Cold snap is here: A meteorologist explains why temperatures in Calgary drop so quickly

A cold snap forced temperatures to -20 C by mid-day Thursday after a daytime high of 8 C the day before, reinforcing a stereotype about Calgary’s weather — it is unreliable.

One day, you’re strolling to the supermarket in a sweater and a light jacket, relishing the sun; and the next, you’re huddling at home, wearing a sweater, or drowning in layers as you run a list of unavoidable errands and wonder: What just happened?

Meteorology has the answer. It mostly concerns Arctic winds, but it’s a bit more complex.

Natalie Hasell, a meteorologist with Environment and Climate Change Canada, points to a phenomenon called the jet stream — a rapid, narrow band of air current high in the atmosphere flowing east to west that also marks the separation between warmer and colder air masses.

“The jet stream is sitting south of Alberta,” she said, meaning very cold air is making its way even into the southern parts of the province. Expect wind chills throughout the day, although the colder air could ease up at night.

The weather will persist until Saturday, when temperatures, as is usually the case in Calgary, swing to the opposite end of the spectrum in the winter season. That’s mainly due to Chinook winds and fluctuations in the path of the jet stream.

“So if you can imagine the jet stream moving over so that we’re not in the trough of the jet stream anymore, but actually under the crest of the jet stream, which is when southern Alberta will have much warmer air,” Hasell said.

By Wednesday, Calgary will go back to being cold again, with temperatures plummeting below -15 C.

“Then the temperatures don’t recuperate very much from there. But that’s looking at the consensus of the models,” Hasell said. “There are some models that are much warmer, some models that are much colder.”

However, this is nowhere near the coldest the city has been during this time of year. On the same day in 1922, Calgarians experienced temperatures below -30 C. Similar weather was felt in 1891, 1919 and 1949.

For those looking for solace amid the cold, we might have a white Christmas.

“A white Christmas, which is defined as two centimetres of snow on the ground, looks very likely for the Calgary area,” Hasell said. “(It’s) not so much likely if you’re close to the American border, though, so depending on where you are in the province, it might be a little too warm or a little too dry in areas further south.”

Precipitation in the city has been picking up after a dry spell in September and October, when Calgary received 0.2 millimetres and 2 millimetres of rain, respectively, a far cry from normal levels of 37 millimetres and 17 millimetres. Precipitation in November neared the average, and as of Dec. 11, we’re on track to meet average levels for the month.