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Avalanche Forecast

Archived

Jan 6th, 2021–Jan 7th, 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 possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Sea To Sky.

Up to 30 cm of new snow Tuesday day night fell with strong to extreme wind from a variety of directions. As a result storm and wind slabs will remain the main concern on Thursday.

Confidence

Moderate - Uncertainty is due to the complexity of the snowpack’s structure.

Weather Forecast

Wednesday Night: Mainly cloudy, light to moderate southeast wind, alpine high -5.

Thursday: Mainly cloudy, light southwest wind, alpine high -5, freezing level 500m.

Friday: Flurries, 3-5 cm, moderate southerly wind, alpine high -3, freezing level 800 m.

Saturday: Flurries, light southwest wind, alpine high -4, freezing level 700 m.

Avalanche Summary

On Wednesday there were several size 2 explosives controlled storm slab avalanches reported running in the alpine and treeline.

Reports on Monday and Tuesday show a few explosives controlled storm slab avalanches running size 1-2 as well as a few human triggered storm slab and loose dry avalanches size 1-1.5. 

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 the weekend storm.

Snowpack Summary

Over 150 cm of recent storm snow from the past week 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.

Terrain and Travel

  • Continue to make conservative terrain choices while the storm snow settles and stabilizes.
  • Be especially cautious as you transition into wind affected terrain.
  • Be mindful that deep instabilities are still present and have produced recent large avalanches.
  • If triggered, storm slabs in-motion may step down to deeper layers and result in very large avalanches.
  • Avoid shallow, rocky areas where the snowpack transitions from thick to thin.

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.