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Avalanche Forecast

Archived

Feb 5th, 2018–Feb 6th, 2018

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

Regions

Kootenay Boundary.

Storm slabs have been reactive to human triggers, resulting in some surprisingly large avalanches. Several weak layers are lurking deeper in the snowpack: Now is a good time to choose simple terrain, free from overhead hazard.

Confidence

Moderate - Due to the number of field observations

Weather Forecast

Tuesday: Isolated flurries (5 cm possible) / Moderate west winds / Freezing level around 900mWednesday: Snow (10-15cm) / Moderate west winds / Freezing level near 1200mThursday: Snow (10-15cm) / Moderate westerly winds / Freezing level around 1400mNote: Confidence is low for forecast wind values and precipitation amounts on Wednesday and Thursday.

Avalanche Summary

On Sunday in the north west part of the region, skiers triggered a size 2.5 avalanche that likely stepped down to the mid January layer. See the MIN report for more details. On Sunday, ski cutting near Nelson produced many soft slab avalanches to size 1.5 on down wind (lee) features that had seen additional snow loading due to wind.Looking forward, expect newly formed storm slabs and the persistent slabs beneath them to remain reactive to human triggers with the potential for very large and destructive avalanches.

Snowpack Summary

Each of the past three days has seen locally heavy snowfall (eg: 15cm in 5 hours at Kootenay Pass on Sunday) coupled with moderate to strong winds at upper elevations. Storm snow totals vary from 25-50 cm and the winds created reactive wind slabs at tree line and above (see Avalanche Summary above). Critical instabilities are buried well below the surface: See this video for a summary of conditions near Nelson. A mixture of weak surface hoar and/or a crust from mid-January is buried beneath all the storm snow at 90-120cm deep. Numerous recent avalanches to size 2.5 have been reported on this layer and large, destructive avalanches at this interface remain a concern.Below this layer lies a second crust/surface hoar interface buried early-January that is now 120-140 cm deep. Several recent avalanches have stepped down to this layer.The mid-December surface hoar layer is buried 130-160 cm below the surface. This layer has continued to produce step down releases and "sudden" test results. It is most pronounced at tree line, but is also present below tree line .

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.