Avalanche Forecast

Archived

Dec 15th, 2021–Dec 16th, 2021

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.

Alpine

Natural and human triggered avalanches likely.

Treeline

Natural and human triggered avalanches likely.

Below Treeline

Natural and human triggered avalanches likely.

Regions

Sea To Sky.

The current persistent slab problem is best managed through conservative terrain choices and disciplined backcountry travel techniques.

Avoid likely trigger spots such as steep, rocky terrain, with a shallow or thin to thick snowpack.

Confidence

Moderate - Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast.

Weather Forecast

WEDNESDAY NIGHT: Cloudy / Light, southwesterly winds / Low of -8 C / Freezing level 200 m.

THURSDAY: Mix of sun and cloud / Light, northwesterly winds / High of -5 C / Freezing level 400 m.

FRIDAY: Increasing cloudiness / Light, northwesterly winds / High of -7 C / Freezing level 300 m.

SATURDAY: Snow; 30-40 cm / Extreme, southwesterly winds / High of -2 C / Freezing level 800 m.

Avalanche Summary

A couple naturally triggered size 2 storm slabs were reported in the alpine on Tuesday.

Several human triggered persistent slab avalanches up to size 2, failing on the early December crust were reported on Monday. See one example in this MIN report.

Snowpack Summary

Recent snow and strong southerly winds formed wind slabs on lee features in open terrain that may remain reactive to human triggers.

The primary layer of concern in the snowpack is a crust down 100-180 cm that was formed in early December. This layer has a weak layer of small facets (sugary snow) above it that has been the cause of recent large avalanches.

This has created a Persistent slab problem that is tricky to manage and likely to surprise riders who choose to ride steep, rocky slopes with a shallow or thin to thick snowpack.

It's tricky to manage because it's unlikely to trigger this layer on many slopes, but the consequence of triggering it is massive.

This scenario requires conservative terrain choices and disciplined backcountry travel techniques.

Terrain and Travel

Be aware of the potential for large avalanches due to the presence of a persistent slab.
Cornice failures could trigger very large and destructive avalanches.
Uncertainty is best managed through conservative terrain choices at this time.
Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
Back off if you encounter whumpfing, hollow sounds, or shooting cracks.

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.