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Avalanche Forecast

Archived

Dec 30th, 2020–Dec 31st, 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 and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural and human triggered avalanches likely.

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

Weather Forecast

WEDNESDAY NIGHT - Flurries, 5-10cm / southwest wind, 30-50 km/h / alpine low temperature near -2 / freezing level 1100 m

THURSDAY - A mix of sun and cloud / light northwest wind / alpine high temperature near -3 / freezing level 1000 m

FRIDAY - Snow, 20-30 cm, with another 20 cm Friday night / southwest winds 30-60 km/hr, gusting to 80 km/hr/ alpine high temperature near -4 / freezing level 1300 m

SATURDAY - Snow, 30-40 cm with another 30 cm Saturday night / southwest wind, 50-70 km/h gusting to 90 km/h / alpine high temperature near -2 / freezing level 1400 m

Avalanche Summary

There were numerous explosives triggered storm slab avalanches reported on Wednesday, mostly size 1, with one size 2 and one size 3.

There was a fatal avalanche in the South Coast Inland region (near the Sea to Sky boundary) north of Pemberton in the Railroad Pass area on Monday that resulted in two fatalities. It was reported to be a size 2 persistent slab avalanche on a west aspect, around 1650 m. The RCMP press release can be viewed here, and the Avalanche Canada report can be found 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 natural 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. 

There were several other human triggered wind slab avalanches up to size 2 reported on Monday as well.

Snowpack Summary

There has been between 40-70 cm of recent snow in the region with strong southwest winds. The fresh snow has likely been redistributed by the wind, forming storm slabs in lee terrain features.

The snowpack is currently complex, and two 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 sit on top of a hard melt-freeze crust. This layer ranges in depth from 20 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. 

There have been several recent human triggered avalanches on these layers. 

The remainder of the snowpack is well-settled.

Terrain and Travel

  • Be careful with wind loaded pockets, especially near ridge crests and roll-overs.
  • Storm slabs in motion may step down to deeper layers resulting in large avalanches.
  • Be aware of the potential for large avalanches due to the presence of buried persistent weak layers.
  • In times of uncertainty conservative terrain choices are our best defense.

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.