Avalanche Forecast

Issued: Feb 9th, 2017 4:23PM

The alpine rating is considerable, the treeline rating is considerable, and the below treeline rating is considerable. Known problems include Storm Slabs and Persistent Slabs.

jsmith, Avalanche Canada

Kootenay Boundary.

Touchy storm slabs reactive to human triggers. Use small slopes with low consequence to test the bond of the storm snow.

Summary

Confidence

High -

Weather Forecast

Friday: 5-10 cm new snow / Strong, southwesterly winds / Freezing level around 1200 m.Saturday: Mostly cloudy with flurries / Moderate, westerly winds/ Freezing level hovering around 1000 m.Sunday: Mix of sun and cloud / Light to moderate, southwesterly winds / Freezing level around 1000 m.

Avalanche Summary

On Wednesday, there was an explosive triggered size 3 persistent slab avalanche south of Nelson. The avalanche was triggered in a thin spot and the crown was 250 cm deep in some places. The weak layer was sugary facets sitting on a crust that was buried in mid-November. This is a good reminder that buried weak layers can remain dormant for months and then awaken like a sleeping dragon. It is possible for storm slab avalanches to step-down to these deeper weak layers resulting in very large and destructive avalanches. Thursday's storm has created touchy storm slabs that will remain likely to human trigger on Friday.

Snowpack Summary

Another 20-30 cm of snow fell Thursday on top of the 45-85cm of recent storm snow which has created touchy storm slabs at all elevations and aspects. Snowfall amounts have been highest around Kootenay Pass. These slabs have been reported as being very reactive to human triggers and are sitting on a variety of surfaces; including scoured surfaces in wind exposed terrain, surface hoar(size 2-3 mm) in sheltered locations, and sun crust on steep solar aspects. 50-100 cm of snow now overlies the weak layer of surface hoar(feathery crystals) and facets(sugar snow) that was buried in mid-January. Areas with a shallower snowpack (less than 150 cm) have a generally weak snowpack structure with sugary facets near the ground. This includes shallow alpine slopes and most of the Rossland range. It is possible for storm slab avalanches to step-down to these deeper weak layers, resulting in large, destructive avalanches.

Problems

Storm Slabs

Thursday's storm started cold and ended warm. This creates a dangerous condition where warmer heavy snow is sitting on-top colder snow, making storm slab avalanches much more likely.

Watch for signs of instability such as whumpfing, shotting cracks, or recent natural avalanching.Use small slopes with low consequence to test the bond of the storm snow.Use conservative route selection, choose moderate angled and supported terrain with low consequence.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Likely - Very Likely

Expected Size

2 - 3

Persistent Slabs

Storm slabs may step-down to deeper weak layers, resulting in large, destructive avalanches.

Avoid likely trigger spots such as steep and rocky areas with a shallow snowpack.Wait to see how the snowpack responds to increased loads before venturing out into bigger terrain

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible

Expected Size

2 - 4

Valid until: Feb 10th, 2017 2:00PM