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Avalanche Forecast

Archived

Mar 8th, 2021–Mar 9th, 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.
Below Treeline
Natural avalanches unlikely.

Regions

Sea To Sky.

Lingering wind slabs are the primary concern. Watch for denser, deeper drifts near ridge-crests and rollovers and give cornices a wide berth from above and below. 

Confidence

Moderate - Uncertainty is due to how quickly the snowpack will recover and gain strength.

Weather Forecast

Dry conditions and diurnal cycles for the next few days

Monday night: Mostly clear, light to moderate southeast wind, freezing level dropping to valley bottom.

Tuesday: Mix of sun and cloud, isolated convective flurries with trace accumulations, light south wind, freezing level rising to 1300 m and dropping to valley bottom.

Wednesday: Mix of sun and cloud, light southwest wind, freezing level rising to 1100 m and dropping to valley bottom.

Thursday: Mainly sunny, light variable wind, freezing level rising to 1100 m and dropping to valley bottom.

Avalanche Summary

On Sunday, small pockets of wind slab were reported releasing naturally and with ski cuts, primarily on northerly aspects near and above 1900 m. Small loose dry sluffs were also observed in steep terrain. 

Explosive mitigation on Saturday produced numerous large cornice and storm slab avalanches, with 50-100 cm crown depths. Explosives also released a very large (size 3) wind slab avalanche on a northeast aspect above 2000 m. 

Reports indicate that a natural avalanche cycle occurred during the storm on Friday, with small to large (size 1-2.5) avalanches releasing in the storm snow across aspects and elevations. See this MIN and this MIN for examples near Rainbow Mountain.

Snowpack Summary

50-100 cm of snow accumulated over the weekend that has been settling rapidly. The recent snowfall was accompanied by strong south winds, forming wind slabs on leeward slopes that remain possible to trigger. Massive cornices exist on ridgelines, which can act as triggers on slopes below.

The mid and lower snowpack is well settled and strong in most areas. A weak layer of facets buried in mid-February may still be found but has been gaining strength.

Terrain and Travel

  • Stay off recently wind loaded slopes until they have had a chance to stabilize.
  • Avoid freshly wind loaded features, especially near ridge crests, roll-overs and in steep terrain.
  • Watch for signs of instability like whumpfing, hollow sounds, shooting cracks or recent avalanches.
  • Cornice failure may trigger large avalanches.

Problems

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.

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.