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Avalanche Forecast

Archived

Dec 29th, 2020–Dec 30th, 2020

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 possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural and human triggered avalanches likely.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.

Regions

Sea To Sky.

A complex snowpack has led to several recent avalanche incidents, including a fatal avalanche north of Pemberton, close to the Sea to Sky boundary. Clues of instability may not be obvious, so stick to low-angle terrain at all elevations and avoid overhead hazard.

Confidence

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

Weather Forecast

TUESDAY NIGHT - Flurries, 5-10 cm / southwest wind, 20-40 km/h / alpine low temperature near -5 

WEDNESDAY - Snow, 10-15 cm / southwest wind, 40-60 km/h / alpine high temperature near -2 

THURSDAY - Mainly cloudy / light southwest winds / alpine high temperature near -4 

FRIDAY - Snow, 20 cm / southwest wind, 30-50 km/h gusting to 75 km/h / alpine high temperature near -2 / freezing level 1500 m

Avalanche Summary

There was a fatal avalanche in the South Coast Inland region (near the Sea to Sky boundary) north of Pemberton in the Goat Peak area on Monday. This avalanche resulted in two fatalities. It was reported to be a size 2.5 persistent slab avalanche on a west aspect, around 1650 m. The RCMP press release can be viewed here.

A size 2.5 avalanche was triggered on Monday by a skier on Rainbow Mountain, which resulted in a full burial and, thankfully, a successful rescue. The avalanche failed on a 60 cm deep crust and facet layer on a northeast aspect around 1800 m. 

A size 3 persistent slab avalanche was reported on Mount MacBeth on Monday, check out this MIN report for details of this observation.

These avalanches highlight 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

There has been up to 40 cm of recent snow in the region, with another 10-15 cm expected during the day on Wednesday. Moderate to strong southwest winds are expected to redistribute this snow, forming storm slabs in 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

  • In times of uncertainty conservative terrain choices are our best defense.
  • Be aware of the potential for large avalanches due to the presence of buried persistent weak layers.
  • Watch for fresh storm slabs building throughout the day.
  • Be careful with wind loaded pockets, especially near ridge crests and roll-overs.

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.

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.