As I've said, these numbers aren't going to be perfect and there is some room for interpretation, but it gives an idea of what's typical.

First Snow versus First Lasting Snow

Finally, here is a comparison of the first snowfall (>1cm) and the first lasting snow.

The biggest gap between the two is 1992. Its first snow was on August 21, but then the first snow that stuck around wasn't until more than 3 months later on December 3 (in between there were a few snowfalls, but they all disappeared).

Looking at all of the years the average gap - mean and median - is a little under a month at around 26 days. Speaking as a winter cyclist, I know that the first snowfall every year scares me into putting the studded tires on my bike. And then I'm frequently disappointed for the next few weeks because there's nothing but bare pavement.

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