Avalanche Forecast

Archived

Apr 6th, 2021–Apr 7th, 2021

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely.

Below Treeline

Natural avalanches unlikely.

Alpine

Natural avalanches unlikely.

Treeline

Natural avalanches unlikely.

Below Treeline

Natural avalanches unlikely.

Alpine

Natural avalanches unlikely.

Treeline

Natural avalanches unlikely.

Below Treeline

Natural avalanches unlikely.

Regions

Northwest Inland.

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

Confidence

Moderate - Uncertainty is due to the limited number of field observations.

Weather Forecast

Tuesday night: Overcast, scattered flurries with trace accumulations, moderate southwest wind, freezing level dropping to valley bottom.

Wednesday: Mix of sun and cloud with isolated light flurries, light west wind, treeline temperatures rising to -4 C, freezing level rising to 800 m and dropping to valley bottom overnight.

Thursday: Increasing cloud, flurries arriving late in the day, southwest wind increasing to strong, treeline temperatures rising to -3 C, freezing level rising to 700 m and dropping to valley bottom overnight.

Friday: Mix of sun and cloud, scattered flurries with up to 5 cm of accumulation, southwest wind decreasing to light, treeline temperatures rising to -2 C, freezing level rising to 800 m and dropping to valley bottom overnight.

Avalanche Summary

Strong winds contributed to intense wind-loading in areas over the weekend. Operators reported large (up to size 2) wind slab avalanches failing in immediate lee features and several natural cornice failures (up to size 2).

Reports from the weekend also included natural wet loose avalanches on south-facing slopes (up to size 2).

Snowpack Summary

Wind slabs formed by light amounts of recent snow and southwesterly winds may linger near ridgetops. Snow surface conditions include widespread wind-affect and a crust on steep south-facing slopes and at lower elevations. Cornices are large and looming at this time of year. Check out these reports from over the weekend from Hankin and Eliot Peak that illustrate the widespread wind-affect and large cornices.

Buried 70-120 cm deep, a spotty layer of surface hoar may exist on sheltered north aspects, but no recent avalanches have been reported on this layer.

The lower snowpack is reported as well settled and strong in most areas. However, weak facets exist at the base of the snowpack in the more shallow snowpack zones across the region. Where it exists, this layer has the potential to be triggered on steep, rocky slopes with a shallow or thin to thick snowpack; especially with large loads such as a cornice fall.

Terrain and Travel

Be careful as you transition into wind affected terrain.
Carefully evaluate steep lines for wind slabs.
Avoid exposure to slopes that have cornices overhead.

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.