Yesterday was an interesting day in the mountains. The snowpack proved to be highly unstable and there were 9 confirmed human triggered avalanches. These were all on the back side of Seattle Ridge. Four of them I was able to witness. These four were along Main Bowl (1st Bowl) and were triggered by two snowmachiners traveling along the ridge; releasing the avalanches one after another while they moved along. The ridge is so broad they could not see the slope below and had no idea they were triggering these slides. To our knowledge, there was no one caught in any slides yesterday.
Most, if not all, of the avalanches triggered were triggered remotely. This means triggered by a person not on the slope, but instead on the ridge above, to the side or at the bottom. The snowpack is set-up exactly for this situation and we can expect remote triggered avalanches as we move forward. The scary thing is, this set-up can allow someone to inadvertently trigger an avalanche onto themselves from below or onto someone else from a safe location. Something for all of us to keep in mind.
The problem is last week’s storm snow fell onto a layer of surface hoar (we are now calling the MLK or 1/21 buried surface hoar). This is a persistent weak layer which is inhibiting bonding between the new and old snow and creating our dangerous snowpack structure. How deep the weak layer and subsequent avalanche will be is dependent upon how much snow fell in that zone. The avalanches yesterday were generally 18″ to 3′ thick. Although yesterday’s activity was all between 2,000 and 3,000′, we saw no evidence or anyone traveling in the higher elevations testing those slopes. Hence, all elevations above 1,500′ should be suspect. Points to keep in mind if headed out today:
1- The snowpack will be just as touchy, we need to know what is and isn’t avalanche terrain before heading out
2- The snowpack will be slow to adjust/stabilize with this notorious persistent weak layer
2- No obvious signs of instability may be seen, yet avalanches could release easily and remotely
3- Sticking to lower angle slopes (less than 30 degrees) with nothing steeper above is a great way to avoid these avalanche concerns
Slab avalanches in Main Bowl (1st Bowl) triggered remotely by riders moving along ridge.
Two slabs triggered remotely by riders along with a cornice break. The slab on the top of the photo is quite thick, up to 5′ at the crown. This area is between Main and Jr’s Bowl (1st and 2nd Bowl) Thanks to these folks for clearing off the weather station!
Snowpack structure at 2,300′, West aspect along Seattle Ridge. Clearly seen is the ‘thin gray line’ of the 1/21 (MLK) buried surface hoar.
Video linked HERE.
South of Turnagain – Johnson Pass/Summit Lake zone: A poor snowpack structure exists in these areas. Multiple mid-pack weak layers of facets and buried surface hoar have been found as well as a facet/crust combination in the bottom of the snowpack. See the video below (link HERE) as Don Sharaf shows the recent weak layer reacting as well as an older weak layer. Avalanches triggered in this area could step down into deeper layers creating a much larger avalanche. Although this region received less snow (6-12″) last week, the snowpack is still very dangerous.
Don Sharaf with the American Avalanche Institute performs an Extended Column Test in the Silvertip area south of Turnagain Pass. A big thanks to the AAS/AAI Pro 2 avalanche course for their information this week.
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