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Avalanche Forecast

Archived

Feb 22nd, 2018–Feb 23rd, 2018

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 possible, human triggered probable.

Regions

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New shallow wind slabs will form during the day Friday around the East Slopes of the Cascades. Identify and avoid steep slopes where the wind is actively depositing snow such as near blowing snow, on snow drifts, and below fresh cornices. A weak buried facet layer exists in many locations. Stay away from large open slopes where large persistent slab avalanches may be triggered.

Detailed Forecast

New shallow wind slabs will form during the day Friday as the next wave of snow and wind impact the Cascades. These avalanches will become larger and easier to trigger as the day goes on. Identify and avoid slopes where the wind is actively depositing snow. Use visual observations such as blowing snow, plumes, fresh cornices, and drifting snow to locate wind loaded slopes as you travel.

A weak facet layer can be found in many locations in the Cascade-East zones. This layer is typically down 1.5-2.5 feet and sits on top of a firm (2/5) crust. This layer has been reported in at least one skier triggered avalanche. You will be more likely to trigger this persistent weak layer in areas where the surface snow is firmer. Information about the distribution and sensitivity of this layer can be difficult to gather. Avoid steep large open slopes where large persistent slab avalanches may occur.

Excellent travel conditions exist on lower angled slopes away from overhead hazard.

Snowpack Discussion

Loose unconsolidated surface snow conditions have been report on sheltered slopes from many locations. In general 18-30 inches of snow now sits over the weak sugary facets buried on 2/13. These facets can be easily located above the most recent (2/5) crust layer.

In exposed terrain near and above treeline, winds over the past week have redistributed snow in some locations. Old wind slabs have been limited to isolated features along ridgelines.

While there are numerous layers in the snowpack, there are currently no significant layers of concern in the mid and lower snowpack.

Observations

North

On Wednesday a skier triggered a size 1.5 slab avalanche near Harts Pass on NE aspect at 6000’. The avalanche was 2 feet deep and failed on weak facets above the 2/5 crust.

Avalanche professional around the Northeast zone Sunday through Tuesday observed snowpack tests indicating that the persistent weak layer could fail and produce avalanches.

Central

NWAC field staff traveled in the Icicle Creek area Wednesday on NE facing slopes up to about 6000 feet. The persistent layer we are tracking was found consistently buried about 2 ft from the surface, though the distribution of this layer was intermittent. In many areas the snow above the facet/crust layer was low cohesion powder, lacking the ability to propagate a slab avalanche. Shallow fresh wind slabs were noted near and above treeline.

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.

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.