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Avalanche Forecast

Archived

Jan 4th, 2021–Jan 5th, 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 possible, human triggered probable.
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.

Avalanche hazard will increase as snowfall resumes and fresh storm slabs build in the afternoon. Be on alert for signs of instability in recent snow.

Confidence

Moderate - Uncertainty is due to the timing, track, & intensity of the incoming weather system.

Weather Forecast

Monday night: Flurries, 5-10 cm, moderate southwest wind, freezing level 700 m.

Tuesday: Snow starting in the afternoon, 5-15 cm, strong southerly wind, alpine high -5, freezing level 1100 m.

Wednesday: Snow overnight then flurries, 15-25 cm, southwest wind easing to light, alpine high -6, freezing level 1100m.

Thursday: Sun and cloud, light southwest wind, alpine high -6, freezing level 900m.

Avalanche Summary

A large, natural storm slab avalanche cycle size 3-4 occurred near Pemberton over the weekend. Natural and explosive triggered storm slabs size 1-2 were reported elsewhere. This MIN report from Friday illustrates the active avalanche conditions during this storm.

Persistent slab avalanche activity has tapered significantly since the cycle early last week, but persistent slabs size 2-2.5 were still triggerable by explosives on weekend. Events from last week include:

  • A fatal avalanche in the South Coast Inland region (near the Sea to Sky boundary) north of Pemberton in the Railroad Pass area last Monday. 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. 

These avalanches highlight the ongoing potential to trigger persistent weak layers in the snowpack. 

Snowpack Summary

Over 100 cm of recent storm snow has seen extensive wind effect at upper elevations, with scoured windward aspects, wind slabs in lee features and growing cornices at ridgetop.

The snowpack is currently quite complex. The layer of greatest concern is a melt-freeze crust from early December, found around 100-200 cm deep in the snowpack. This crust may have surface hoar or sugary faceted grains sitting above it. There have been several recent large natural and human triggered avalanches on this layer and new snow loads as well as large loads from storm slab avalanches have potential to trigger these layers.

The remainder of the snowpack is well-settled.

Terrain and Travel

  • Watch for avalanche hazard to increase throughout the day.
  • Choose conservative terrain and watch for clues of instability.
  • Be careful as you transition into wind affected terrain.
  • Be mindful that deep instabilities are still present and have produced recent large avalanches.

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.