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Avalanche Forecast

Archived

Feb 19th, 2021–Feb 20th, 2021

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 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.

Regions

South Rockies.

Strong westerly wind might have formed reactive wind slabs in the southeast of the region. Persistent slabs continue to be easily triggered by 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 the speed, direction, or duration of the wind and its effect on the snowpack.

Weather Forecast

FRIDAY NIGHT: Cloudy with clear periods, moderate west wind, temperature low -13 C.

SATURDAY: Mostly cloudy, trace of new snow, moderate west wind, temperature high -7 C.

SUNDAY: Mostly cloudy, 5 cm new snow, strong to extreme southwest wind, temperature high -2 C, freezing level rising to 1700 m. 

MONDAY: Cloudy, 10 cm new snow and rain at lower elevations, strong to extreme southwest wind, temperature high +1 C, freezing level at 1900 m.

Avalanche Summary

Several explosives triggered slab avalanches up to size 2 and several natural and skier triggered dry loose and wind slab avalanches up to size 1.5 were reported on Thursday. 

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. 

Snowpack Summary

5-15 cm recent snow sit on top of wind affected snow surfaces in the alpine and open areas at treeline and a facet layer which is up to 20-30 cm thick in sheltered areas. 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 newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Watch for signs of instability like whumpfing, hollow sounds, shooting cracks or recent avalanches.
  • 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.