Wednesday 30 September 2015

The Return of Thunderstorms; a Humid and Stormy Summer in 2015

After 3-4 years of ''thunderstorm drought'' in the Winnipeg area, Summer 2015 featured the return of more normal amounts of thunderstorm activity. There were 20 thunderstorm days at Winnipeg airport from June to August, close to the 1981-2010 normal of 19 days. It was also the first summer to have above normal thunderstorm activity since 2007 when we also had 20 thunderstorm days. Numerous severe thunderstorm events produced large hail, tornadoes and flooding rains across southern Manitoba, some of which will be discussed later in this post. Humidity was also a big story this past summer with unusually high dewpoints, despite only seasonal temperatures overall.

Normal Temperatures

With an average mean temperature of 18.7°C, summer 2015 was just 0.3°C above normal. Each month was within one degree of normal. It was a typical summer in terms of temperatures as a result.

There were 8 days over 30°C from June to August at Winnipeg Int'l airport, slightly below the normal of 12 days. In fact, the airport station did not reach its first 30°C of the year until July 26, the second latest on record since 1872. We came close to 30°C several times before, reaching 29°C 9 times before July 26. It seemed like the airport had a bit of a cold bias. The Charleswood station reached 30°C on 16 days this summer and had reached its first 30°C on May 26. The Forks reached its first 30°C on July 10 and reached 30°C on 13 days.

High Humidity

With an average dewpoint of 13.6°C, summer 2015 tied with 1996 for 4th most humid since 1953. This was 1.0°C above the normal of 12.6°C.

Top 10 most humid summers (Jun-Jul-Aug) since 1953 (by average dewpoint temperature):
1.   14.3°C (2010)
2.   14.1°C (2012)
3.   13.7°C (2005)
4.   13.6°C (1996, 2015)
6.   13.5°C (1955, 1995, 1998)
9.   13.4°C (2001)
10. 13.3°C (1963, 1983, 2003)

In the last 20 years, 9 years saw a top 10 most humid summer since 1953. Summers have become increasingly humid since the 1990's and this is easily seen in the graph below. The graph shows the top 20 most and least humid summers (red and blue dots) and the 30-year running mean (green line). Whether or not the increasing humidity is a trend that will continue is unknown. With only 63 years of dewpoint records, there simply isn't enough data to see if there were similar humid periods in the past.

Graph showing a trend of increasingly humid summers in Winnipeg. 1980's normal summer average dewpoint was about 11.5°C. Today's normal is now close to 12.8°C.

July and August were particularly humid. With an average dewpoint of 16.1°C in July, it was the second most humid July and month on record since 1953. Only July 2012 was more humid with an average dewpoint of 16.5°C. August tied with 2003 for 14th most humid with an average dewpoint of 13.7°C.

22 days from June to August saw dewpoints reach 20°C, the second most on record since 1953. The most was in 2001 when 26 days saw dewpoints reach 20°C. The 1981-2010 normal is just 10 days. 14 of these 20°C dewpoint days occurred in July, tied with 1957 for most on record in July since 1953. 8 daily high and daily high minimum dewpoint records were broken. Most impressive was on Aug 15 when the dewpoint reached 24.5°C, smashing the old record of 21.7°C in 1972 for the day. It was also the latest occurrence of dewpoint over 24°C, 10 days later than the previous record set in 2001.

Summer of Tornadoes

2015 featured Manitoba's most active tornado season in more than 5 years. At least 11 tornadoes and waterspouts occurred. There may have been more because some tornadoes go unreported and also because storms on July 18 and 27 produced multiple tornadoes. At least 2 tornadoes/waterspouts occurred on July 18 (according to photos) and at least 3 tornadoes occurred on July 27 (according to Justin Hobson's chase story).

Manitoba easily had the most reported tornadoes in Canada in 2015. Ontario was second place at ''just'' 6 confirmed tornadoes (as of Sep 29). Alberta had about 5 and no tornadoes were reported in Saskatchewan. Thankfully, no injuries or fatalities occurred in Manitoba from tornadoes this summer.

In Manitoba, 3 tornadoes and waterspouts were reported in June, at least 6 in July and 2 in August. The July 18 and July 27 tornadoes/waterspouts were the most documented, attracting attention from around the world. On July 18, a landspout tornado near Matlock travelled over Lake Winnipeg, becoming a waterspout. The storm then produced at least one other waterspout. The two waterspouts occurred simultaneously, as seen in the following photo by Wray Pearce.

Twin Water Spouts over Lake Winnipeg at Matlock
Two waterspouts over south basin of Lake Wpg July 18; by Wray Pearce
On July 27, a supercell thunderstorm travelled from Tilston to Virden, producing multiple tornadoes. Some were quite violent as seen on storm chasers' videos. One tornado reached close to a kilometre wide. Luckily, the strong tornadoes dodged all major communities and narrowly missed Virden. Only a few farms experienced some damage. Environment Canada sent a damage survey team to investigate. The worst damage they saw was from a high-end EF-2 tornado (winds close to 200 km/h). It is entirely possible that the tornadoes may have been even stronger, but when/if they were they weren't hitting anything significant. Photos and video of the tornadoes can be seen by following this link.

