Avalanche Forecast

Archived

Dec 14th, 2021–Dec 15th, 2021

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.

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

TUESDAY NIGHT: Mostly cloudy / Moderate, southwesterly winds / Low of -9 C / Freezing level surface.

WEDNESDAY: Cloudy with isolated flurries; 0-3 cm / Moderate, southerly winds / High of -5 C / Freezing level 700 m.

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

FRIDAY: Mix of sun and cloud / Light, northwesterly winds / High of -7 C / Freezing level 300 m.

Avalanche Summary

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

Large amounts of recent snow and strong southerly winds have formed deep wind slabs on lee features in open terrain.

Pay attention to conditions that change with exposure to wind and monitor for cracking or drum-like sounds.

An impressive storm over the weekend brought 40-70 cm of new snow to the mountains with strong southwest winds. Below the storm snow (down 60-100 cm), there is a layer of weak facets (sugary snow) above a crust that formed in early December.

Over the weekend, snowpack tests at upper treeline elevations produced sudden results at this interface (see this, this, and this MIN report). On Sunday, operators reported large, explosive-triggered avalanches releasing on this layer in alpine start zones. This persistent slab problem warrants careful evaluation and conservative route-finding as the recent snow consolidates into a more cohesive slab over the weak layer.

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.