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Avalanche Forecast

Archived

Dec 28th, 2020–Dec 29th, 2020

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

Regions

Sea To Sky.

A close call occurred on Rainbow Mountain on Monday when an avalanche on a persistent weak layer resulted in a full burial. Remain conservative in your terrain choices to account for significant variability from one slope to another.

Confidence

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

Weather Forecast

Monday night: Dry with clear skies. Cooling temperatures.

Tuesday: 5-10 cm new snow. Freezing level around 600 m. Winds light to moderate southwesterly.

Wednesday: 15-20 cm new snow. Freezing level around 800 m. Winds moderate southerly. 

Thursday: Flurries. Freezing level around 900 m. Winds light southwesterly.

Avalanche Summary

A large (size 2.5) avalanche was triggered on Monday by a snowmobiler on Rainbow mountain in alpine terrain, which resulted in a full burial and, thankfully, a successful rescue. The avalanche failed on a layer 60 cm deep. At this time the nature of the failure plane is unknown, but it is reasonable to assume it failed on one of the two weak layers discussed in the snowpack section. This avalanche points to the ongoing potential to trigger persistent weak layers in the snowpack.

A small (size 1) avalanche was also triggered on Monday by skiers on a north aspect in the Whistler backcountry, most likely in recent storm snow.

Snowpack Summary

15-30 cm of recent snow has been redistributed by strong south to east winds, depositing deep pockets of storm slab into lee terrain features.

Two potential concerning weak layers may be present in the snowpack: 

  • The shallower layer, around 60 to 100 cm deep, includes feathery surface hoar crystals. This layer may be found in sheltered terrain features at treeline and lower alpine elevations but has been reported as spotty across the region. 
  • The more widespread weak layer includes sugary faceted grains that overlies a hard melt-freeze crust. This layer ranges in depth from 10 cm to 120 cm due to the wind scouring and loading the snow in different terrain features over the past two weeks. The layer has been reported as being widespread up to around 2000 to 2200 m. 

To date we have mostly seen sporadic avalanche activity on these layers, but they remain possible to trigger where they exist in the mountains.

The remainder of the snowpack is well-settled.

Terrain and Travel

  • Be careful with wind loaded pockets, especially near ridge crests and roll-overs.
  • Conditions may have improved, but be mindful that deep instabilities are still present.
  • Surface hoar distribution is highly variable. Avoid generalizing your observations.

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.