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Avalanche Forecast

Archived

Dec 21st, 2018–Dec 22nd, 2018

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

Regions

South Columbia.

Avalanches are most likely on steep and convex terrain features, especially those that have been loaded by the wind.

Confidence

High.

Weather Forecast

FRIDAY NIGHT: Mostly clear skies, moderate wind from the northwest, alpine temperatures drop to -14°c. SATURDAY: Clear in the morning then increasing cloud throughout the day, light wind from the south, alpine high temperatures around -6°c. SUNDAY: Scattered flurries with 5-10 cm of snow, moderate wind from the southwest, alpine high temperatures around -6°c. MONDAY: Mostly cloudy with isolated flurries, light wind from the southwest, alpine high temperatures around -6°c.

Avalanche Summary

Isolated natural avalanche activity was reported on north aspect in the alpine during the windy storm on Thursday. Skiers were able to trigger small slabs on all aspects in the 10-30 cm of most recent snow. None of the avalanches reported on Thursday or Friday involved persistent slab problems, but earlier in the week there were a few large persistent slab avalanches (size 2-2.5) that were triggered remotely from adjacent slopes (most recently on Wednesday in the Valhallas). We can't rule out the possibility of deeper layers still being reactive to human triggers, as these layers are still adjusting to the weight of the new snow.

Snowpack Summary

Strong wind from the southwest has formed fresh wind slabs in the alpine and around treeline. A week of stormy weather has deposited 70-100 cm of snow above a weak layer of facets (sugary snow), surface hoar (feathery crystals), and a sun crust (on south aspects) that formed during the dry spell in early December. This layer has been responsible for large persistent slab avalanches over the past week, particularly on north and east facing slopes above 1900 m. The potential exists for smaller avalanches to step down and trigger this deeper layer, resulting in very large avalanches. Another weak layer from mid-November is now buried up to 120 cm, but has been less active recently. It could potentially remain a concern on steep south-facing slopes where the surface hoar sits on a sun crust. At the base of the snowpack is a crust that formed in late October. The probability of triggering this layer is low, but the most suspect areas would be steep rocky alpine features with a shallow snowpack.

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.