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Avalanche Forecast

Archived

Feb 18th, 2021–Feb 19th, 2021

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

Northwest Coastal.

20-30 cm new snow by the end of the day on Friday combined with strong southwest wind will increase the avalanche danger quickly throughout the day. The storm and wind slabs will be most reactive where they sit on cold facets, surface hoar and hard wind affected surfaces.

Confidence

Moderate - Uncertainty is due to the timing, track, & intensity of the incoming weather system.

Weather Forecast

THURSDAY NIGHT: Cloudy, 10-15 cm new snow and up to 30 cm in the very south of the region, strong southwest wind, temperature low -5 C.

FRIDAY: Cloudy, 10-15 cm new snow, strong southwest wind, temperature high -3 C, freezing level at 700 m.

SATURDAY: Cloudy, 15-20 cm new snow, strong to extreme southwest wind, temperature high 0 C, freezing level rising to 900 m.

SUNDAY: Cloudy, 25-35 cm new snow, strong southwest wind, temperature high +1 C, freezing level at 1000 m.

Avalanche Summary

Whumpfing was reported by a party on Wednesday (see this MIN report). Many natural wind slab avalanches of size 2-2.5 were observed on lee terrain features and cross-loaded slopes. Several small dry and wet loose avalanches were reported on steep solar aspects. 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 and this layer might become reactive with the increasing snow load.

Snowpack Summary

20-40 cm of recent snow sit on top of a variety of older snow surfaces consisting of facets and crust, hard wind slabs, scoured slopes and sastrugi. Below treeline, 20-50 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 40-80 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 changing conditions today, storm slabs may become increasingly reactive.
  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • 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

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.