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Avalanche Forecast

Archived

Feb 17th, 2021–Feb 18th, 2021

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

Regions

Northwest Coastal.

Strong to extreme southwest wind is in the forecast. The wind will distribute the 5-10 cm new snow falling throughout the day and form fresh reactive wind slabs in lee terrain features in the alpine and at treeline.

Confidence

Moderate - Uncertainty is due to the speed, direction, or duration of the wind and its effect on the snowpack.

Weather Forecast

WEDNESDAY NIGHT: Increasing cloud coverage, 2 cm of new snow, south wind increasing from light to extreme, temperature low -9 C, freezing level at valley bottom.

THURSDAY: Cloudy, 5-10 cm of new snow and 20 cm in the very south of the region around Kitimat, strong to extreme southwest wind, temperature high -3 C, freezing level at 700 m.

FRIDAY: Cloudy, 20-25 cm of new snow, strong southwest wind, temperature high -4 C, freezing level at 700 m.

SATURDAY: Cloudy, 15-20 cm of new snow, strong to extreme southwest wind, temperature high -4 C, freezing level at 800 m.

Avalanche Summary

Several dry loose avalanches up to size 2.5 on steep terrain were reported on Tuesday. 

Natural wind slabs to size 2 have been reported on various wind loaded and cross loaded aspects over the weekend. These wind slabs were formed by the recent outflow winds. Wind slabs have also been reactive to human triggering with ski cuts in the size 1-1.5 range.

On Saturday explosives control triggered cornice and wind slabs, mostly size 1-2 with the odd larger result with big cornices.

It is worth remembering that skiers were able to trigger unsupported pillows failing on surface hoar down 25-40 cm near Terrace last week. Although there are no recent reports of avalanches failing on this layer, it is still propagating in some snowpack tests.

Snowpack Summary

10-20 cm of recent snow sits on top of a variety of older snow surfaces consisting of facets and crust, hard wind slabs, scoured slopes and sastrugi. Below treeline, 10-30 cm of soft snow sits above isolated pockets of surface hoar and a crust which is more prominent on solar aspects. The late January interface is down 30-70 cm and consists of surface hoar in sheltered locations, a crust on solar aspects, and facets and stiff wind affected snow at upper elevations. 

The mid-pack seems to be well settled. Deep persistent layers appear to have mostly become unreactive, except for the Bear Pass area and the far reaches south of Kitimat. 

Terrain and Travel

  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Recent wind has varied in direction so watch for wind slabs on all aspects.
  • Potential for wide propagation exists, fresh slabs may rest on surface hoar, facets and/or crust.
  • Pay attention to cornices and give them a wide berth when traveling on or below ridges.

Problems

Wind Slabs

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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.