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Avalanche Forecast

Archived

Mar 8th, 2022–Mar 9th, 2022

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

Regions

Northwest Coastal.

Another sunny day! Wind slabs will form with moderate to strong wind from the northeast. Two buried persistent weak layers that consist of feathery surface hoar and a crust with sugary facets are still of concern.

Confidence

Moderate - Uncertainty is due to the speed, direction, or duration of the wind and its effect on the snowpack.

Weather Forecast

Tuesday night: Clear, moderate to strong northeast wind, alpine low -8 °C, freezing level at valley bottom.

Wednesday: Sunny, moderate northeast wind switching to moderate west wind around noon, alpine high -6 °C, freezing level at 500 m.

Thursday: Cloudy, 5-10 cm new snow, moderate to strong southwest wind, alpine high -5 °C, freezing level at 500 m.

Friday: Cloudy, 20 cm new snow, extreme southwest wind, alpine high -2 °C, freezing level at 1000 m.

Avalanche Summary

There have been a few, skier triggered slab avalanches to size 2 on the two buried persistent weak layers described in the Snowpack Summary in the last few days.

A skier triggered a small wind slab from a distance away on a northeast facing alpine slope on Monday. The avalanche released on feathery surface hoar that was buried in late February. Several natural wind slab avalanches and cornice failures up to size 2 were observed in the alpine. 

On Sunday, a large (size 3) wind slab released naturally in steep alpine terrain and many small natural wind slab avalanches up to size 1.5 were reported. One was triggered by a failed cornice. Skiers triggered small (size 1) wind slabs on south, east and north facing slopes in the alpine. Loose wet avalanche activity to size 1.5 was observed on steep sun-exposed slopes.

Snowpack Summary

There's a lot going on in the upper snowpack. Careful assessment is recommended. 

The snow surface is heavily wind affected in wind exposed areas. There is a crust on the surface below around 1000 m and on solar aspects below around 1200 m.

Feathery, weak surface hoar crystals buried around February 26th are found down 25-50 cm, most prominently at treeline elevations. This layer has been reactive to skier traffic in the last few days. 

A thick crust buried in mid-February is now found down 60-100 cm. Skiers triggered avalanches on this layer last weekend.  

The lower snowpack is effectively bridged by the layers above, and we do not expect avalanches to be triggered in the lower snowpack at this time. 

Terrain and Travel

  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Minimize your exposure time below cornices.
  • Carefully assess open slopes and convex rolls where buried surface hoar may be preserved.
  • Be aware of the potential for large avalanches due to the presence of a buried crust.

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.

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.