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'Broad trough' causes unseasonably cool temperatures for May

Lone Sheep Publishing
By Lone Sheep Publishing
June 8th, 2022

The Southeast Fire Centre monthly weather synopsis said that a “broad trough”dominated the upper flow pattern for most of May causing temperatures to remain unseasonably cool for most days. 

The monthly weather synopsis said mean monthly temperature was more than two degrees below normal and was the coolest mean monthly temperature since 1996. 

“Not since 1977 did temperatures fail to reach 25 degrees at some point during the month of May,” the monthly weather synopsis said.

Total rainfall was 18% below average despite four more days than normal with measurable rain.

The La Niña influence

Senior Meteorologist Brett Anderson for AccuWeather, an American company providing commercial weather forecasting worldwide, said a moderate La Niña phase is expected to persist into the summer, 

Anderson said during a period of La Niña influence, western Canada is typically the portion of the country that experiences the most significant impacts.

La Niña is one of many teleconnections meteorologists study to determine the weather outlook on a seasonal scale.

La Niña conditions tend to influence the orientation and position of the jet stream over North America.

In this case, the projected position of the jet stream this summer is forecast to funnel more moisture over much of British Columbia.

“It’ll be wetter than normal across the northern part of the province with near-normal rainfall expected in the southern portion,” Anderson explained.

Anderson expects air temperatures will be near normal for much of British Columbia, as water conditions in the Canadian Pacific are expected to be cooler than normal. Cooler waters just offshore will likely exert a notable influence on the air that flows inland over the province.

The hottest portion of the summer in Vancouver, British Columbia’s most populous city, occurs in mid-to late July when high temperatures in the low 70s F (21-22 C) are common. The summer of 2021 saw the city climb well above normal each month in the summer and end the season at 4.8 F (2.7 C) above the average for the time period.

In terms of precipitation, over the course of a typical summer, Vancouver averages about four inches (100 mm) of rain from June through August.

This works out to about 1.25 to 1.50 inches (32 to 38 mm) of rainfall each month during meteorological summer.

Environment Canada is calling for few days of mixed sun and cloud before showers take over for the remainder of the week and into the weekend.

Temperatures are forecasted for the high teens or low 20s.

Broad trough: 

The global-scale average wind flow pattern at high elevations over the mid-latitudes is from west to east when averaged over the course of a year.  However, the day-to-day upper wind flow pattern is usually not straight out of the west as an endless series of waves of varying size circum-navigate the globe.  When the West Kootenay is positioned under the portion of the wave called the ‘trough’, cooler air is drawn towards the area from the cooler northern latitudes.  A broad trough refers to the broad east-to-west extent of the feature.

Categories: General

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