Turnagain Pass

A combination of several feet of new snow, powder starved people and clearing skies this weekend all equal a chance for an avalanche incident. Between 2-4′ of new snow fell over the region Thursday night through yesterday morning. Many natural avalanches were seen, yet at the same time, many popular areas were obscured in clouds and little information exists. Therefore, we know the new snow that rapidly loaded slopes avalanched immediately in some areas, but we are suspect that more slopes remain intact and could be lying in wait for a human to tip the balance. How quickly the storm snow will bond to the old surface is uncertain and today (and tomorrow) are days to let the slopes adjust. 

Mid-elevation storm totals from Thursday through 6am Saturday:

• Turnagain Pass: 26-36 inches 
• Girdwood: 32-42 inches 
• Portage: 26-36 inches 
• Summit Lake: 12-16 inches

If choosing to head to the backcountry, keep in mind that triggering large slabs is likely and remote triggering from a ridge or below a slope is also likely. These are unmanageable and dangerous avalanches. Whumpfing (collapsing) and shooting cracks will be keys the snowpack is very unstable. We need to watch our terrain (keeping to the flats and low angle slopes) and watch the other groups around us. Types of storm snow avalanches that could be triggered today:

–  Wind slabs:  Slabs up to 2-4+ feet thick could be found and triggered on slopes that saw wind loading during the storm. They are most likely to be triggered along ridgelines and in cross-loaded gullies.

–  Storm slabs:  Soft slabs (2-3 feet deep) are expected in areas out of the wind, similar to the photos below. With various weak layers under the new snow, this can cause a storm slab that ‘seems’ to be bonding with the old snow to remain unstable as the weakness may lurk just below the old/new interface.

–  Cornice falls:  Cornices have grown and cornice falls are likely. They are a concern in themselves, but could also trigger a wind slab or storm slab below.

Clearing skies? Sunshine can quickly de-stabilize slopes with enough solar warming. This could occur today if skies open up, but is more likely to be an issue tomorrow. Be suspect of slopes that are ‘baking’ in the sun, including being under these as they could release naturally.

Natural storm slab avalanches that released yesterday morning due to the rapid loading of 2-3′ of new snow. These are small slopes along the cut bank of Granite Creek on the Southern end of Turnagain Pass. Larger terrain out of view in the clouds likely experienced similar avalanche activity.

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Several widespread persistent weak layers exist within our snowpack region-wide, including buried surface hoar and facets 1-2′ below the old surface. It’s been over 6 weeks since a significant storm has added weight to these layers and a lot of uncertainty exists around how well they will adjust to the new load. Avalanches failing in these layers are possible, along with a storm related avalanche ’stepping down’ into these layers. In this case a much larger avalanche is possible. Areas on the South end of Turnagain Pass and the Summit Lake area are most suspect for having deeper weak layers release.

The slabs that released in the lower elevations yesterday, pictured above, were technically failing in facets below the storm snow (pictured below). The snowpack structure changes with elevation and upper slopes could be overloading other weak layers. Nonetheless, weak layers exist at all elevations.

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