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A cloud larger than Mount Everest spun over Calgary on Thursday. It was a supercell thunderstorm.

A tornado reportedly touched down just outside the city

A timelapse shows an ominous supercell thunderstorm moving over Calgary, Alberta, Canada on July 23, 2020. (Video: Craig Blumhagen)
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Imagine looking into the sky and watching an object bigger than Mount Everest spin. It seems apocryphal, right? But for the 1.4 million residents of Calgary, Canada, that was exactly the sight that loomed overhead Thursday as an ominous — and beautiful — supercell thunderstorm spun through.

The rotating thunderstorm prompted a tornado warning during the midafternoon hours. At 2:36 p.m., Environment Canada warned of a confirmed tornado, stating “an eyewitness reported a possible tornado near Springbank at 2:30 p.m. [MDT].” Springbank is about 10 miles west-northwest of Calgary.

“Damaging winds, large hail, and locally intense rainfall are also possible,” read the warning.

Kyle Brittain, a meteorologist and Calgary bureau chief at the Weather Network, captured the likely tornado west of Calgary at 2:32 p.m. local time.

Another Twitter account posted a video that appears to show the tornado on the northwest side of the city around the time the tornado warning was issued. Other tornado warnings were issued for storms in Saskatchewan.

It appears that the tornadic thunderstorm first developed around or shortly after lunchtime east of the Canadian Rockies between Peter Lougheed Provincial Park and Banff National Park. Differential heating caused by the irregular terrain can help pockets of air to rise, resulting in “convective initiation,” or the blossoming of showers and thunderstorms.

That region of Canada is its own miniature tornado alley, but it sees its violent storms later in the year than the United States does. The storms follow the clash of the seasons as warm air builds north.

That keeps most rotating thunderstorms relegated to the Gulf Coast during February and March, the southern Plains during April and May, the High Plains in May and June, and southern Canada and Montana during June and July.

In mid-June, Calgary was hit by a supercell that dropped tennis-ball-size hail on the city and brought significant flooding that inundated vehicles. At least one confirmed tornado occurred nearby.

In June 2019, a number of picturesque supercells spun up over Banff National Park, their barber-pole swirling updrafts drifting whimsically across the prairie landscape.

Towering corkscrew-shaped storms swirled over Alberta in June 2019

Thursday’s remarkable supercell in Calgary was an “LP,” or low-precipitation, supercell. That means its structure was such that any rain and hail fell primarily outside of its updraft, leaving the corkscrew plume of rising air visible from all directions.

That’s the eerie rotating column videos reveal sweeping into Calgary. Ahead of the counterclockwise-rotating storm, warm southerly “inflow” races in to feed moisture to the tempest. On its backside, a shot of cool, dry, dense air from aloft cascades to the surface. That’s known as the “rear flank downdraft.”

The delicate intermingling of both opposing air masses can help tighten rotation beneath the updraft, squeezing it into a slender column that becomes a tornado.

The most visually striking supercells are often the low-precipitation ones. Farther south in the southern United States, supercells are more common — and frequently more potent — but lower cloud bases and increased precipitation obscure their structure.

Check out these stunning shots of Thursday’s epic storm: