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Avalanche Forecast

Archived

Feb 16th, 2021–Feb 17th, 2021

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

Regions

Purcells.

Watch for hard wind slabs in unusual places at upper elevations and avoid steep rocky areas with a thin to thick snow cover if you travel in avalanche terrain on Wednesday.

Confidence

Moderate - Confidence is due to a stable weather pattern with little change expected.

Weather Forecast

TUESDAY NIGHT: Mainly cloudy, isolated flurries with a trace of snow, light northwest wind with moderate gusts at ridgetop, alpine temperatures around -15 C. 

WEDNESDAY: Mix of sun and cloud, light northwest wind, alpine temperatures around -13 C. 

THURSDAY: Mostly clear, light south wind with strong gusts in the afternoon, alpine temperatures around -11 C.

FRIDAY: Mainly cloudy, isolated flurries with a trace of snow, light west wind, alpine temperatures around -10 C. 

Avalanche Summary

Since Monday, there have been several reports of small dry loose avalanches initiating from rocks on solar aspects or in steep terrain.

Over the weekend, observers reported large cornice falls and several small to large (up to size 2) natural and skier-triggered wind slab avalanches on westerly aspects above 2400 m. An explosive triggered avalanche reportedly stepped down to a deeper layer. 

Earlier in February, there were a few reports of large avalanches (size 2.5-3) on southeast aspects in the alpine, likely releasing on a layer of facets on a crust (see a photo here), and a large human-triggered avalanche (size 2.5) on surface hoar near Quartz Creek.

Snowpack Summary

Winds have varied in strength and direction over the past week, loading cornices and forming shallow wind slabs at upper elevations that may be possible to trigger. Watch for these wind slabs in unusual places as complicated cross-loading and reverse-loading patterns are prevalent. Wind slabs may overlie weak, sugary, faceted snow, meaning that they could break wider than expected and will likely be slow to bond. 

In wind-sheltered areas, the upper snowpack is becoming soft and faceted with the cold temperatures. A weak layer that formed in late January is now 30-60 cm deep. In the northern Purcells this layer has been reported as a surface hoar layer at treeline and below treeline elevations, but a combination of facets on crusts could exist at all elevations throughout the region. 

The northern Purcells also have an older surface hoar layer that can still be found 60-100 cm deep at treeline. Additional weak layers may exist near the bottom of the snowpack and should be considered in steep rocky areas with a shallow or thin to thick snowpack.

Terrain and Travel

  • Recent wind has varied in direction so watch for wind slabs on all aspects.
  • Approach lee and cross-loaded slopes with caution.
  • Be aware of the potential for large avalanches due to the presence of buried persistent weak layers.
  • Avoid shallow, rocky areas where the snowpack transitions from thick to thin.

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.