Turnagain Pass

It was an active day for human triggered avalanches yesterday. All but one of these slides were loose snow avalanches or sluffs. The one that was not was a fresh, but hard, wind slab avalanche triggered in the Girdwood Valley zone on the headwall of the Raven Glacier. This wind slab was shallow, 6″ to a foot thick, and hard – allowing the person onto the slope before it released. See the write up in the link.

As far as sluffs go, they were long running and fast. As one observer remarked “sluffs got uncomfortable large”.  This is due to 6-8″ of new light snow that fell on a few inches of loose faceted snow and surface hoar (acting as ball bearings). The reason they are running so far is a hard base and/or crust underneath is acting as a slick bed surface. For today, sluffs should be just as concerning and far running – keep a close eye on your sluff.

Great video sent in by Kevan Dee of skier triggered sluffs on Sunburst yesterday:

Long running and fanning sluffs on the back Southerly slopes of Sunburst (Jonathan Rupp).

WIND SLABS:

Although sluffs are a concern, wind slab avalanches created by moderate/strong winds today are a much less manageable situation. With 8-10″ of loose snow on the surface, it will not take much wind to form slabs. Furthermore, slabs will be sitting on a slippery base, making them very touchy. If the winds really crank, we could see natural slabs as well. Due to the limited amount of snow available for transport, slabs should be around a foot thick. They are likely to initiate sluffing as they avalanche and large amounts of debris can be generated by even a small wind slab.

Watch surface conditions and wind patterns closely for areas that could becoming windloaded. Look for areas of stiffer snow over softer and quick hand pits are good ways to assess the bonding of any wind slab found.

Winds transporting snow along higher peaks yesterday (Andy Moderow).

Even in the trees and at lower elevations, shallow slabs and sluffs are possible. This is a shallow soft slab skier triggered avalanche on a steep but small slope in the Tincan Trees (Zachary Liller).

Weak snow (facets and depth hoar) in the lower layers of the snowpack lingers as an issue in our advisory area. It has been 9 days since the last human triggered avalanches on these layers. However, triggering a deep slab is still a possibility if you find the wrong spot and a concern due to the depth of slab overlying the weak snow. The likelihood for someone to find and trigger a large slab is decreasing as the snowpack adjusts, but the poor snowpack structure remains and the consequences are scary. Both near miss avalanche accidents last week were triggered in thinner areas of the snowpack, on slopes that already had tracks on them. The tricky part about this particular avalanche problem is that it is very difficult to assess. Obvious signs like whumpfing and shooting cracks are unlikely and stability tests may not be reactive. Just because a slope has tracks on it does not mean it is safe. Likely places to trigger a deep persistent slab will be near rocks or in thin zones where affecting the weak layer is more of a possibility. Likely triggers are large: snowmachines, groups of people or cornice falls. Identify and avoid terrain traps (like large gullies).