Kootenay Boundary

Newly formed storm slabs are expected to be touchy on Tuesday. The region continues to deal with a tricky persistent slab problem and very large avalanches remain possible. Conservative terrain selection remains critical.

Unsettled conditions are expected on Tuesday in the wake of the storm. Periods of light snow and sunny breaks are both possible. 

Monday night: Snowfall 5-15 cm, light to moderate SW wind, treeline temperature around -10 °C. 

Tuesday: Periods of light snow up to 5 cm and a chance of sunny breaks in the afternoon, light to moderate SW wind, treeline high around -10 °C. 

Wednesday: A mix of sun and cloud with a chance of flurries in the morning, light to moderate N wind, treeline high around -8 °C. 

Thursday: Snowfall 5-10 cm, moderate to strong SW wind, treeline high around -10 °C. 

Early reports from Monday include natural loose dry avalanches up to size 2 and explosive triggered storm slabs up to size 2. On Sunday, a skier triggered a size 1 wind slab on an E aspect at treeline which was 30 cm thick. 

On Tuesday, recently formed storm slabs are expected to remain reactive to human triggering. These new slabs sit on a weak interface and may be very reactive, especially in wind loaded terrain. The persistent slab problem remains a serious concern and the weight of the new snow may increase 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 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. Also on Friday, another skier triggered avalanche was reported at Apex mountain, just west of the region. This one was a remotely triggered wind slab and the skier was caught but uninjured. 

On 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 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.

The new storm snow buried a variable snow surface which is likely 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 and 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 in the past week. The depth of the crust ranges widely from 70-160 cm and has most recently been reactive in wind affected terrain near ridgetops. 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.

The lower snowpack is composed of several early-season crusts. It is these basal crusts that are the primary concern for persistent slab avalanches in thin snowpack areas near Rossland.

• 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.