Sunday, December 9, 2012

Jackson Hole Dec 8-9 Avalanche Cycle: Lessons Learned

Winter was slower to arrive than most of us like in Jackson Hole. Rain fell on the valley floor for much of late November and early December. However, while the valley rain was hard to endure, weather above 7500' was another story--by December 8th there was 60" (152cm) of settled maritime snow at the base of Rendezvous Bowl (9580') at the Jackson Hole Mountain Resort.

Things started changing the first week of December as a strong, cold storm system from the SW hammered Jackson from December 5th-8th giving us about up to 32" of dry, right-side-up powder as well as snow to the valley floor. Winter had finally arrived. Wind slabs formed promptly during the storms outgoing wind event with winds reaching 60+mph out of the SW.

On December 7-9th, I was teaching an avalanche course.  Our field day on the 9th proved to be a very educational experience.

We began the day at the top of Teton Pass at 7:15am. A strong westerly wind was honking through the Pass. These winds diminished shortly after we arrived. We immediately reviewed our findings from our tours to Olympic and Avalanche Bowls on the the prior day. For our intended tour (25-35 degree, east and north facing slopes between 8000-9000'), we rated the danger as moderate (2 out of a scale to 5) our main concern was windslab on north and east facing at or around 9000'. No known avalanches had been reported at this time in the backcountry, though the ski patrollers at the Jackson Hole Mountain Resort had been able to trigger many slides in the storm snow at the upper elevations of the forecast zone (from 9,000-10,400'.) Our intended tour would remain in more protected, lower elevation terrain than where the patrollers were getting results.

The day before we had also seen evidence of a deep slab instability. This deep slab instability is comprised faceted October snow that is sitting on the ground (HS at 9000' is 90-120cms.) The only reported avalanche on this layer to date was on a slope above 10,000' on an steep, unsupported, rocky, NE facing slope--extreme terrain by any standard.

Additionally, on the previous days' tour, one group had found buried surface hoar down 70cms. I never saw it myself. However, this layer was unreactive to Extended Column Tests and there were no known reported avalanches on this layer.

So, my summary of the avalanche problem consisted of:
1. Wind Slab
2. Persistent Deep Slab (minimally reactive to date.)
3. Buried Surface Hoar (un-reactive to date.)

On the morning of December 9th, we toured south of Teton Pass, feeling quite happy about the new snow and the Moderate danger (2 on a scale of 5) in the backcountry--despite the fact that we had persistant weak layers in a snowpack that had just received over 2' of new snow. Well, all I can say is "hindsight is 20/20."

On the way out to avalanche bowl, I began receiving reports of natural avalanche activity from the last 24 hours--Unski-a-bowl, a NE facing bowl above 10,000 and Teepee Glacier, a SE facing slope above 11,000' had slid during the cycle. Little did I know, Cody Bowl had also slid--six feet deep.

Arriving at Avalanche Bowl (9000', ENE) we chose a location to practice stability tests. We chose a 26 degree slope to minimize our exposure. Soon afterwards, a very experienced skier arrived with similar intentions. This person chose a steeper slope to dig on, a 38 degree NE facing slope. On approach, the skier triggered a small but potentially fatal avalanche (SS-ASu-D1.5/R2-O, down 70cm, 20m wide, ran 150m.) No one was caught.

In retrospect, I knew this surface hoar layer was present. Yet, while I did not have any incident, I still feel I did not give it enough respect--I over estimated the importance of my stability tests which all pointed to mostly stable snow. Additionally, at 7:15am on the 9th, I mistook no reported avalanches to mean no avalanches.

Interestingly, after the slide occurred we finished our stability tests and still got no results on three separate ECT tests (ECTN x 3).



We were eventually able to make the layer respond in a Propagation Saw Test (PST). During this test we got a PST 15/100 end.





Here are some photos which help indicate the size of the slab.

70cm crown in Avalanche Bowl, Teton Pass. The slab was about 70 feet wide and ran for about 400 feet.


Skiers, after tickling the dragon, aborting their mission to dig a pit after triggering SS-ASu-D1.5/R2-O. The slab avalanche may be hard to see in the shaded area of the picture.


In the last few years I have come to trust the Extended Column Test. Karl Birkeland has done research that demonstrates that this test can be administered on low angle slopes, has lower rates of false negatives than other pits, and--of course--the ECT tests for propagation whereas Compression Tests only look for failure in the weak layer. In this case, the ECT did not reveal the instability in three different ECT's in three different locations in Avalanche Bowl. In a fourth pit, the day before, an instructor had found the layer but had found it to be ECTN.

It also is a good reminder that surface hoar commonly surprises professional avalanche workers in the backcountry. When you find it, don't forget about it. So, take note of the spatially variable "12/5 buried surface hoar layer" (the day I believe it was buried.)

When we got back to the car we found out that the Powder Eight Face on Cody Peak slid on the October facets at the ground. The crown was 6' deep in places.

This experience reminds me yet again--safe travel habits that provide you with a margin of safety are the only things that will keep you safe when you underestimate the hazard. One thing that is a gaurantee is that you and everyone else out there (regardless of experience level) WILL underestimate the danger at some point. Having a healthy amount of humility is our strongest defense.

So, ski one at a time. Minimize your exposure to objective hazard. Avoid consequential terrain during or immediately after a storm. Practice your beacon skills. And always, always, always ask yourself, "what if I am wrong this time?"

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