: Sun, trees and snow

Jet stream brings weather extremes to the Kootenays

By John Boivin, Valley Voice, Local Journalism Initiative Reporter

It wasn’t so much the broken temperature record that excites Southeast Fire Centre Fire Weather Forecaster Jesse Ellis about December’s weather.

It was the way that the record was broken.

On Dec 1, temperatures hit +14.6C, breaking a new record for the warmest December day ever (the previous had been +12.5 in 1996). 

With the thermometer hitting -18.9C on the last day of the year, it also made for the widest coldest day-warmest day difference in December in 40 years.

The wild swing is due in part to the waves of the jet stream growing more pronounced – the wave top reaches a higher latitude than normal, while the trough bottom goes farther south. In December, that reached a peak, Ellis says. 

“By the end of the month, a large upper ridge strengthened over the Pacific and a deep upper trough set up over western North America,” says Ellis, noting it was likely the deepest ridge-trough waveform for the planet’s northern hemisphere at that time. He says the prevailing northerly or northwesterly flow between these two features helped support the southward shift of a modified Arctic airmass into the area. That brought much colder temperatures by the end of the month (as well as a period of strong northerly winds).

“That’s why we had the cold air pushing south, and the really cold temperatures at the end of the month,” explains Ellis.

Other than the “luck of the draw” of the warmest first day, and the wild swing of the jet stream, there was little else of note in December, Ellis says. 

The lowest temperature reached in December was far below the record set in 1968 (18.9 C last month vs the record of 30.6 C).

But the colder weather did affect overall temperature readings. The mean monthly temperature of -4.2 was also lower than average, and Ellis notes it prompted more rain than snow to fall than usual.

“With the cooler than average temperatures, the usual pattern… allowed there to be 40% more snow than normal despite total monthly precipitation being 22% below average,” Ellis notes.  “Total precipitation falling as rain was less than 10% of normal.”

Ellis says the weather pattern is shifting into the classic La Nina pattern, and we should see cooler, sunnier weather in the coming weeks.

‘We’re going to see a significant pattern in the mid-month, a week when we have high-pressure dominating,” he says.

Top photo credit: Sun, trees and snow – by John Brighenti via Flickr (CC BY SA, 2.0 License)