Avalanche Forecast
Regions: Stevens Pass.
The storm may have tapered off, the but avalanche danger has not. Easterly winds through the pass formed slabs in unusual locations on Saturday. The wind drifted snow may be resting on a layer of weak, sugary facets over a crust, a recipe for slab avalanches. Stay off of steep slopes in wind loaded areas.
Discussion
Snow and Avalanche Discussion:
A little less than a foot of low density fluff fell overnight Friday, then easterly winds cranked-even below treeline. Avalanche professionals were able to trigger small wind slabs very easily in the morning, and these became more reactive towards the afternoon as the wind continued. Prior to the storm, the very cold temperatures turned a layer of low density snow into facets. These facets rest over a mostly supportable crust, and are fairly widespread above 5,000ft. An observer reported a significant collapse on a South aspect at 5,100ft on Saturday. This was in a location that was wind loaded with a slab about 16†thick. They headed down to change their underwear, and reported that the ski quality was variable to poor after the winds did their thing. As the storm snow becomes more cohesive, conditions may actually become more dangerous with time as the slab may begin to communicate a fracture across the terrain more easily on Sunday than on Saturday. This is a fairly unusual situation for the Cascades, and one to be taken very seriously as the cold temperatures and snowfall continues into the week ahead.
Facets are angular snow grains that bond poorly to other grains around it. Once formed, buried, and preserved, they tend to persist in the snowpack. They are often the culprit of delayed action avalanches that result in avalanche accidents.Â
Snowpack Discussion
Coming Soon.
Avalanche Problems
Wind Slabs
You may find a little less than a foot of new snow from Saturday, with deeper snow in drifts. The easterly winds cranked at low elevations, transporting both new and old snow into slabs of 4 finger hardness. Looks for these slabs on leeward sides of ridges and in wind exposed terrain at all elevations. They may be on westerly facing aspects, an unusual loading pattern. Use snowpack tests to help assess for slab structure, or just simple ski pole tests. Is there strong over weak? Then there is slab structure. Check for a layer of weak facets resting on top of a crust beneath the new snow. Use caution on and beneath any steep slopes where you find strong over weak. Slabs may be triggered from a distance if they rest over facets. In the few places out of wind effected locations, the low density of the snow may not have enough cohesion to act as a slab. Exposed, easterly aspects are likely to be scoured to the crust, creating difficult travel conditions.
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.
Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..
Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.
Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Likely
Expected Size: 1 - 1