On December 27th 2021, I remotely triggered an avalanche of serious destructive magnitude while traversing a north-to-south ridgeline, at approximately 9,600 feet in the west Teton backcountry, on skis. The deep persistent slab avalanche broke with a 6-10 foot crown, ran 1,400 feet, uprooted many trees and failed on the “December 5th weak layer”. Read on for a comprehensive review of the avalanche, the current state of the Teton snowpack and a few tidbits on remote triggers.
A Brief Snowpack Intro
Over the past two weeks the Tetons have received a monumental amount of snow accompanied by very strong winds. In the last seven days the avalanche danger has been rated considerable above 7,500 feet. The previous four days (Dec. 23-27) brought approximately 40 inches alone, with about three inches of SWE (snow-water equivalent). The avalanche concern was two fold; wind slabs on northern and easterly aspects and a classic persistent slab problem on all aspects above 8000-9000 feet, the result of a buried solar layer formed by two weeks of warm and sunny weather following a preceding storm on December 5th. Though this rapidly buried deep layer seemed surprisingly resistant to natural and human triggered activity, recent gale force winds prompted worry for colossal failures on leeward slopes where several feet of snow was added to an already significant overhead loading.
Treasure Mountain Avalanche
On December 27th my partner and I set off from the Teton Canyon trailhead at approximately 10:30 AM. The air temperature was very cold and the skies were thick overcast. Light snow was falling. Our plan was to use the roundabout “Lake Valley Traverse” by way of Chicken Knob, as detailed in the Targhee Backcountry Ski Atlas, to access the west facing Eddington Chutes on Treasure Mountain. With careful route finding we managed to reach the summit of Treasure around 2:30 PM, avoiding exposure to avalanche terrain. Consistent trail breaking through 2-3 feet of snow slowed us down monumentally. At 10,000 feet the air temperature was below 10 degrees, with little wind and overcast skies – surprisingly docile.
To access the Eddington Chutes area we traversed north on a narrow alpine ridge from the USGS Treasure Mountain summit. Colossal cornices and wind loading loomed on the steep easterly slopes above Hanging Valley, so we hung mostly on the west side of the ridge using the many trees and boulders as anchoring points. Eventually the ridge got quite narrow and terminated in cliffs too precarious to navigate without a rope. The slope angle directly east was gentle, well less than 30 degrees and stabilized by an isolated grove of trees. My partner and I assessed the slope and determined that it wasn’t avalanche terrain. To bypass the cliffs I slipped through a break in two cornices and slid into a safe cluster of pines, but the second my skis touched the easterly slope we felt, and heard, an immense collapse.
What we experienced was astonishing. Starting from the bases of my skis, an enormous shooting crack launched 500 feet south, out of the safe zone and onto the steeper heavily loaded slopes of “Happy Days”, the prominent slope directly northeast of Treasure Mountain’s summit. Here, a deep persistent slab avalanche propagated on the December 5th weak layer spanning the full width of the line. At its’ deepest the crown was about ten feet, with a consistent depth of at least six feet. The torrential freight train of snow was mythical, riding the full 1,400 feet into Hanging Valley and spraying residual debris up the west facing slopes of USGS Peak 10,251. My partner, who had a better vantage point than I, claimed several large trees were uprooted, “bent at 45 degrees.” The slope I skied onto didn’t budge, and no humans were implicated. We chose not to perform a search of the path because of the virtually non-existant chance that there were parties below. We were the only humans atop Treasure Mountain that day, and no ski tracks were observed entering Hanging Valley, which is not easily accessible from below.
After photographing the carnage, we pivoted and were surprised to see our shooting crack also extended several hundred feet to the north. A second but much weaker avalanche propagated on a 30 degree easterly slope, running for about 500 feet before dissipating on lower angle slopes below. The crown was approximately four feet. Shell shocked and chocked with nervous energy we traverse gingerly back to west facing slopes and enjoyed 2,500 feet of fall-line tree skiing in a healthy two feet of supportable powder. As expected, no signs of wind loading or persistent layer activity were observed on this aspect. Though the turns were immaculate, every face shot was shadowed by the sinister display of just how destructive, unruly and merciless Mother Nature can be.
What does this tell us?
The ability for a single skier to cause a third of a mile wide failure remotely, from a protected and shallow angled slope, points to incredible fragility and volatility in the snowpack. Crown depths of up to ten feet indicate the immense amount of wind transported snow that has been deposited onto north, east and in some places south, facing aspects. The buried December 5th persistent weak layer is clearly not dormant. Though this layer is becoming harder to trigger by humans alone, substantial snowfall and subsequent wind loading has proven a worthy instigator of destructive avalanches. Even after the snowpack gets a chance to heal, large, rapidly developed cornices will be lurking over most high elevation east facing aspects. We rated our first observed avalanche R4D3, a product of incredible crown depth, a slide distance well exceeding the predictable path and destructive force capable of uprooting large pines. The second avalanche to the north, with a crown depth of four feet and an approximate path length of 500 feet, was R2D2. On the December 28th avalanche bulletin, the Bridger-Teton Avalanche Center said “today would be a good day to let the snowpack heal.” I couldn’t argue that sentiment more. In the long term, this event urges caution on all steep, wind loaded or otherwise stressed slopes, where a dangerous sleeping dragon, born in early December, has not yet entered hibernation.
Key Takeaways – Remote Triggers
I would wager that most backcountry travelers reading this article lack a firm understanding of remotely triggered avalanches. I certainly did. I was aware of the concept from avalanche courses and Bruce Tremper books, but I never could’ve fathomed the weight of a single 180 pound skier producing a quarter-mile shooting crack from a low-angle, lightly treed safe zone, let alone producing two remotely triggered destructive avalanches. Even after my firsthand experience, the thought is difficult to imagine. Avalanche.org offers this simplified summary of remote triggers:
“Someone does not need to be on the avalanche to trigger the avalanche. Especially in a snowpack with high propagation potential, a person can initiate a fracture from some distance away. We call these “remote” triggers. It’s common to remotely trigger an avalanche from the ridge above a slope, a gentler slope next to the avalanche and especially from a flat or gentle area below the avalanche. Needless to say, if you remotely-trigger an avalanche, the snowpack is extremely unstable and you need to choose your routes very carefully.”https://avalanche.org/avalanche-encyclopedia/remote-trigger/
Basically, the more unstable a snowpack becomes, the higher the probability of remote triggers. According to a chart published on avalanche.org, until the snowpack reaches a point of relatively high instability, the likelihood of remotely triggering an avalanche is near zero. My key takeaway from this experience is a newfound respect for the ability to trigger avalanches from safe terrain. On days of exceptional danger, I will assume a constant vigil for areas of potential remote trigger, and take care to avoid adjacent terrain in the vicinity of vulnerable avalanche paths.
As always, a huge shoutout to my main squeeze Icelantic Skis, makers of great slippery wooden sticks immune to early season rocks. The Natural 111 was my lightweight powder machine for the day, and boy did they rip it up. In the market for a burly, but still ultralight pair of powder crushers capable of everything from Teton Pass powder laps to mid-winter ski mountaineering? Look no further. Check em’ out at icelanticskis.com