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Avalanche Forecast

Archived

Dec 24th, 2018–Dec 25th, 2018

Alpine
Natural avalanches possible, human triggered probable.
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.

We're into a week of stable weather, but a persistent slab problem still warrants conservative terrain choices. Read more in the new forecaster blog here.

Confidence

Moderate -

Weather Forecast

MONDAY NIGHT: Cloudy with clear periods, light wind, alpine temperature drop to -12 C.TUESDAY: Mix of sun and cloud, light wind from the west, alpine high temperatures around -10 C.WEDNESDAY: Mix of sun and cloud, light wind from the west, alpine high temperatures around -12 C.THURSDAY: Mix of sun and cloud, light wind from the northwest, alpine high temperatures around -12 C.

Avalanche Summary

Natural avalanche activity has started to taper off, but large persistent slabs continue to be reactive to human and explosive triggers. On Sunday, a large (size 2.5) persistent slab avalanche was triggered by a snowmobiler in Singing Pass. The avalanche occurred on a wind loaded northwest-facing slope in the alpine. See details in the MIN report. Over the weekend explosive in produced numerous large persistent slab avalanches (size 2-4) in alpine terrain. The avalanches occurred on all aspects and failed on several different weak layers including the early December weak layer and weak facets at the bottom of the snowpack. Over the past two weeks, several large persistent slab avalanches were remotely triggered from skiers on adjacent slopes, particularly in the Golden area.

Snowpack Summary

5-15 cm of low density snow sits above wind slabs in the alpine, and in some isolated areas above small surface hoar (feathery crystals).A weak layer that formed during the dry spell in early December is now 50-100 cm deep. The layer is composed of facets (sugary snow), surface hoar (feathery crystals), and a sun crust (on south aspects). Another similar weak layer is buried 80-150 cm. Finally, the base of the snowpack has weak facets layers at alpine and treeline elevations. All of these weak layers have been producing large avalanches over the past week. Human triggering any of these layer is most likely on slopes that didn't previously avalanche and on slopes that have variable snowpack depth (such as rocky alpine features).

Problems

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.

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.