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Avalanche Forecast

Archived

Feb 20th, 2021–Feb 21st, 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 and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
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

Lizard-Flathead.

Reactive wind slabs have formed in lee features and avalanche hazard will likely increase through the day. Smaller wind slabs could easily step down and trigger larger persistent weak layers. Seek out sheltered and low angle terrain for the best and safest riding.

Confidence

Moderate - Uncertainty is due to the speed, direction, or duration of the wind and its effect on the snowpack. Uncertainty is due to how buried persistent weak layers will react with the forecast incoming weather.

Weather Forecast

SATURDAY NIGHT: Flurries, up to 5 cm, moderate southwest wind, temperature low -11 C.

SUNDAY: Cloudy, 5-10 cm, strong southwest wind, temperature high -2 C, freezing level rising to 1500 m.

MONDAY: Cloudy, 20-30 cm with rain at lower elevations, strong southwest wind, temperature high 0 C, freezing level at 1700 m.

TUESDAY: Cloudy with clear periods, trace of new snow, moderate northwest wind, temperature high -8 C, freezing level dropping to valley bottom.

Avalanche Summary

On Friday, skiers remote triggered a size 2 persistent slab avalanche on a north east aspect at 1550 m. A reactive wind slab also developed, small (size 1-1.5) avalanches were easily triggered by by skiers in lee features, a size 2 natural wind slab avalanche was also reported on a northeast aspect.

A small persistent slab avalanche was triggered remotely by a rider below treeline (see this MIN report) on Thursday. Whumpfing and shooting cracks were reported by several parties. 

More reports of easily triggered persistent slab avalanches came in on Wednesday, including these large avalanches at treeline on Mt. Fernie. There were also several small loose dry avalanches which were triggered by skiers. 

On Tuesday, a natural cornice failure triggered a small avalanche on the slope below in the alpine. 

On Monday, skiers triggered size 1 avalanches at treeline and in the alpine on northeast aspects. One avalanche was triggered from a short distance away (see this MIN report). On Sunday skiers triggered avalanches up to size 2.5 on open features at the treeline/alpine interface in the Lizard Range and Tunnel Creek. These failed on the late January persistent surface hoar layer (see this MIN as an example). 

Reports on human and remote triggered persistent slab avalanches up to size 2.5 continue since last week. Locations like Mount Fernie, 2000, Liverwurst and McDermid are a few location examples. Some avalanches were triggered from a distance away. 

Snowpack Summary

Southwesterly winds have redistributed loose snow and formed slabs in lee features. Fresh snow now covers old wind slab, a thin sun crust, or layers of faceted snow. Below 1600 m a hard melt-freeze crust is underneath 20-40 cm of recent snow. 

A persistent weak layer lurks 40-70 cm below the surface. In some places it consists of surface hoar, in other places just facets, or crust/facet combinations. This weak interface has been responsible for the majority of recent avalanches. 

A solid mid-pack sits above a deeply buried crust and facet layers near the bottom of the snowpack (150-200 cm deep), which is currently unreactive. 

Terrain and Travel

  • The trees are not the safe-haven they normally are at this time. Terrain at treeline is primed for human triggered avalanches.
  • Carefully assess open slopes and convex rolls where buried surface hoar may be preserved.
  • Watch for signs of instability like whumpfing, hollow sounds, shooting cracks or recent avalanches.
  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.

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.