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Avalanche Forecast

Archived

Jan 19th, 2022–Jan 20th, 2022

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

Regions

Purcells.

Reactive wind slabs may build on Thursday with new snow, strong wind and warmer temperatures.

Be mindful of the lingering deep persistent slab problem. Caution on shallow snowpack areas and thin to thick steep rocky zones.

Confidence

Moderate - Uncertainty is due to the extreme variability of wind effect on the snowpack.

Weather Forecast

Thursday Night: Increasing cloud cover later in the evening with isolated flurries. Ridgetop wind light from the southwest and alpine temperatures near -10. 

Thursday: Cloudy with new snow 5-10 cm. Moderate to strong ridgetop wind from the southwest. Alpine temperatures warming to near -3 and freezing levels valley bottom.

  

Friday: Mix of sun and cloud. Ridgetop wind light from the West. Alpine temperatures near -5 and freezing levels 1100 m.

Saturday: Sunny with some clouds. Alpine temperatures near -4 and freezing levels 1200 m.

Avalanche Summary

On Wednesday, explosive cornice control triggered isolated wind slabs up to size 2 on the slopes below. These did not pull deeper slabs.

Over the last week, the early December persistent weak layer has produced a few large natural avalanches (2-3) in the center of the region. Explosive controls have also produced a few size 2 avalanches that failed on the same persistent weak layer, one being remotely triggered. These occurred on thin to thick features on northeast-facing slopes from between 1600 m and 2000 m, where the layer was buried 100 cm + deep. Although avalanche activity has tapper off, it remains a concern across the region.

Snowpack Summary

The recent storm snow has been redistributed on alpine lee slopes and open areas at treeline forming reactive wind slabs on leeward slopes. Around 1900 m and below, a crust is now capping the dense 10-30 cm of snow which has settled significantly with the past mild temperatures. The crust varies from thin/breakable to thick/supportive, according to the aspects and elevation. Below the recent snow, a layer of feathery surface hoar and melt-freeze crusts are found, which seems to be still reactive in isolated areas.

The most notable layer of concern in the snowpack is a crust that was formed in early December and is now down 90-160 cm. This layer was reactive last weekend producing many large size 3 avalanches from all aspects and elevations. Activity has tapered, however, warming, cornice fall, and human triggers from steep, rocky slopes, with a shallow or thin-to-thick snowpack may wake it up again. 

Terrain and Travel

  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • In areas where deep persistent slabs may exist, avoid shallow or variable depth snowpacks and unsupported terrain features.
  • Be aware of the potential for large avalanches due to the presence of a deep persistent slab.

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.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.