Observation: Turnagain

Location: Seattle Ridge

Route & General Observations

Seattle Ridge Snowmachiner Triggered Avalanche 4pm January 8, 2013


Start Zone Elevation: 2500′
Toe of Debris Elevation: 1,100′
Vertical Fall of debris: 1,400′

Crown Width: 1000′
Crown Height/Depth: 2-8′

Start Zone slope angles: 30-40 degrees avg

East Aspect

“Repeat Offender” aka snowmachine uptrack

This avalanche was remotely triggered by a single snowmachine around, we think
the 2200′ elevation. Fortunately no one was caught, buried, injured or killed
in this avalanche. The hazard was rated at CONSIDERABLE above treeline on this
day, following a 15 day stretch of steady snowfall. The avalanche occurred as
visibility was increasing towards the end of the day.

A quick pit around 2200′ in snow that had not slid revealed a total snow depth
of 295cm. Roughly 30cm of 2mm faceted grains are intact at the bottom of the
snowpack at this site. Our ability to affect this layer in a location like this
is minimal at best. The distance from the snow surface to the weak layer is
close to 9 feet.

When areas of thinner slab are impacted, cracks are propagating long distances
along these basal weak layers (see Tincan avalanche on 1/4 for another example
of this). These layers are of varying thickness and are well developed. There
are still many areas holding this basic set up of very strong thick snow over
very weak snow. The likelihood of triggering these full depth avalanches may
steadily decline but the consequences could be grim.

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