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Avalanche Forecast

Archived

Dec 13th, 2021–Dec 14th, 2021

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

Regions

Sea To Sky.

Forecast snow and wind overnight on Monday are expected to form reactive wind slabs on lee features at treeline and above. 

The presence of a persistent slab problem requires continued conservative terrain choices.

Confidence

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

Weather Forecast

MONDAY NIGHT: Snow, 10-20 cm / Strong, southeasterly winds / Low of -7 C / Freezing level surface.

TUESDAY: Mostly cloudy with flurries; 0-3 cm / Light, northwesterly winds / High of -5 C / Freezing level 400 m.

WEDNESDAY: Mix of sun and cloud / Moderate, southerly winds / High of -5 C / Freezing level 500 m.

THURSDAY: Mix of sun and cloud / Light, northwesterly winds / High of -5 C / Freezing level 500 m.

Avalanche Summary

On Sunday, operators reported numerous small to large (size 1.5-3) human and explosive-triggered avalanches releasing within in the storm snow on a variety of aspects. 

We also received reports of four large (size 2-2.5) explosive-triggered persistent slab avalanches failing on the early December crust/facet layer. These occurred on northwest and northeast aspects around 2000 m. A large (size 2.5) explosive-triggered cornice fall was also reported. 

Snowpack Summary

Large amounts of recent snow and extreme southeasterly winds have formed deep wind slabs on lee features in open terrain. Extreme winds can form reactive wind slabs much further down slopes than is typical.

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

  • 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.
  • 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.

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.