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Avalanche Forecast

Archived

Feb 11th, 2017–Feb 12th, 2017

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.

Regions

Kootenay Boundary.

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

Confidence

Moderate - Due to the number of field observations

Weather Forecast

Seasonal temperatures and isolated flurries through the weekend. Significant alpine warming Tuesday afternoon. SUNDAY: Cloudy / Light - moderate southwesterly winds / Freezing level around 900 m. MONDAY: Sunny with cloudy periods, warming with highs to +1Celsius / Moderate southeasterly winds / Freezing level around 1200 m. TUESDAY: Mostly cloudy / High temperatures to +3 Celsius / Light-moderate southwesterly winds/ Freezing level near 2000m.

Avalanche Summary

Near Nelson, several persistent slab avalanches to Size 3 (average depths ranging from 60cm to 100cm) were triggered on virtually all aspects above 1900m with explosives on Saturday. See Whitewater Ski Patrol's great video (here) posted on the Backcountry Skiing Canada web site for a concise summary of details to Feb 9th, and some excellent video footage of a Size 2.5 avalanche running on the Goat Slide path near Whitewater (with crown height to 2.5m).

Snowpack Summary

Another 20-30 cm of wet snow fell up to 1900m on Thursday and ended with a 1-2cm thick crust on all aspects below this elevation. Since then, up to 30cm additional snow has fallen (mostly near Nelson) and brings the recent storm snow total to 65-100cm, creating touchy storm slabs at all upper elevations and aspects. Snowfall amounts have been highest around Kootenay Pass. Slabs have been reported as 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. Snowpack tests near the Valhallas have given moderate, propagation-likely results down 50-70cms on the Feb 3rd interface. 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

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.