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Avalanche Forecast

Archived

Dec 16th, 2019–Dec 17th, 2019

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

Regions

South Columbia.

Storm snow accumulating over the next few days will gradually increase the load on a buried layer of surface hoar crystals. As a result, expect to see an increase in the likelihood of human triggered avalanches.

Confidence

Moderate - Uncertainty is due to the track & intensity of the incoming weather system.

Weather Forecast

MONDAY Night: Flurries, accumulation 5-10 cm, moderate west wind, alpine high temperature -9 C.

TUESDAY: Flurries, accumulation 5-10 cm, moderate southwest wind, alpine high temperature -6 C.

WEDNESDAY: Scattered flurries, light to moderate southwest wind, alpine high temperature -7 C.

THURSDAY: Scattered flurries, light to moderate southwest wind, alpine high temperature -8 C.

Avalanche Summary

On Sunday there were several reports of skier triggered size 1 storm slab avalanches up to 30 cm deep on north south and east aspects at treeline.

On Saturday there were two reports of skier triggered persistent slab avalanches size 1-1.5. One was on a south aspect at 2200m and was about 25-45 deep.

Snowpack Summary

25-40 cm of recent snow has accumulated above a widespread layer of large, feathery surface hoar crystals. New snow is expected to continue to accumulate above this layer over the next few days and possibly make it more sensitive to human-triggering.

A weak layer formed in late November is now buried around 1 m below the surface. This is the layer of concern relating to the listed persistent slab avalanche problem. The weak layer may present as surface hoar, a crust, facets or a combination, depending on elevation and aspect. Below this, a variety of crusts from late October are buried deeper in the snowpack.

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.