The Bottom Line: Overall we are still observing lingering poor structure in the deeper snowpack. Having just dropped down from Considerable danger, we are in the mindset of “scary moderate.” Over the last week, the NW and N winds blew hard, loading up SW through S through E aspects with wind slabs and cross-loading other aspects, gullies, and features. Most alpine areas are going to be a mix of sustrugi, hard slabs / wind board, and slabby powder in protected areas. Variability in these surface wind slabs is high. The most dangerous areas will be wind loaded solar aspects steeper than 32 degrees where you may still find tender slabs resting on weak old snow. Likewise, cross-loaded gullies will have wind slabs and pillows that could take you for a nasty ride. Some nice softer snow remains in the trees.

Photo: Example of a wind loaded slope with a wind slab avalanche that stepped down to deeper persistent layers below. This one was probably triggered by cornice fall. Photo from Flathead Avalanche Center.

Wind slabs can be avoided by sticking to sheltered or wind-scoured areas. Also from the wind, cornices will be unsupported, scoured, and weak. Tread carefully around them and limit your exposure to slopes below them. A cornice drop could cause a slide, with potential for a step down.

The severe consequences of our avalanche problems remain, due to deep persistent layers. Be hypersensitive of terrain- it is an active way to manage our exposure to the risk of an unlikely but high-consequence deep slab avalanche.

When on or near avalanche terrain, be diligent on terrain selection, group management, and human factor.  Travel one at a time or with enough spacing so that only one person is exposed to an avalanche path at any one time. Be prepared for an avalanche that could step down to deeper instabilities, creating a slide that could break very wide, and take out all the “safe” zones on a slope. Be extra careful not to group up in places that an avalanche can reach. 

When seeking out wind protected zones, remember that wind-protected pockets may be harboring weak, buried surface hoar layers. Protected means preserved. These mid-pack layers appear to be what failed in this weeks avalanches observed.

Buried Surface Hoar Layers (These layers have become less reactive in snowpits, but smaller slides could still step down and trigger them):

• Jan 25th melt freeze crust is about 2-3 feet deep (with recent winds and snow, there is a lot of variability on depth of this widespread layer) sitting on top of it are near surface facets, and buried surface hoar from Feb 1-4.
• January 10th surface hoar is now buried about 4 feet deep.
   

March 2nd: Snowpit at 2700ft on Flower mountain found weakness about 75cm deep, and some healing of deeper weak layers. But we are not confident on how well this pit maps to other areas. The takeaway is that while some healing is happening, it is variable across the landscape.

Cornices have become very large and weak. They are undercut, overhanging, and slowly sagging as they grow. Strong sunshine and increasing winds will put a lot of stress on these dangerous beasts (especially as temperatures rise). Stay way back from any snow ledges or cornices on ridgelines or summits. Some may fall naturally, so do not travel below them. More likely will be human triggering of cornices from above. When a cornice fails, it will break way further back on the ridge than expected and take most of the flat terrain out with it. Any cornice falls are likely to trigger very large and deadly deep-slab avalanches. Consequences are very high right now.

Old Melt Layers and Crusts:

• Dec 31 surface hoar layer is present in the Transitional and Pass zones, about 5 ft deep. This layer has been active in pits, and caused some natural avalanches in early January.
• A buried persistent weak layer about 7+ feet deep (The “Big Warmup” Layer, Formed Nov 17th) is still a concern. Numerous natural avalanches observed 1/27 likely on this layer.
• A wide safety margin is necessary. This setup could produce avalanches that break wider than expected, and are most likely to be triggered from shallow trigger points like rocks or small trees. You could ride the same slope numerous times until that right spot produces a large destructive slide.

“Big Warmup” facet layer in the Transitional Zone at 2500ft on Jan 2 with test results ECTP18 that demonstrated propagation potential on this layer.

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Pit at Old Faithful, 2500’, East aspect on Jan 12. A 5″ thick layer of facets down 170cm. Also, observation noted faceted surface conditions.

Depth Hoar at the Ground (Above 3,000′):

• October snow followed by long cold snaps created depth hoar at the ground in most areas.
• You are most likely to human-trigger this layer from shallow spots around rocks, or trees, or have a surface avalanche step down to the ground on this layer. Any slides that break to the ground are likely to be deadly.

Photos shows 6-9mm advanced depth hoar at the ground down 180cm with ECTP 25 down 80cm on facets below a crust in the Haines Pass Zone at 4,000ft on a NE-aspect.