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Avalanche Forecast

Archived

Jan 18th, 2021–Jan 19th, 2021

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

Regions

Sea To Sky.

Avalanche conditions are improving, but lingering cornices, wind slabs, and deep persistent slabs warrant careful terrain selection.

Confidence

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

Weather Forecast

MONDAY NIGHT: Clear skies, light to moderate north wind, treeline temperatures around -1 C, freezing level around 1300 m.

TUESDAY: Increasing cloud and wind throughout the day as temperatures drop, sunny in the morning with light west wind and treeline temperature around -2 C then some light snowfall in the afternoon with strong southwest wind as treeline temperatures drop to -4 C.

WEDNESDAY: Mix of sun and cloud, light variable ridgetop wind, treeline temperatures around -5 C.

THURSDAY: Cloudy with some light flurries, light wind, treeline temperatures around -5 C.

Avalanche Summary

A few small wind slab and wet loose avalanches were reported on Sunday and Monday. Explosive control targeting cornices on Friday and Saturday produced size 2 results, some triggering deep slabs on the rocky slopes below. 

There has been significant deep persistent slab activity this month. During a widespread avalanche cycle last week, several storm slabs stepped down to deeper weak layers resulting in very large (size 3-4) deep persistent slab avalanches (see these photos from Panorama Ridge). On Jan 9, a large (size 3) avalanche was remotely triggered by skiers on Cowboy Ridge near Whistler. The avalanche occurred on a north to northwest aspect at around 1900 m. The crown was up to 200 cm deep and was approximately 200 m wide. See this MIN for photos and more information. A similar avalanche was remotely triggered by skiers on a nearby westerly slope a few days before (see this MIN).

Snowpack Summary

Ski quality in the extensively wind affected alpine has been widely referred to as chalky. Soft snow may be found in sheltered areas around treeline and may overlie a spotty layer of surface hoar. A widespread surface crust exists below 1900 m and on south-facing slopes.

The snowpack is currently quite complex. A layer of sugary faceted grains above a hard melt-freeze crust from early December may be found down 100 to 200 cm. Just over a week ago, several large natural and human-triggered avalanches on this layer occurred, with the hot spots being around the Whistler backcountry. Near the base of the snowpack, there may be faceted grains above a crust from early-November, which has also been identified as the failure layer in sporadic avalanche releases.

Terrain and Travel

  • Use ridges or ribs to avoid areas of wind loaded snow.
  • Use extra caution around cornices: they are large, fragile, and can trigger slabs on slopes below.
  • Avalanche hazard may have improved, but be mindful that deep instabilities are still present.
  • In times of uncertainty conservative terrain choices are our best defense.

Problems

Cornices

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.

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.