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Avalanche Forecast

Archived

Apr 6th, 2021–Apr 7th, 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.
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

Northwest Coastal.

Watch for cornices and reactive slabs in wind-drifted areas.

Confidence

Moderate - Forecast precipitation (either snow or rain) amounts are uncertain.

Weather Forecast

Tuesday night: Overcast, scattered flurries with 5-10 cm of accumulation, strong southwest wind, treeline temperatures around -8 C and freezing dropping below 300 m.

Wednesday: A mix of sun and cloud, isolated flurries with trace accumulations, moderate west wind, treeline temperatures around -3 C and freezing level rising to 800 m and dropping to valley bottom overnight.

Thursday: Overcast, flurries arriving late in the day, increasing southwest wind, treeline temperatures rising to -2 C, freezing level rising to 700 m and dropping below 500 m overnight.

Friday: Overcast, 15-25 cm of snow, strong southwest wind, treeline temperatures -3 C, freezing level rising to 700 m and dropping to valley bottom overnight. 

Avalanche Summary

In the aftermath of the weekend storm, there have been reports of several natural and explosive-triggered avalanches (size 1.5-2) in the recent snow triggered by incremental loading, warming, and wind-loading. Below 600 m, observations of loose wet avalanches continue and include one instance of a large (size 2) wet slab that failed to the ground from a steep, shallow feature. 

Strong sun over the weekend initiated numerous natural loose wet avalanches on steep, sunny slopes. Large natural and explosive triggered glide slab avalanche activity has been ongoing for the past few weeks. Glide cracks releasing as full depth glide slab avalanches are extremely difficult to predict. Best practice is to avoid slopes with glide cracks.

Snowpack Summary

Ongoing flurries and strong southwest winds will continue to build wind slabs in lee features in exposed areas, especially where the wind is redistributing dry snow. Expect to find wet flurries and rain below 1000 m. 

Over the past week, 40-60 cm of snow has fallen at upper elevations. While this snow is well settled, wind slabs may linger on steep open slopes. Sun crusts may be found on sun-exposed slopes. Cornices are large, looming, and capable of triggering large avalanches when they fail. Previous weak layers are now deeply buried and have not resulted in any recent avalanches.

At lower elevations, the recent snow has mixed with rain, forming a hard crust in the freezing temperatures overnight. 

Terrain and Travel

  • Expect slab conditions to change drastically as you move into wind exposed terrain.
  • Watch for wind-loaded pockets especially around ridgecrest and in extreme terrain.
  • Avoid exposure to slopes that have cornices overhead.
  • Stay off recently wind loaded slopes until they have had a chance to stabilize.

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.