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Avalanche Forecast

Archived

Dec 18th, 2021–Dec 19th, 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 unlikely, human triggered possible.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

North Columbia.

Touchy storm slabs that formed on Saturday will remain reactive to human triggers through the weekend; especially in wind affected terrain.

The new snow will increase the likelihood of triggering the persistent slab problem. Learn more HERE.

Confidence

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

Weather Forecast

SATURDAY NIGHT: Snow; 5-15 cm / Moderate west wind / Low of -16

SUNDAY: Clearing skies with a mix of sun and cloud / Light northwest wind / High of -13

MONDAY: Sunny / Light southwest wind / High of -16

TUESDAY: Increasing cloudiness / Moderate northwest wind / High of -15

Avalanche Summary

Fresh storm slabs formed on Saturday will likely remain reactive to human triggers through the weekend, especially in wind affected terrain.

A naturally triggered size 2.5 persistent slab avalanche was observed in the Monashees on Wednesday on a West aspect at 2150 m. This avalanche failed on the early December facet/crust combo. It was 100 cm deep, a good example of the high consequence of triggering this layer.

Snowpack Summary

25-35 cm of new snow and strong southwesterly winds on Saturday formed touchy storm slabs that will remain reactive to human triggers though the weekend; especially in wind affected terrain.

Below the new snow, the defining feature of the snowpack is a widespread crust that reaches as high as 2200m and now sits 80-150 cm below the surface. 

In some places overlying snow is well-bonded to the crust but in others weak faceted grains have been observed above it. The facets above the crust are most prominent at treeline where the crust is thinner. 

The recent snow will increase the likelihood of triggering this layer.

Terrain and Travel

  • Be aware of the potential for large avalanches due to the presence of a persistent slab.
  • Look for signs of instability: whumphing, hollow sounds, shooting cracks, and recent avalanches.
  • Continue to make conservative terrain choices while the storm snow settles and stabilizes.
  • Storm slabs in motion may step down to deeper layers resulting in large avalanches.
  • Avoid shallow snowpack areas, rock outcroppings and steep convex terrain where triggering is most likely.

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.