Saturday, 1 August 2015

Warm and Very Humid July

July 2015 was perfect for spending time outdoors and enjoying the many activities we look foreward to in the summer. Generally, temperatures were close to or slightly above seasonal on most days, but consistently high humidity made it feel hotter. This high humidity helped fuel an active month for thunderstorms and heavy rains across southern Manitoba.

The average mean temperature at Winnipeg airport was 20.2°C in July, slightly above normal (+0.6°C) and the warmest since 2012. Despite being above normal, only 2 days exceeded 30°C at Winnipeg airport. The 1981-2010 normal is 5 days. In fact, the first 30°C day of the year wasn't until July 26, the second latest on record since 1872.

Top 5 latest dates to reach 30°C for the first time in a year
1. Aug 16 (2008)
2. Jul 26 (2015)
3. Jul 24 (1882, 1927)
5. Jul 19 (1920)

The lack of 30°C days at the airport was a little surprising and masked the fact that it was a hot July. Most locations around the city were hotter. Charleswood recorded 10 days over 30°C.

The humidity was the main story of July. 14 days saw a dewpoint over 20°C at Winnipeg airport, tying with 1957 for the most in July since 1953. The 1981-2010 normal is 6 days. In addition, the average hourly dewpoint observation was 16.1°C, the second highest in July since 1953. The 1981-2010 normal is 14.2°C.

Top 5 most humid Julys since 1953 (according to average hourly dewpoint observation)
1. 16.5°C (2012)
2. 16.1°C (2015)
3. 15.5°C (1966, 1989)
5. 15.4°C (1955, 1983, 2007, 2010)

Top 6 most 20°C dewpoint days in July since 1953
1. 14 days (1957, 2015)
3. 12 days (1955, 2001, 2005)
6. 11 days (1966)

The dewpoint peaked at 24.1°C on July 12, breaking the old record of 23.9°C in 1955 for the day. This occurred during an 8-day streak with maximum dewpoints over 20°C. Two other maximum dewpoint records were broken during the month (22.5°C on July 27 and 22.4°C on July 28).

Dark as night during a t-storm near 7 pm July 4 in Wpg
Thunderstorms were also a big story in July with 8 thunderstorm days at Winnipeg airport, the most in July since 2005 and the most for any month since June 2007. The worst of the storms missed Winnipeg however. Rainfall was slightly below normal for the month as a result. Many parts of southern Manitoba got over 100 mm of rain in July, including 178 mm in Portage la Prairie. Even isolated portions of southwest Winnipeg got a little over 100 mm. Some of the heaviest rains were in the Morris to St. Pierre Jolys area where over 200 mm fell.

Another memorable event was the July 27 southwestern Manitoba tornado event. Full summary of the event is on A Weather Moment.