Friday 2 October 2015

Yet Another Summer-Like September

2015 continued the trend of warming Septembers in southern Manitoba. Winnipeg Int'l A averaged 15.8°C, the 6th warmest September on record since 1872. This was also the warmest since 2009 and the second warmest since 1948. The following table lists the top 10 warmest Septembers since 1872:

Top 10 warmest Septembers in Winnipeg since 1872 (by average mean temperature)
1.  17.7°C  (2009)
2.  16.8°C  (1948)
3.  16.6°C  (1940)
4.  16.3°C  (1897)
5.  16.1°C  (1906)
6.  15.8°C  (2015)
7.  15.6°C  (1931, 1967)
9.  15.4°C  (1920, 2013)

This September was the 4th consecutive warmer than normal September and the 17th warmer than normal September in the last 22 years. The last time we had a September that was over 1.0°C below normal was in 1999. Since then, 9 Septembers (out of 17) averaged over 1.0°C above normal. The 30-year September average ending this year (1986-2015) rose to 13.3°C, an increase of 0.4°C since the 1981-2010 normal and an increase of 1.0°C since the 1971-2000 normal. This was also the warmest 30-year period for Septembers since 1872.

Henteleff Park on a warm and sunny September evening

The following graph shows how average mean temperature in September has varied since 1872. The top 30 warmest and coldest Septembers are indicated with red and blue dots. Interestingly, we have not had a top 30 coldest September since 1993. Since then, 9 Septembers were in the top 30 warmest. The green line shows the previous 30-year average which gives an idea of how the September normal has changed. It shows a rapid warming in the last decade. This doesn't mean the warming will continue forever because as you can see, the normal goes through up and down periods.


Early September Heat Wave Brought Record Humidity

Right off the hop, September greeted us with a heat wave in its first few days as temperatures exceeded 30°C for 3 consecutive days. Some locations exceeded 32°C on these 3 days, reaching official heat wave status. However, the main story was the humidity as record high dewpoints moved into southern Manitoba. In Winnipeg, dewpoint reached 23.1°C on September 3, the second highest dewpoint in September since 1953. This came very close to the all-time September high of 23.4°C on September 2, 1983. The minimum hourly dewpoint observation for the day was 18.7°C, an all-time high minimum dewpoint for September. The previous record was 17.2°C on September 3, 1960 and September 10, 2005. Three other high maximum and high minimum dewpoint records were broken on Sep 4th and 5th.

The heat and humidity combined to bring some of the highest humidex values on record in September. Humidex exceeded 40.0 on both the 2nd and the 3rd. This was the most 40+ humidex days on record in September. In fact, there had only been 4 other occurrences of humidex over 40 in September since 1953. Humidex peaked at 42.0 on September 2, the second highest in September since 1953. The following table lists the only occurrences of humidex over 40 in September since 1953:

Occurrences of humidex over 40 in September since 1953:
1.  45.9  (Sep 2, 1983)
2.  42.0  (Sep 3, 2015)
3.  41.4  (Sep 5, 1978)
4.  41.2  (Sep 2, 1960)
5.  40.4  (Sep 6, 2013)
6.  40.1  (Sep 2, 2015)

Interestingly, half of the occurrences of humidex over 40 in September occured in the last 3 Septembers.

Humid Month Overall

With an average hourly dewpoint observation of 9.8°C, September 2015 was the 3rd most humid since 1953. Three days had dewpoints exceed 20°C, tied with 1970 and 2005 for second most since 1953.

Drenching Thunderstorms September 4-5

Heavy thunderstorms on September 4th and 5th marked the end of the early-month heat wave. The event started in the early morning between 1 and 8 am on September 4. Nocturnal thunderstorms dropped large hail south and east of Winnipeg with dime to toonie-size hail reported from around Niverville to Cooks Creek to Selkirk. Large hail also fell northwest of Minnedosa around Strathclair and Shoal Lake where quarter to loonie-size hail was reported. The hail was enough to cause some minor crop damage according to reports on social media.
Strathclair area Sep 4. By Ricky Memryk   
Additional thunderstorms moved up the Red River Valley in the afternoon on September 4, producing strong winds and flooding rains. The Winnipeg area was one of the hardest hit areas as heavy thunderstorms moved in mid afternoon, plunging the city into night-like darkness. Generally 30 to 50 mm of rain fell in less than 1 hour, causing significant street flooding. Wind gusts up to 83 km/h were also recorded at Winnipeg Int'l A. The rain and wind combined caused visibilities to drop below 1 km. Behind the storm, additional heavy rains fell until evening. Daily totals were generally between 40 and 60 mm in Winnipeg. The airport recorded 41.1 mm, breaking the old record of 36.8 mm in 1872 for the day. Similar totals close to 50 mm were also recorded in the southern RRV near Letellier.
Winnipeg Sep 4. By Felicia Wiltshire
Heavy thunderstorms moved up the RRV once again late day on September 5. Generally 15 to 30 mm of rain fell south and east of Winnipeg. Locally higher totals near 40 mm fell near Letellier. Additional rains later in the month brought monthly totals to over 110 mm in the Letellier area.


  1. Great write-up Julien. Very interesting how consistently warm Septembers have been over the past 2 decades. Summers getting longer? Nice thought. Now if we can combine that with consistently shorter winters.. I'd be a happy man! :)

    1. Thanks Rob! Since Augusts have also warmed slightly (not as much as Septembers), it does seem like summers have been lasting longer into the fall. On the other hand, we haven't had a decently warm May in a long time so it seems like they are also starting a tad later.