Thursday, 7 January 2016

#4, #3 & #2 - Top 10 Weather Stories of 2015 in the Winnipeg Area

#4 - May Long Weekend Storm System

The strongest low pressure system of the year, a Colorado Low, slammed southern Manitoba during the May Long Weekend. Heavy rain, damaging wind, large waves on the lakes and even snowfall occurred, making for one of the worst May long weekends in recent memory weather-wise.

The rain in Winnipeg began late on Saturday May 16 and continued throughout the entire day on Sunday May 17. In total, 35-50 mm fell in the city. 31.3 mm fell at Winnipeg airport on May 17 alone, breaking the old rainfall record of 22.9 mm in 1903 for the day. The heaviest rains fell southwest of Winnipeg and in southwestern Manitoba. 50-90 mm fell from Morden to Carman to Melita. Significant overland flooding occurred as a result. Many farm fields were underwater.

The rain was accompanied by very strong winds. Winds were sustained between 47 and 63 km/h at Winnipeg airport for 22-consecutive hours, from 6 am on May 17 to 3 am on May 18. Wind gusts were stronger, gusting at times between 80 and 95 km/h. A peak gust of 93 km/h was recorded in Winnipeg. The wind damaged property, caused power outages and uprooted trees. In addition, larges waves and storm surge occurred on Lake Manitoba and Lake Winnipeg. Some local residents on the south shore of Lake Winnipeg said it was one of the worst storms they've seen on the lake.

Behind the system on Monday May 18, snow and strong wind occurred. Temperatures dipped to freezing in the evening on May 17, causing rain to changeover to snow. Snow continued throughout the night, ending in the morning. The cold and wind combined to produce wind chill values of -9. About a dusting to 3 cm of snow fell in Winnipeg, depending where you were. 2.5 cm was measured in Charleswood on May 18 alone, just shy of the old snowfall record of 3.0 cm in 1963 for the day. A snow depth of 2 cm was recorded in the morning observation, breaking the old record of trace cm for the day. The snow melted in the afternon as temperatures rose above freezing. This was the latest spring snowfall in Winnipeg since 2002 when a trace cm fell on May 23 and the latest snowfall accumulation since 1969 when 0.3 cm fell on June 12. Prior to this year, snow had fallen on May 18 only 4 times since 1872. Heavier snow fell west of the city with a swath of 10-15 cm from Boissevain to McGregor to Teulon to the Gimli area.

A full summary of the storm was posted on A Weather Moment.

#3 - Warm and Moist Fall

It was a very warm fall across southern Manitoba (likely is partly thanks to a strong el nino). All three months finished in the top 30 warmest on record since 1872 in Winnipeg: September was 6th warmest, October tied 30th warmest and November tied 16th warmest. As a whole, it was the 4th warmest fall on record since 1872 with an average mean temperature of 7.3°C, tied with 1953 and 2009. Brandon also had a 4th warmest fall on record (since 1890), averaging 6.2°C.

Top 10 warmest falls (Sep-Oct-Nov) in Winnipeg since 1872 (by average mean temperature):
1.  8.6°C (1963)
2.  7.9°C (1931)
3.  7.5°C (1923)
4.  7.3°C (1953, 2009, 2015)
7.  7.2°C (1948)
8.  7.1°C (1914, 1920)
10. 7.0°C (1922)

The warmth in September was already discussed previously in the top 10 series and will not be rediscussed here.

In October, the most significant warmth was during the Thanksgiving weekend. Temperatures reached the low to high 20's on Saturday the 10th and Sunday the 11th. Although Winnipeg didn't break any records, many parts of southern Manitoba did. The warmest temperatures were seen along the US border where records highs near 28°C occurred in Pilot Mound, Morden and Sprague.Winnipeg reached a maximum of 24.6°C on the 11th. Even some weak thunderstorm activity occurred in the evening on the 11th, ahead of a strong low pressure system which produced damaging winds the following day. October as a whole saw 22 days above 10°C, above the normal of 17 days.

