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Avalanche Forecast

Archived

Feb 27th, 2020–Feb 28th, 2020

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

Regions

South Rockies.

Warm temperatures and sunny breaks could weaken the upper snowpack. Really watch your exposure to overhead hazard, it would be easy to be surprised by weird avalanches on Friday.

Confidence

Low - Uncertainty is due to the timing or intensity of solar radiation and its effect on the snowpack. Uncertainty is due to the fact that deep persistent slabs are particularly difficult to forecast.

Weather Forecast

THURSDAY NIGHT: Cloudy, moderate southwest wind, freezing level drops to 1000 m overnight with alpine temperatures around -4 C.

FRIDAY: Mix of sun and cloud, freezing level climbing to 2000 m in the afternoon, moderate wind with strong gusts from the southwest. Increasing cloud and light snow starting in the evening.

SATURDAY: Cloudy with 3-8 cm of new snow, moderate to strong wind from the southwest, freezing level drops from to 2000 to 1400 m.

SUNDAY: Mix of sun and cloud, light wind from the west, freezing level reaches 1500 m in the afternoon.

Avalanche Summary

On Thursday a few small cornice falls were observed near Elkford, one of which triggered a size 2 slab on the slope below. A few small (size 1-1.5) slab avalanches were triggered by skiers on Tuesday and Wednesday, including a small storm slab that may have failed on surface hoar. 

The most recent deep persistent slab activity we know about is described in this MIN report from February 11 and this MIN report on February 9. This layer may become reactive again with warming temperatures and the appearance of the strong late winter sun.

Snowpack Summary

The 10 to 35 cm of snow from Sunday/Monday has been absolutely hammered by wind Wednesday as evidenced in this MIN submission. Wind was strong enough to get into below treeline features even. Wind stiffened slabs now rest on a variety of old surfaces including a firm crust on solar aspects, soft faceted snow, possibly surface hoar in sheltered areas, and old wind slabs near ridge crest. 

A thick rain crust sits 30-60 cm below the surface and can be found up to 2100 m. Avalanche activity was last observed at the faceting interface between this crust and overlying snow on February 17. 

The mid-pack is well settled and strong, but the base of the snowpack contains basal facets that are most prominent in shallow rocky start zones.

Terrain and Travel

  • Avalanche hazard is expected to to increase througout the day, think carefully about your egress.
  • Avoid freshly wind loaded features, especially near ridge crests, roll-overs and in steep terrain.
  • Use extra caution around cornices: they are large, fragile, and can trigger slabs on slopes below.
  • Recent wind has varied in direction so watch for wind slabs on all aspects.

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.