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Avalanche Forecast

Archived

Feb 10th, 2019–Feb 11th, 2019

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

Regions

Stevens Pass.

Dangerous avalanche conditions are to be expected. This storm pattern has been unusual, and will continue to produce low density snow along with storm slabs for Monday. We are tracking a commonly found persistent weak layer. This layer may produce deeper and more destructive avalanches and they may be triggered from a distance. Seek out slopes less than 33 degrees if you find slab structure over facets. 

Discussion

Snow and Avalanche Discussion:

Prior to the storm cycle that started on February 8, 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. Observers have reported multiple collapses on a wide range of aspects in recently wind loaded areas the past two days. As the fluffy snow becomes more cohesive above this layer and the storm snow continues to pile up, the size of the potential avalanches are increasing, and once the slab begins to communicate a fracture across the terrain, watch out. 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. Tell your friends to be safe out there, and hold them to it. This forecast may be updated at 0630 on Monday based on overnight storm totals. 

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 deep and destructive avalanches that result in accidents. 

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.