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Avalanche Forecast

Archived

Jan 12th, 2022–Jan 13th, 2022

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

Regions

Purcells.

A rising freezing level up to 2500 m Wednesday night in the east of the region will result in very dangerous avalanche conditions with warmth, some sunshine and a persistent weak layer in the snowpack.

Confidence

Moderate - Uncertainty is due to how buried persistent weak layers will react with the forecast incoming weather. Uncertainty is due to difficult to forecast freezing levels.

Weather Forecast

Wednesday night: Mostly cloudy, up to 3 cm new snow, moderate to strong southwest wind, treeline low around -2 °C, freezing level rising to around 2500 m in the east of the region and staying around 1100 m in the west of the region.

Thursday: Mix of sun and cloud, trace of new snow, moderate southwest wind, treeline high around 0 °C, freezing level around 2200 in the east of the region and around 1400 m in the west of the region. 

Friday: Mix of sun and cloud, trace of new snow, light southwest wind, treeline high around -3 °C, freezing level around 1000 m.

Saturday: Mix of sun and cloud, trace of new snow, strong southwest wind, treeline high around -3 °C, freezing level around 1200 m.

Avalanche Summary

On Tuesday, a large (size 2) natural storm slab avalanche was observed. Skiers and explosives triggered many small avalanches up to size 1.5.

On Monday, two cornices failed naturally and triggered a storm slab resulting in size 2 and 2.5 avalanches. The cornices did not step down to the early December persistent weak layer. Explosives triggered a small wind slab avalanche of size 1.5. 

On Sunday, a natural size 2 wind slab avalanche that released in extremely steep terrain was observed. A few small (up to size 1.5) wind slab and dry loose avalanches were triggered by skiers.  

An increasing amount of large, persistent slab avalanches is being reported since Sunday. Almost all the reported avalanches over the past week ran on the early December weak layer. Deeply buried persistent problems like these don't go away overnight, and it remains a serious concern. 

  • On Monday, a skier triggered a size 2 persistent slab avalanche below treeline on an E aspect. The avalanche ran on the early December layer.
  • On Sunday, two size 3 and 3.5 deep persistent avalanches were triggered with explosives up to 1 m deep. They released at an elevation around 1900 m on SE and SW aspects. And a persistent slab avalanche 2 m deep and 200 m wide was reported in this MIN report. 
  • Last Tuesday just east of the region in the Rockies near Invermere, a size 3.5 (very large) persistent slab avalanche was triggered remotely by a skier on an alpine ridge top. The resulting crown was 2-3 m deep and the avalanche ran about 1 km to the valley bottom. More details in this MIN report. 

Notable persistent slab avalanches in neighbouring regions feature in our latest blog, Photos of recent persistent slab avalanches in the southern interior.

Snowpack Summary

Up to 20 cm new snow fell with southwest wind and warming temperatures. The new snow sits on wind affected surfaces in exposed and open areas. In sheltered areas the new snow might sit on a weak layer of feathery surface hoar. Below that 40-90 cm of previous storm snow sits on variable and potentially weak snow surfaces including widespread facets, wind affected snow, and/or surface hoar up to 3 mm in sheltered 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. Activity on this layer has been sporadic. It has generally been triggered on steep, rocky slopes, with a shallow or thin-to-thick snowpack. This layer has created a low likelihood, high consequence scenario which is best managed through conservative terrain choices and disciplined backcountry travel techniques.

Terrain and Travel

  • Minimize overhead exposure; avalanches triggered by warming or cornice fall may be large and destructive.
  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Be aware of the potential for large avalanches due to the presence of a persistent slab.
  • 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.

Cornices

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.

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.