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Avalanche Forecast

Archived

Feb 21st, 2020–Feb 24th, 2020

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

Regions

North Rockies.

An unevenly distributed surface hoar layer in the upper snowpack could catch you out. It's deep enough to cause problems. Factor this into your decision making by choosing smaller terrain features unless you are certain it does not exist in your area.

Confidence

Moderate - Uncertainties in both the snowpack structure and the weather forecast limit our confidence.

Weather Forecast

Friday night: Around 10 cm new snow with strong westerly winds.

Saturday: Mostly dry. Treeline temperatures around -6C. Strong southwesterly winds.

Sunday: Dry. Sunny in the morning but clouding over later on. Treeline temperatures around -5C. Moderate southeasterly winds.

Monday: 2-5 cm new snow. Treeline temperatures around -6C. Light southwesterly winds.

Avalanche Summary

On Wednesday, several size 1.5-2 avalanches were reported from north-facing slopes up to 1 m deep, most likely on a layer of buried surface hoar in the upper snowpack. 

On Monday, a size 2 slab avalanche was reported, this also released on a layer of surface hoar.

A few large cornice released have been noted from the last several days, however I suspect these will become less likely with cooling temperatures and less sunshine.

Snowpack Summary

One or more buried surface hoar layers are buried in the upper snowpack at depths of around 45-65 cm. Buried surface hoar has been confirmed in the Pine Pass, Torpy & Renshaw zones, it may be more widespread throughout the region too. It's now sufficiently deep to result in a large avalanche if triggered, so this layer should be factored into your decision-making process.

The mid-and lower snowpack layers are generally well-bonded, except for shallow and/or rocky start zones which could harbour weak basal snowpack layers. 

Terrain and Travel

  • Watch for signs of instability like whumpfing, hollow sounds, shooting cracks or recent avalanches.
  • Avoid freshly wind loaded features, especially near ridge crests, roll-overs and in steep terrain.
  • Caution around slopes that are exposed to cornices overhead.

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.