In November, warm conditions continued. An incredible 25-consecutive day streak of above normal temperatures occurred from October 26 to November 19. During this time, 10-consecutive days from October 27 to November 5 never dropped below freezing. Warmth and high humidity returned mid November before winter-like conditions arrived. Temperatures exceeded 10°C from the 14th to 16th. A high of 13.1°C on the 15th was just a couple degrees shy of the old record of 15.6°C in in 1939. Unusually high humidity also occurred with dewpoints reaching 9.4°C on the 16th and 9.3°C on the 17th, both daily record highs. These were also the latest occurrences of dewpoint over 9°C on record since 1953. Thanks to the high humidity, a record high minimum temperature of 5.3°C was achieved on November 16, the warmest daily minimum for so late in the year.

In the end, dewpoint temperatures averaged 3.2°C in fall 2015 (September to November). This made it the second most humid fall on record since 1953. September dewpoint temperatures averaged 9.8°C, the third highest on record. November dewpoint temperatures averaged -2.9°C, the highest on record, beating the old record of -3.0°C in 1981.

Thanks to warm conditions, the first accumulative snowfall did not occur until November 18 in Charleswood, Winnipeg's official station for snowfall reports. This was the 4th latest first snowfall accumulation of the season since 1872. It was also a month later than the 1981-2010 normal of October 18. In addition, no snowfall was recorded in October, only the 16th time this has occurred since 1872.

#2 - August 22-23 Severe Thunderstorms and Heavy Rains

Significant thunderstorms and heavy rain pummelled Winnipeg and the surrounding area on August 22 and 23. Due to extreme nature of the event and the large area affected, the event is considered number two in the top 10 series.

The event began overnight on August 22 when severe thunderstorms developed north of a warm front. The bulk of the storms remained north of Winnipeg and so the city was spared the worst. The city did get a spectacular lightning show however. Cottage-country north of Winnipeg and a few towns northwest of the city took the brunt of the storms. Very large hail fell from Westbourne to Teulon to Matlock to Beaconia to Silver Falls. The largest hail stone reported was 10 cm in diameter, located near Silver Falls. Significant and widespread damage occurred as a result. Locally 40-50 or more mm of rain also fell within an hour, causing overland flooding. The following table gives a chronology of storm reports from the event.

Table: Chronology of storm reports from the overnight thunderstorms of August 22
Approx time
2:00 am
Nickel to golf ball size hail.
4:15 am
Quarter size hail with significant accumulation. Tree, garden and vehicle damage occurred. 43 mm of rain in 1 hour caused overland flooding. Mounds of hail still remained on the ground hours after the storm.
4:45 am
5 cm diameter hail.
5:15 am
Grand Marais
Golf ball size hail.
5:30 am
Baseball size hail punched holes in roofs and totalled almost every vehicle left outside during the storm. 50+ mm of rain also fell within an hour.
5:45 am – 6:00 am
Pine Falls, Powerview and Silver Falls area
Nickel to softball size hail. One photo showed a 10 cm diameter hailstone.

Heavy thunderstorms developed again midday in the Red River Valley. Winnipeg was particularly hard hit by these. 50 mm fell in less than 1 hour in parts of the south end. Nickel to toonie 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 caused a few fires and wind snapped branches.

The afternoon thunderstorms also dumped some large hail in other areas. Nickel to quarter size hail fell in Oakbank, Winkler and Jessica Lake. Training thunderstorms continued to pummel southeastern Manitoba late afternoon and evening, dumping significant rainfall. Rain from these also spread into the Winnipeg area and Red River Valley. Wrap-around rain around the low pressure system continued to drench south and southeastern Manitoba in the overnight and morning on August 23. Two-day rainfall totals were significant. Widespread totals of 50 to 110 mm occurred throughout the Red River Valley, southeastern Manitoba and the Interlake. In Winnipeg, close to 75 mm fell in southern sections of the city. At Winnipeg airport, about 46 mm fell. 38.4 mm of this fell on August 22 alone, breaking the old record of 38.1 mm in 1959 for the day.

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