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Avalanche Forecast

Archived

Feb 18th, 2021–Feb 19th, 2021

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

Regions

South Rockies.

Slabs on the late January persistent weak layer continue to be easily triggered by humans. For skiers and riders in the Elk Valley the Lizard-Flathead forecast with danger rating Considerable at treeline is more appropriate.

Confidence

Moderate - Uncertainty is due to how quickly the snowpack will recover and gain strength.

Weather Forecast

THURSDAY NIGHT: Cloudy with clear periods, trace of new snow, moderate westerly wind, temperature low -13 C.

FRIDAY: Mostly cloudy with sunny periods, trace of new snow, moderate westerly wind, temperature high -7 C.

SATURDAY: Mix of sun and cloud, trace of new snow, moderate southwest wind, temperature high -5 C.

SUNDAY: Mostly cloudy, 3 cm new snow, moderate to strong southwest wind, temperature high -1 C, freezing level rising to 1700 m. 

Avalanche Summary

On Tuesday and Wednesday a few wind slab avalanches up to size 2 were triggered by explosives and released naturally. 

Several wind slab avalanches up to size 2 were triggered by skiers and released naturally at treeline and in the alpine on Monday. Human triggered avalanches were reported north of Crowsnest Pass. The avalanches were triggered on east aspects on mellow slopes in the alpine and at the treeline/alpine interface (see this MIN report). The failure plane of these avalanches is likely the late January persistent weak layer consisting of surface hoar, sugary facets or a crust/facet combination. 

Several skier triggered wind slab avalanches up to size 1.5 were reported on Sunday on usually and reverse loaded terrain features. 

Snowpack Summary

A few centimetres of new snow sit on top of wind affected snow surfaces, especially in the alpine and open areas at treeline, and a facet layer which is 20-30 cm thick in places. A persistent weak layer lurks 40-65 cm below the surface. In some places it consists of surface hoar, in other places just facets, or crust/facet combinations. 

A solid mid-pack sits above deeply buried decomposing crust and facet layers near the bottom of the snowpack. Though unreactive under the current conditions, steep rocky slopes and shallow snowpacks should still be approached with caution.

Terrain and Travel

  • 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.
  • Recent wind has varied in direction so watch for wind slabs on all aspects.

Problems

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.

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.