Kootenay Boundary

Recently formed storm slabs are expected to remain reactive to human triggering on Wednesday. The region continues to deal with a tricky persistent slab problem and very large avalanches remain possible. Conservative terrain selection remains essential.

A break in the weather is expected on Wednesday before the next system arrives overnight bringing snowfall and wind for Thursday. 

Tuesday night: Periods of light snowfall 2-5 cm, light variable wind, treeline temperature around -10 °C. 

Wednesday: A mix of sun and cloud with a chance of flurries in the morning, light NW wind, treeline high around -10 °C. 

Thursday: Snowfall 10-15 cm, moderate to strong SW wind, treeline high around -8 °C. 

Friday: Periods of light snowfall, moderate SW wind, treeline high around -4 °C. 

On Monday, visibility was limited but reports include natural storm slabs and loose dry avalanches up to size 2. Skiers and explosives were also triggering storm slabs and loose dry avalanches up to size 2. Just north of the region in the South Columbia, a natural size 2.5 persistent slab avalanche was reported on a NE aspect at 2200 m failing on the early-Dec layer down 80 cm. 

On Wednesday, recently formed storm slabs are expected to remain reactive to human triggering. These new slabs may sit on a weak interface which may result in them persisting longer than normal. The early-Dec persistent slab problem remains a serious concern and the weight of the new snow may have increased the reactivity of these deeper layers. Smaller storm slab avalanches also have the potential to step down to deeper layers resulting in very large avalanches. 

There has been an alarming pattern of large, persistent slab avalanches being reported each day last week. This blog post goes into additional details on the problem.

On last Friday, a skier triggered a size 3.5 avalanche on Whale's Back near Nelson. The crown was over 500 m wide, the slide wiped out 30 year old timber and the debris piled 6 m deep. The previously tracked slope was triggered from on top of a ridge/rib feature, where the snowpack is likely thinner. This is a good example of smart terrain travel as the skier who triggered it from the rib was not caught in the avalanche. 

On last Thursday, evidence of a natural size 2.5 persistent slab avalanche was observed on a northeast aspect at 2000 m in the Bonnington Range. On last Wednesday, a skier triggered size 2 storm slab stepped down to the early December crust at Kootenay Pass and a naturally triggered size 3 persistent slab was reported in this MIN post from near Rossland. On Tuesday, explosive control work at Kootenay Pass produced numerous persistent slab avalanches failing on the early December crust, up to size 3.5. On Monday December 27, several riders were involved in a human triggered size 2.5 persistent slab avalanche near Nelson. The avalanche occurred on a southeast aspect at approximately 1900 m. The treeline slope was heavily wind effected and the rider triggered the avalanche from a thin spot where the layer was only around 60 cm deep. Here is the MIN report of the incident.

Most of the region received around 40-50 cm of new snow from the recent storm except for areas around Nelson which received 60-70 cm. This new storm snow buried a variable snow surface which is potentially weak and includes widespread facets, wind affected surfaces, and/or surface hoar up to 5 mm in sheltered areas. The new snowfall was accompanied by moderate to strong winds at higher elevations and the new storm slabs are expected to be most reactive in wind loaded areas. 

The early December crust/facet layer has been responsible for numerous very large persistent slab avalanches over the past week. The depth of the crust ranges widely from 80-160 cm and has most recently been reactive in wind affected terrain near ridge crests. Nearly all of the recent avalanches on this layer have been triggered from shallow spots in the snowpack where the crust is less than 100 cm deep. Avoid steep, rocky slopes with a shallow or thin-to-thick transitions in the snowpack.

• Continue to make conservative terrain choices while the storm snow settles and stabilizes.
• Be aware of the potential for large avalanches due to the presence of a persistent slab.
• Avoid steep, rocky, and wind effected areas where triggering slabs is more likely.
• It's critical to stay disciplined and choose only well supported, low consequence lines.