Turnagain Pass

Timing is everything…

It’s that time of year where clear skies freeze up the snow surface at night, just for the sun to warm them up and turn hard crusts into slop later in the day. Hence, a LOW danger in the morning transitions to a MODERATE or CONSIDERABLE danger in the late afternoon – and even into the evening hours. A big thanks to Dan Koepke for sharing his story from Sunburst yesterday, where he talks about this exact experience; watching the snowpack warm up during the day and around 4:30pm a slab avalanche is triggered on a Southerly facing slope that runs to the valley floor. This slab was ~150′ wide and broke 12-18″ deep, it stepped down to the ground in places (we are very glad Dan was able to ski out of this avalanche). You can see his full report HERE.

Video of the Sunburst avalanche, wet snow debris in motion (lack of powder cloud). Note the older debris from last week on the left that becomes partially covered. (Koepke)

Photo of Sunburst avalanche crown, ~16″ average in depth at the top before stepping down ~50′ below the crown to near the ground in places. (Koepke)

WET AVALANCHES (Southerly facing slopes) – late in the day:

The Sunburst slide above is a good example of what kind of terrain (Steep Southerly) to avoid late in the day. Our snowpack consists of several weak layers of facets and buried surface hoar. Because of this, wet slabs are possible and having them step down to deeper weak layers is also possible. Wet loose snow avalanches have also been occurring, these have been relegated to steep slopes with a shallow snowpack so far. If heading out for a fun day in the sun keep in mind these things:

• Once the snowpack becomes so wet it is unsupportable and ‘punchy’ to skis, snowmachines or boots – it’s high time to head to a cooler aspect. 
• The steeper the Southerly slope, the more it will warm and the more dangerous it will be (due to more warming but also because it’s simply steeper).
• Buried weak layers in the pack can make a small avalanche or sluff become bigger by propagating across the slope and/or stepping down. 

SPRINGTIME SHED CYCLE?
This is when the whole snowpack becomes wet and begins to litterally ooze and slide off the mountains. So far there is still cold snow at the upper elevations; some thinner and lower elevations are ‘shedding’ however, such as the Southeast face of Seattle Ridge. We will be watching the snowpack and and air temperatures and talking about this in the future. 

The Southeast face of Seattle Ridge – this zone is typically the first area in Turnagain Pass to begin shedding its snowpack.

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On the shaded and cool side of the mountains (Northerly aspects) dry snow still exists. While wet snow issues are not a problem here, deep slab issues are. There are several weak layers anywhere from 2-5′ below the surface. The most noteable is the March 27 buried surface hoar; this layer was buried by 3-6′ of snow during the 10-day April Fools storm. We have not seen or heard of any avalanche activity on this layer for 6 days now, but with warm temperatures and a ‘persistent weak layer’ present, this remains a concern. Shaded aspects (NE – NW – W) in the mid and upper elevations that haven’t avalanched already are the most suspect places for triggering a deep slab. 

This problem is one of low probability but high consequence as these are large and potentially unsurvivable slides. As the snowpack continues to adjust, triggering will become more stubborn and less likely with time. Keep these point in mind:  

• It will take someone hitting a ‘thin spot’ in the slab, or a large trigger such as a snowmachine and/or groups of people.
• These slides can be triggered remotely, for example, from a ridge or bench on the top/side or below. 
• There may be no signs of instability before the slope shatters 
• Several tracks may be on a slope before it releases
• Stability tests may not produce any noteable results

Two new small glide avalanches released on Southeast facing Seattle Ridge yesterday. On Wednesday, a larger glide released on the South aspect of Eddies ridge. We expect this trend to continue with the warm days ahead. Keep an eye out for glide cracks, full depth cracks in the snow, and limit time underneath them. Most cracks appear to be small and in areas less traveled, but a glide avalanche can also release with little warning when a crack is not visible prior.