Turnagain Pass

Many natural avalanches likely occurred during yesterday’s first pulse of heavy snowfall; that was between 1.5-2.5 feet of snow from yesterday morning through midday! This photo below is a natural avalanche in upper Girdwood Valley in the Crow Creek area. With such little visibility, it was difficult to assess the extent of natural avalanche activity.

Natural avalanche in the upper Girdwood Valley caught on camera in action yesterday. 11.27.20. Photo: George Creighton

Before yesterday’s storm, Thursday offered a brief break in weather and blue skies. Skiers along Sunburst ridge noted a large natural avalanche that likely occurred at the tail end of Wednesday’s (11.25) storm and strong winds. This slide began as a wind slab that stepped down to the ground to take the entire snowpack.

Wind slab avalanche that stepped down into the facets at the bottom of the snowpack on north facing Sunburst around 2,500′. The wind slab portion can be seen in far upper right corner of photo. 11.26.20. Photo Nick D’Alessio

Today’s RED FLAGS are ‘Rapid Loading’ and ‘Recent Avalanches’. It’s simple right now. Let the mountains do their thing while we stay away from, and out from under, steep slopes.

Too much snow too quick = large and dangerous avalanches.

This is all because the Thanksgiving parade of storms continues. After an exciting last week of heavy snowfall and large avalanches, another system rolled in yesterday morning as mentioned above. Add to that another foot of snow fell overnight and up to 10″ more could fall through the day. Furthermore, the easterly ridgetop winds are howling – blowing in the 40-50mph range with gusts over 80mph.

Snowfall totals in the past 28 hours (estimated for mid and upper elevations):
– Turnagain Pass:  3-4 feet (3″ of water equivalent)
– Upper Girdwood Valley:  2.5-3 feet (2.5-3″ of water equivalent)
– Summit Lake:  ~1.5 feet (1″ of water equivalent)

Storm snow avalanches have, and are likely, to continue to occur naturally. These will be in the form of wind slabs, storm slabs and cornice falls. Slabs could be anywhere from 2-6′ thick depending on the amount of new snow and wind loading. All of which could generate a significant amount of snow and send debris well into valley bottoms.

Deep snow immersion:  With so much snow be aware, even in areas in the trees (i.e., Tincan Trees) a small slope can push snow over you, making it hard to escape if you get caught up and head down. Additionally, falling into tree wells or other hollows in the snow can cause great problems as snow can fill in over you and make escape difficult to impossible without help. If thinking of heading out today, be sure to watch your partners!

Too much snow..?! A skier wallows in yesterday’s storm snow in low anger terrain near Sunburst ridge.

Making the avalanche problem worse is that the existing snowpack is unstable to begin with. There are weak sugary facets at the bottom of the pack (basal facets) and a foot or so above this is a layer of buried surface hoar in some areas. Both these layers could fail with the overloading of new snow, creating a much larger avalanche. The basal facets are looking to be the most reactive of these layers and we are headed into what could be a future tricky deep persistent slab avalanche issue.

Note the amount of new snow over a shallow old snowpack. This pit is showing the basal facets that sit at the bottom. The codes (ECTP14, etc) are snow science speak meaning the snowpack is unstable. 

CNFAIC new forecaster Andrew Schauer points at the facets under the snowpack on Seattle Ridge Thursday (11.26).