Severe Thunderstorm Events

After 4 years of marginal thunderstorm seasons in parts of southern Manitoba, 2015 saw a spike in severe thunderstorm activity. Other than tornadoes, hail and flooding rains were also big stories of 2015. Of the three Prairie provinces, Manitoba saw the most severe hail and thunderstorm rain reports. The majority of severe hail reports were in August partly thanks to significant nocturnal thunderstorm events late month. In fact, half of all severe hail reports this summer were in August. As for severe thunderstorm rain events, almost three quarters of them occurred in July. The increase in activity this year was despite a very slow start to the season. No severe thunderstorm reports were received in May. In fact, Winnipeg Int'l A didn't get its first t-storm until June 7, the 3rd latest start to the season since 1953. Below is a summary of the major t-storm events of the summer across Manitoba.

@bscomet22: RT @Conway30789074: Roseisle, MB / June  27/ 2015
Roseisle June 27. Photo by @Conway30789074
June 27 - A supercell thunderstorm developed west of Carman in the mid afternoon and quickly dived south into North Dakota. Significant hail and wind damage occurred with the storm from Roseisle to Miami to south of Winkler. Roseisle was hardest hit with wind-driven baseball-size hail (wind gusts estimated at 100 km/h). Some trees were knocked down, leaves were shredded and crops were damaged. Hail also smashed windows, dented vehicles and punched holes into home siding. Miami lived a similar fate with golf ball-size hail, 100 km/h winds and over an inch of rain. Wind was the concern near Blumenfeld and Haskett (south of Winkler) with yard sheds and swings knocked over and trees/branches knocked down. While all this was happening, another strong thunderstorm was developing north of Arborg. As it moved south, it dropped tennis ball-size hail in Fraserwood and loonie-size hail in northeast Winnipeg.

tree smashes car
Winnipeg July 4. Photo by John Pelechaty
July 4 - A widespread area of thunderstorms formed along a trough of low pressure from southwestern Manitoba to the Interlake throughout the afternoon. Training storms in the Woodlands, Fraserwood and Gimli areas dumped over 50 mm of rain in just 1 hour. The storms in Fraserwood lasted close to 4 hours, dumping up to 95 mm according to an unofficial spotter. Overland and street flooding were widespread in the area. Large hail was reported in Arborg, Macdonald, Minnedosa and Elkhorn. Elkhorn was hardest hit with golf ball-size hail, causing vehicle and plant damage. Storms then organised into a bow echo which raced through Winnipeg and the RRV in the early evening. Wind gusts of 90-100 km/h were reported in southwest Winnipeg and southeast of the city. Uprooted trees, fallen branches and other damages occurred. More storms developed behind the line in the late evening, dropping up to golf ball-size hail around Steinbach.

St-Pierre-Jolys Manitoba storm thunderstorm weather
St. Pierre Jolys July 15. Photo by Tristan Field Jones/CJOB

July 15 - Numerous severe thunderstorms developed through southwestern and south-central Manitoba in the afternoon. Large hail was not a big story but toonie-size hail did fall east of Dauphin. Heavy rain in slow-moving storms and damaging winds were the main story. The west side of Brandon was hard hit late afternoon with 50-60 mm of rain in just 1 hour, causing severe street flooding. Training storms also reportedly dumped about 100 mm in a few hours east of Deleau. Separate storms produced wind gusts of 100 km/h near Morris and dumped 50-75 mm of rain around St. Pierre Jolys and St. Malo in just 2 hours.
Vita Aug 12. By Shirley Zaporozan

August 12 - Severe nocturnal thunderstorms impacted the Onanole area near Riding Mountain Nat'l Park around 7 am. Hail up to golf ball-size damaged vehicles and damaging winds knocked over dozens of trees. The wind also tore part of a home's roof and blew it 300 metres away. Residents in the area thought they had witnessed a tornado, but given the situation it is more likely that it was a microburst. Storms then moved into the Neepawa and Gladstone areas between 8 and 11 am, dropping quarter to golf ball-size hail which damaged crops and vehicles. Widespread thunderstorms continued their trek southeastward throughout the afternoon, dropping loonie-size hail near Plum Coulee and loonie to golf-ball size hail around Vita and Caliento.

August 22 - Heavy thunderstorms brought flooding rains on August 22 to Winnipeg. Up to 50 mm fell in less than 1 hour in parts of the south end. Quarter-size hail, wind gusts to 80 km/h and frequent lightning also occurred, making for quite a storm to remember. Numerous streets and underpasses flooded and some were impassable. Water seeped into and flooded parts of St Vital Mall. Lightning also caused a few fires. The daily total rainfall at the Winnipeg airport was 38.4 mm, breaking the old record of 38.1 mm in 1959 for the day. Over 50 mm fell in the south end, making it one of the wettest days since 2010.

In addition, severe nocturnal thunderstorms also occurred during the overnight hours early on August 22. Winnipeg was spared the worst with only minor rainfall amounts and frequent lightning. Cottage country north of the city was not as lucky. A swath of quarter to softball size hail fell from Teulon to Matlock to Beaconia to Pine Falls. Major damage occurred, especially in the Grand Marais to Beaconia region where one person reported that every single vehicle outdoors during the storm was severely damaged. 50+ mm of rain also fell in the area within 1 hour, making for quite a storm. The largest hail reported was a 4-inch diameter hail stone near Powerview.

Altona Aug 28. Photo by Colleen Otto Harder.
August 28 - Large and damaging nocturnal hailers occurred southwest of Winnipeg. Toonie-size hail and larger fell from Austin to St. Claude to Roseisle to Plum Coulee to Altona. The Plum Coulee to Altona region was hardest hit with hail up to baseball-size. Major damage occurred in Altona with broken windshields and siding, flattened and shredded plants/trees, damaged crops and dented vehicles. The storm dumped about 26 mm of rain, causing some street flooding.


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