Avalanche Forecast

Archived

Feb 11th, 2019–Feb 12th, 2019

Alpine

Natural and human triggered avalanches likely.

Treeline

Natural and human triggered avalanches likely.

Below Treeline

Natural and human triggered avalanches likely.

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.

Heavy snowfall and winds will create very dangerous avalanche conditions for Tuesday. An avalanche cycle is expected tonight into tomorrow. This unusually cold and snowy storm pattern continues to stress a deeper weak layer, even at low elevations. Travel in avalanche terrain is not recommended, as avalanches may run large from upper elevations. Avalanches may be triggered from below.

Discussion

Snow and Avalanche Discussion:

Morning Update: Avalanche Warning. HIGH Danger at all elevations

Observers continue to report multiple collapses on a wide range of aspects, and even a remotely triggered slide at Snoqualmie Pass. The collapses have been on a wide range of aspects, but appear to be most prevalent where the snow had previously been drifted onto. A skier was able to trigger a small slab that ran on the February 8th layer of facets over a crust near Lanham Lakes. These signs are bullseye pieces of information. Ding, ding, ding...

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. As the fluffy snow continues to pile up, it is becoming more and more cohesive. 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 picks up Monday night into Tuesday. Tell your friends to be safe out there, and hold them to it. Honestly, tomorrow is not a good day to venture out of bounds and into avalanche terrain.

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.