Turnagain Pass

Overnight saw a quick shot of snowfall that should give us a much needed refresher. This storm was on the warm side however. Along with new moist snow that fell at the mid and upper elevations, rain has fallen up to 200-500′ in places. For storm totals at mid-elevations as of 6am this morning:

Girdwood Valley:  8-10+” snow
Turnagain Pass:  5-7″ snow
Summit Lake:  6-8″ snow

This is clearly not a large storm for our standards, but what is notable is the variety of weak layers we have within, and on the surface of, our pre-existing snowpack. Although the new snow is moist and ‘sticky’ it will likely have a hard time bonding right away onto the surface – storm snow avalanche issues are below. What is possibly more concerning is what is lurking around 1-2′ deep in our snowpack. This is a layer of buried surface hoar from Jan 21 that remains intact in many areas and could be waiting for a slab to form on top of it to start producing large avalanches. This storm has likely created that slab. Hence, today is a day to be extra cautious and evaluate the snowpack carefully. Avalanches releasing in the Jan 21 buried surface hoar could be up to 3′ thick. Also, small avalanches within the storm snow could ‘step down’ and release a much larger and unmanageable slide. This issue is less of a concern on slopes that were heavily tracked out over the past two weeks, and much more on a concern on slopes seeing moderate to no traffic. 

Photo below is from a small wind slab triggered yesterday releasing on the recently buried surface hoar. This was on the North Chutes of Cornbiscuit. Note the crack in the bed surface at the bottom of the photo. This crack is believed to have stepped down to the Jan 21 buried surface hoar, yet that layer did not slide. With more load overnight, we may see avalanche ‘step down’ to this older weak layer, creating much larger than anticipated avalanches. (Thanks to Mike Records for the photo).

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Storm snow instabilities associated with the new snow overnight and today will be likely. These come in many forms listed below and due to the lesser snow amounts, are expected to be on the smaller side. The most concerning for us however, are wind slabs as they will likely be larger and a less manageable storm snow problem. Bonding between the new and old snow is not expected to be good as the new snow has fallen on a new crop of surface hoar and near surface facets.

Wind Slabs:  Moderate to strong winds coupled with 6-10″ of new moist snow have likely formed soft wind slabs up to a foot thick or even two feet in places. Because these slabs are likely sitting on weak old snow, they are expected to be quite sensitive and easy to trigger. 

Storm Slabs:  Out of the wind, on slopes that have more than ~5″ of new snow, expect to see soft storm slab avalanches composed of the new snow.

Loose snow sluffs:  Sluffs on steep slopes are likely with the recent new snow.

Wet sluffs:  Below 1,000′ where rain is falling on snow, small wet sluffs are likely in the steep terrain.

Roof-a-lanches:  Watch for your roofs to avalanche with the warm temperatures and rain.

Today’s snowfall is only a small load on top of our generally weak snowpack structure in the Alpine (above 3,000′). However, even a small load and warming temperatures could help tip the balance for someone to trigger a large deep slab that breaks in the bottom half of the snowpack. It’s good to remember that multiple layers of old buried surface hoar, facets and crusts exist deep in the pack and near the ground. These are not likely to ‘wake up’, but it’s worth keeping in mind, as outliers can happen as with last week’s Twin Peaks slide.