2018/10/01

September Review / October Preview

It's time to pull off the band-aid to take a look back at a very, very, very cold September.

Edmonton is obviously not known as a warm city, and it is certainly true that sometimes autumn can show up very early. But today we're going to see that September 2018 was not just a normal, cold September. September 2018 was especially unseasonable, and it was something that Edmonton hadn't seen in 50 or 100 years, if ever. Last week we looked at Edmonton's History of cold Septembers if you are interested in a bit more background.


September High Temperatures

We're at the point in the year where our temperatures are falling rapidly. At the beginning of September the average High temperature is about 20°C, and by the end of September it drops to 14°C. At the end of October it will be down to 5°C, and then -2.5°C by the end of November.

But when we look at the High temperatures for September 2018, the "average" had very little to do with anything. For the whole month there were only 3 days - the 5th, 6th, & 7th - which were above the average, and those are highlighted in red.

The other 27 days were all below average, and are highlighted in blue. We had 8 daytime Highs that were 10°C or more below average. And with all of those really cold days we set one all-time record for coldest High, on September 13th at 1°C. And we also set 6 other coldest-since-1996 Highs.

On this chart it's worth noticing how many of our September Highs weren't just below average for September, but also fall below the averages for October too.



When we look at the numbers, September 2018 is like nothing in recent memory. Our average High was way down at 11.1°C, compared to 18.8°C last year or 20.7°C in 2013. The 2nd coldest September in the last 20 years had been 2004, and it was way up at 14.7°C.

This September only had 3 days which hit 20°C, while last year we had 15, and the average is 10. We also had 15 days with Highs of 10°C or less, while the 2nd-coldest year here was 2006 with 6 days, and the average is only 2 days. And we had 7 days of 5°C or less, with the 2nd-coldest recent years at 2 days, and the average at only 0.4 days.


October High Temperatures

Normally we leave next month's temperatures until the very end of these month-in-review posts, but this time it's worth taking an early look at October.

Our September average High of 11.1°C was just a little bit below last October's average of 11.2°C. And in the last 20 years a warm October has had average Highs around 13°C or 14°C, while a cold October is down around 5°C or 6°C. So September 2018's Highs would fit right in as a slightly warm October. Not even a really warm October - just a slightly warm one.

September's 3 days at 20°C, 21 days below 15°C, and 5 days below 5°C also wouldn't be out of place in a warm~ish October.


September Low Temperatures

When we switch over to Low temperatures it's a similar story. There were a total of 4 days with Lows above the average - the 6th, 7th, 8th, & 26th. We didn't set any records for all-time Lows, but 8 nights hit coldest-since-1996 temperatures.



Looking at the numbers, we had 8 nights below freezing. In the last 20 years we've averaged only 1 frost in September, and the 2nd-most frosts was 2003 with 5. The last time that we had 8 frosts in September was 1985 which also had 8, and before that 1972 had 10. The September with the most recorded frosts was 1926, with 15.

In the past 20 years the average Low has ranged from a warm year like 2009 at 8.7°C down to a cold one like 2010 at 4.7°C. And this year our average was almost 2°C colder than a cold year, down at 2.8°C.


October Low Temperatures

Looking at October Lows, in recent years the monthly average Low has ranged from -2.7°C in 2002 to +2.5°C in 2010. And so this September's very cold average Low of 2.8°C would be a little bit warm for October.

In October the average number of frosts is 13. This September we had 8, which is a lot for September, but would be on the low side for October. Recently the Octobers of 2015, 2014, 2010, 2007, & 2003 were all similar to September 2018 with 7-9 frosts. This September we didn't have any nights which hit -5°C, but in October its typical to have a few of those.


September Ranking

So we've seen that September 2018 was a lot like an October, and that it was much colder than other recent Septembers. But how does it compare to history? In this chart we have the average September temperatures going back to 1880.

The orange & blue bars in the chart are for the Mean daily temperature (the average of the Highs & Lows), and at 6.917°C this was the 5th coldest September ever recorded. And it was essentially in a tie with 4th-place 1972, which was at 6.913°C. The colder years were 1934 at 6.8°C in 3rd, 1965 at 6.3°C in 2nd, and the very-chilly 1926 at 5.4°C as Edmonton's coldest September. The only other recent" September in the bottom-10 was 1985 in 9th place at 7.9°C.

During the 20th century the average-Mean temperature for September was 10.8°C, and so this year we were 4°C below that. And over the years September's average has risen, and recently it's been up around 13°C, so 2018 was 6°C colder than that.

In this chart the High & Low temperatures are also shown as orange & blue lines:
  • 2018's average-High of 11.1°C was the 2nd coldest recorded, falling behind 1926's 10.3°C, but just ahead of 1934's 11.6°C. 
  • 2018's average Low of 2.8°C is about 27th coldest, because Lows have trended upwards quite a bit over the years. The last time that we had an average Low this cold was 1972, at 2.3°C.

One final bit of trivia is that 2018's average-High of 11.1°C is actually below the average-Mean that we've seen in recent years, which had been up around 13°C.


2018 so far...

Looking back over 2018, we had a really late spring with a very cold April. But then at the beginning of the summer we had a really warm May with July-like temperatures (and I hoped that everyone was able to take advantage of it). And now we've had an unfortunately early end to the summer with an October-like September.



And this is what that looked like on a day-to-day basis.

In April we had a number of days down at the very bottom of the range. But then in May temperatures jumped to the very top of the range, and they stayed there through June. July was pretty normal, and then in August the smoke rolled in. And the summer of 2018 came to an abrupt end in our bottom-of-the-barrel September.


Precipitation

We're going to start by talking about Precipitation in general (rain & snow-equivalent), and we'll switch to just talking about snow later on.

This chart shows the total number of days with recorded Precipitation at the International and at Blatchford. And as gloomy as things were in town, Blatchford only record precipitation on 13 days, while at the International it was 18 days. This chart goes back to 1995 as a reference, and in those years the previous highest was 1996 at 16 days. The International's the records go back to 1961, and
2018's 18 days also sets a new overall record beating 1964 and 1978 which both recorded 17 days.


Total Precipitation

In terms of total Precipitation for the month, the International recorded 79mm, while Blatchford was down at 58mm.

The International's 79mm was right up at the top of the recent range for September, coming in just below 80.4mm in 2006. But going back a bit further, in 1984 the International recorded 116.3mm and in 1978 it recorded 141.5mm. So 2018 was a lot, but was still well down from the records.

The International's 79mm also made September the month with the most precipitation so far this year, beating out June's 76mm.


2018 Cumulative Precipitation

And with all of that precipitation in September that brings the International's total for the year to 387.7mm, which is just above average for this time of year. Blatchford has only recorded about three-quarters of that, with a total of 302.1mm.


Snow

It's time to talk about September snow.

You might think "Edmonton is cold and always gets snow in September" but in this chart each of the bubbles represents and early snowfall, and September has usually been pretty empty. We looked at this in more detail in First Frost & First Snow.

But September 2018 is definitely pretty crowded compared to most years (2017 was too), so lets take a closer look.


Snowy Days


The International recorded 8 days with snow this September. That's a lot. Going all the way back to 1961 the previous highest year was 1985 with 5 days of snow. And since 1995 the average had been less than 1 day. For recent years 2017 had been the previous leader with its 3 snow days, but 2018 has changed all that. 8 snow days is right at the average for our real-winter months like November-March.


Total Snow

In terms of total snow, this September the International recorded 38cm. That's a lot. It's more than we got in any of the months last winter. And it's basically twice the average amount for any of the real-winter months: in November the average snowfall is 19cm, in January it's 21cm, in March it's 20cm, etc.


A New Snow Record

And so, 38cm at the International doesn't just set a new record - it blows away the competition. The previous snowiest September at the International was 1965, with 12.9cm. And this September we had 38cm. And that's a bit ridiculous.

This chart switches between total Precipitation, Rain & Snow for September. In 2018 Rain & Snow were fairly evenly split, with 40.6mm of rain and 38.4cm of snow. Typically though, September is much more of a rain-month than a snow-month.



Going even further back into the records, the snowiest September ever recorded at Blatchford was 1926 with 28.5cm. Environment Canada doesn't track snowfall at Blatchford anymore, but the International's 38cm is well ahead of any of the years here.

And here we see that even going back a century or more September has not usually been a particularly snow month in Edmonton.


Summary

Today we had a lot to talk about, because there was a lot going on this September:
  • 2nd-coldest High temperatures and 5th-coldest Mean daily temperatures.
  • More snowy days than we've ever seen in September, more snow than we've ever seen in September, and even more snow than we see in most winter months.
So on the one hand, we might never see another September like this. But on the other hand, the September that we just had was basically a fake-October, and we're about to enter real-October. So we can probably expect another month of what we just had.

If all of that is getting you down, last week we looked at the history of Edmonton's Cold Septembers, and asked: "Does a cold September mean a cold winter?" And the answer was "No". After this historically cold September, the chances that we'll have a warm winter are still about as good as a coin toss.

1 comment:

  1. Wow. Lot to digest here. Will make several comments.

    A: Right now you are using monthly average as your base atom of information. Consider doing an analysis of extremes. Heat waves and cold snaps tend to be under a month long. And in a given month you may get both. I've seen months with both a February Thaw and ad cold snap.

    In 2017 November costed along much like this November has done, with single digit highs, and single or double digit lows. Fairly typical November. Then it went down to -35 in a few days with no snow cover. This is a tough event on plants.

    How common is this sort of event?

    To dig this out you will need dailies instead of monthy data.

    Here's another stat to dig out. If we use either your 1 century base (1900-1999) or the more typical 30 year base, what is the average standard deviation of the daily temp compared to the base? Or perhaps better, the root mean square of the standard deviation. This might be a good measure of "Is the climate getting more variable.

    Animations: I really wish your animations would cycle once then stop, and had a step button. They move too fast for me to comprehend, then move too slowly.

    ReplyDelete