Cold Septembers (...and the really cold September of 2018)

There's still a week left in September, and when it is finally over we will do the whole, ugly, month-in-review.

But for today, we're going to look at other cold Septembers and see how this one compares (so far).

The big question is: does a cold September mean a cold winter?

At Edmonton Weather Nerdery we never try to forecast things - that's for the actual experts. But we will see if historically there's any connection between a cold September and the winter that follows. We did something similar last year in: Does a cold November mean a cold Winter?

High Temperatures

This chart is just a reminder of our High temperatures so far.

At the beginning of September the average High is around 20°C, and over the course of the month that slides down to 13°C. We're in the part of the year where average temperatures start to drop pretty quickly.

But for this month we've only had 3 days above the average - the 5th, 6th and 7th. All the rest have been below, and sometimes waaaaay below. We set an all-time record for coldest-High on September 13th at 1.3°C, as well as 5 other recent (since 1996) coldest-Highs. Those temperatures are about 15°C below average.


Low Temperatures

For the Low temperatures the range isn't as large at the Highs, but we've still spent most of the month well below average.

One thing to notice here is that the historic (since-1880) record Lows are shown as the light blue line, and we have generally still been about 5°C above those.


Warmest and Coldest Septembers

So how does 2018 compare to previous years? This chart shows Edmonton's warmest & coldest Septembers going back to 1880. This uses the daily Mean temperature (rather than the High or Low) averaged for the month.

This chart uses the 20th century average as a reference, because it's a handy comparison when looking at long-term trends. In Edmonton in September the average daily mean temperature for September for the 20th century was 10.8°C. In recent years we've usually been above that average, with 2011, 2012 and 2013 all ranking in the top-10 warmest Septembers at around 14°C. A few of the recent cold~ish Septembers were 2002, 2004, 2005 & 2010 which were down at around 10°C.

As of September 22nd this year is sitting at 7.7°C, which makes it Edmonton's 8th coldest September on record. That also means it has averaged about 6°C colder than years like 2011-2013, and about 4°C colder than last year. The "as of Sep 22" calculation also makes the assumption that the rest of this month will be average, which it probably won't be, and so on the 30th we'll see if we've fallen far enough to catch 7th place 1941 at 7.4°C

The last time that we had a September this cold was 1972, which was Edmonton's 4th coldest at 6.9°C. And 1983 through 1986 were also all pretty chilly, with 1985 being the most notable in 9th place at 7.9°C.

Edmonton's coldest recorded September was 1926, way down at 5.4°C. That was twice as far below the average as 2018 has been.


Highs & Lows

The previous chart compared the daily Mean temperature for September. The daily Mean is the average of the Highs and the Lows, and in this chart those have been split up to see where 2018's cold temperatures are coming from.

Looking at the line for the Highs, the averages have warmed up just a little bit since 1880. The current average is about 17°C, and this year we've been way down around 11°C. The last time our average High was that cold was 1972. Looking at the red reference line which appears in this chart we can see that 2018 is actually in the bottom-5 coldest Highs, with only 1972, 1965, 1934 and 1926 below it.

The Low temperatures have been noticeably trending upward over time. An average Low of 7°C has been quite typical recently, whereas a century ago 3°C was more common. And 3°C is about where we are this year, which makes it the coldest average Low since 1985.

One of the things to take from this chart is the difference between the Highs and the Lows. 2018's Highs and Lows are 6°C and 4°C below average, respectively. But because of how those averages have changed over time the Highs for 2018 are 5th coldest while the Lows are 31st coldest.


The Other Months

This chart shows the average Mean temperatures for July, August, September, October, & November.

We've seen that right now September's average Mean is around 12°C. July's is 17°C, and that drops just a bit to 16°C for August. But then it's a pretty big step down to 12°C for September, and then it plummets down to 5°C for October. And lets not even talk about November.

September 2018 at 7.7°C is still warmer than an average October, but what that means is that even a really warm October won't make up for this cold September. And that's the worst thing about a cold September - it's our last chance for summer. October might still have a warm day or two, but at this point summer is sadly over.

But what about winter?


Does Cold September = Cold Winter?

In one of the previous charts we saw that Edmonton's 10 coldest Septembers have been (from coldest to warmest): #1 1926, #2 1965, #3 1934, #4 1972, #5 1901, #6 1884, #7 1941, #8 2018 (tentatively), #9 1985, & #10 1895.

This chart shows those 10 cold Septembers, and the months that came afterwards. The bars represent how warm or cold each month was relative to the average for that month. The chart slowly cycles through the bottom-10 Septembers, and here are some highlights:

• The winter of 1926-1927 was consistently cold, although September was the most abnormally cold month.
• 1965-1966 had a really warm October, a cold~ish November, and a really, really cold January.
• 1934-1935 also had a lot of cold months, although that February was very warm.
• 1972-1973 had a cold December, but a really warm January and March.
• 1901-1902 had a really warm October, December, and January, and no other really cold months.
• 1884-1885 was mostly very cold (although it was also 134 years ago), but it had a warm November and a very warm March.
• 1941-1942 had a warm November & March, and a really warm January.
• 1985-1986 had a spectacularly cold November, but then a really warm December, January, & March.
• 1895-1896 had a cold January and a really warm February.

We can see what that all adds up to in the next chart:


Cold September ≠ Cold Winter

The severity of our winters has changed a lot over time, and so the definition of a "cold" or "warm" winter needs to adapt a bit depending on the year. In this chart we can see how the winters for those other bottom-10 Septembers ranked compared to their neighbours:

• 1926-1927 & 1965-1966 were both colder than the average for the time, but neither were particularly extreme.
• 1934-1935, 1884-1885, & 1895-1896 were all around the average for their eras.
• 1901-1902, 1941-1942, 1972-1973, & 1985-1986 were all quite a bit above average.

And so of these 9 other cold Septembers, 2 had cold winters, 3 had average winters, and 4 had warm ones. So yay?


Recent Cool~ish Septembers

The bottom-10 Septembers that we looked at are all quite a long time ago, so here we're looking at 5 more recent years which had cool~ish Septembers: 1992, 1996, 2004, 2005, 2010:

• 1992-1993 had a really cold December. cool January, & cold February.
• 1996-1997 had a really cold November & December, and a really warm February.
• 2004-2005 had a cool October, but then the rest of the winter was warm.
• 2005-2006 had a really warm winter, including a spectacularly warm January.
• 2010-2011 had a cold December & March.

And here's how they compared to other winters:

And so for those 5 recent Septembers:

• 2004-2005 & 2005-2006 were both followed by warm winters.
• 1992-1993, 1996-1997, & 2010-2011 were all followed by really cold winters.

And that's a little less good, with a 40%/60% warm/cold split.


Summary

Here at Edmonton Weather Nerdery we don't make predictions. But based on these reviews of previous cold Septembers, for the question "Does a cold September mean a cold winter?" it appears that the answer is "Not Really."

For the 5 cool~ish recent Septembers we looked at, 3 had a cold winter and 2 had a warm one. Those odds aren't great, but when we looked at other historically-cold Septembers things flipped. Of the other 9 coldest Septembers in Edmonton's records only 2 had cold winters, 3 were average, and 4 were warm. And for 2018-2019 we'll just need to wait and see.

Next week we'll be back with the full month-in-review to see where September finally ended up.