The July 12 Tornado West of Nanton

Our view of the tornado from near Stavely.

On July 12, 2020, a tornado occurred deep in the rolling terrain of the northern Porcupine Hills about 17km WSW of Nanton on a roughly west-to-east track, between 107-120PM MDT. It impacted one property, ripping the roof off a barn, and destroying a shop - among other damage in the yard.

Meteorological Background Environment and Observations

Severe weather was anticipated in parts of southern and central Alberta on July 12. The parent tornadic supercell occurred in the southern foothills on the cyclonic shear side of a 50 knot 500mb jet streak, with continued cooling aloft during the day. The area was also beneath the left exit region of the upper jet, and north of a stalled cold front/post-frontal dryline, where low level flow had locally begun to turn upslope (southeasterly). Being early in the day, MLCAPE values were marginal (well below 1000J/kg) but ample for robust, low-topped updrafts amid 50 knots of 0-6km shear (with slightly less effective shear) - a profile that favoured highly-tilted storms, and deep updraft rotation. In many ways, the event transpired in a similar fashion to the Priddis tornado event of July 22, 2015.

We observed the tornado from the southeast, near Stavely. Immediately prior to witnessing the funnel, the RFD of the parent storm wrapped in, evaporating cloud matter and bringing about a clear slot. The tornado occurred with this first occlusion, which lasted several minutes, with the next cycle bringing about ominously intense rotation in the wall cloud - perhaps as a result of the storm interacting with the northward-surging dryline. However, the second cycle did not produce a tornado, possibly due to the cold RFD we observed, which was more established than in the first cycle. No subsequent mesocyclonic occlusions occurred, as the storm rapidly lost control of its outflow thereafter.

Modeled data (didn't grab the obs in time). Surface low in NW SK, with W-E boundary draped back across southern AB.

The storm from the SE at 107PM, at the approximate start of the tornado. The funnel is visible within the clear slot. The parent storm is a highly-tilted, low-topped supercell. 

The storm from the SE at 144PM, after which the wall cloud really began to rotate for a few minutes. 

ECCC's initial summary, which classified the tornado as a landspout (I would contest this conclusion!)

The Damage

The property owners were home at the time of the tornado, but were caught off guard at its arrival, since it had been sunny at their location immediately south of the storm. They also lacked a view up the slope to their immediate west, where the tornado entered their yard. They ran inside just in time, as the tornado destroyed a shop, before ripping the roof and gable ends entirely off a well-built barn. The barn was built in 1912, but was sturdy, and housed active horse stables. Aerial footage reveals that just a single roof panel had remained collapsed into the upper level of the barn, with the rest being scattered both aft and to the side of the tornado track. Some of the roofing debris was carried many tens of metres away from the barn. The top plate to which the roofing trusses were affixed appeared to be a double-stacked 2x6.

The barn also shifted on its foundation, with the SW corner being lifted off, and the north wall being warped and having steel anchoring material pop out of the concrete foundation. The barn is now leaning, which is apparent both outside and within the barn. Outside the barn, a portion of the roofing pile drove into the grass aft of the tornado track, becoming deeply embedded into the ground. In the shadow of the immediate downwind (east) aspect of the barn, four horses survived the ordeal, with only one of them receiving some minor scratches.

According to damage indicators, it seems that this classifies as a small barn (under 250 square metres), with a DOD of 5 (uplift or collapse of more than 50% of roof panels). Expected wind speeds associated with this damage intensity are in the neighbourhood of 150km/h (EF1), with a lower bound of 125km/h (EF0) and an upper bound of 185km/h (EF2).

Here is a link to the story on The Weather Network.

Destroyed shop on the SW side of the yard.

Another shop had a chimney ripped off

The roof and gable ends were ripped off this barn.

A "before" photo of the barn from the property owner

Roofing trusses embedded deep into the ground. 

There's a spooky lean to everything inside the barn.

Anchoring material popped out of the concrete foundation on the north side.

Roofing debris tossed NNE of the barn several tens of metres.

Impaled board through upper exterior wall

Aerial view of the yard.

Closer aerial shot of the barn.

Despite being in the vast, open rangeland characteristic of this area, the narrow little tornado still managed to find the barn. The impacted property exists in the middle of the image.