Human-triggered avalanche: Central Gully, Huntington Ravine

Overview:

On February 26th, a pair of skiers were descending below the choke in Central Gully in Huntington Ravine when one of the skiers triggered a large avalanche (D2). The skier was not caught or injured, but shared lessons learned from their experience. 

Weather/Snowpack:

The avalanche hazard rating for Wednesday, February 26 was CONSIDERABLE with a wind slab avalanche problem. Wind slabs were possible up to size D2 (large avalanches that could bury, injure, or kill a person) on northeast, east, southeast, and south aspects.  The avalanche forecast “Bottom Line” read: 

You could trigger a large avalanche in wind-drifted snow on steep east, and south-facing slopes above 3000 feet today. To reduce your risk, steer clear of slopes with obvious drifts and where the wind has stiffened and smoothed the snow surface. Limit your time underneath steep, connected snow slopes to reduce your exposure to natural avalanches.

In the previous 24 hours, a low pressure system out of the west moved slower than forecast which increased snow total overnight to 3.7 inches which elevated the avalanche hazard on the 26th. The snow initially fell with little to no wind and winds shifted to the northwest and quickly increased to 50-70 mph. This setup was something forecasters discussed on the morning call and wrote about in the forecast discussion: 

Look out for strong-over-weak structure in new wind slabs. This snow initially fell with light winds on a firm old snow surface, this soft cohesionless snow is the weak layer of concern. As wind speeds increased, winds began to transport this light snow and build wind slabs, these slabs are the stronger layer on top. You can identify this structure by digging down to the old snow surface. The depth will help you to understand the size and using informal tests like hand shears or formal column tests will help you to understand the sensitivity.

Natural avalanches are a possibility today, especially if snow is still actively loading. The biggest red flag to look for is blowing snow onto loaded snow slopes. If you encounter this, consider limiting time under well developed snow slopes as paths in most east-facing terrain are large and fully developed. 

Continued snow showers throughout the day added 1.7 inches of snow exceeding the trace to 1 inch of snow forecast. The additional snowfall increased the size of the building wind slabs, and the continued active loading prolonged their reactivity.

This reactivity during the day was noted by multiple field observations. Two professional observations (obs 1, obs 2) found shooting cracks, strong-over-weak structure, and were able to trigger small avalanches (D1) on representative test slopes. 

Events:

Skier 1 and Skier 2 got an early start to the day passing Harvard Cabin in orange alpine glow. Ascending the Huntington Ravine Fireroad they found 1-3 inches of new wind blow snow and discussed the likelihood of encountering touchy wind slabs building on top of the firm pencil-hard old snow surface and the plan to assess for this instability.

Arriving into the Huntington Ravine floor both skiers noticed erratic wind and blowing snow with limited visibility. In drifts low in the ravine they found 4-6 inches of soft snow atop the pencil-hard layer. This new snow was 4F hardness over a softer F hard snow layer. This hardness change was very subtle, and with only light cracking observed underfoot the pair of skiers continued up the Fan. 

Along the Fan they found variable surfaces, with some scoured to the pencil hard layer, ankle deep drifts, and drifts deeper than their boot tops. At the top of the Fan, in an alcove underneath Pinnacle Gully, the pair discussed the deeper snow drifts they were finding and if they would ski the Fan from there or continue higher to climb Central Gully. Skier 2 pulled out their shovel to do a shovel shear test in the 18 inches of drifted snow. They did not find any shear on the new snow/pencil hard old snow surface. Skier 1 then performed a ski cut on a steep snow drift in the alcove and found no cracking or red flags other than sending a small sluff downhill. 

Not finding instabilities in this location, the pair decided to ascend higher into Central Gully. At this point the pair put on crampons and early in the climb found waist deep drifts of snow that became shallower again. At this point the pair of skiers were by another predetermined decision point, a small ice cave underneath and to the left of the choke in Central Gully. As they were finding scoured, and firm old snow surfaces they made the decision to climb higher and through the choke and agreed if they continued to find this surface they would ski Central from the top. 10 feet above the choke, they found a denser wind slab that was 18 inches thick. This was easily isolated and had a planer shear on the firm, old snow surface. 

The pair of skiers quickly decided that they wanted to down climb the choke of Central and avoid the firmer and reactive wind slab above them. Back in the ice cave underneath the choke, the pair transitioned to ski and planned to ski one at a time. When Skier 1 descended parallel to the wall of Pinnacle Gully and skied over a convex feature of the apron, a denser slab fractured and Skier 2 yelled “avalanche”. Skier 1 was able to turn towards Pinnacle and out of the path of debris. The avalanche propagated all the way to the choke of Central and this unsupported snow caused the whole gully to release. The skiers then skied out on the pencil-hard bed surface and noticed that nearly the whole ravine had been stripped to that layer, with the debris depth averaging 4 feet. 

They then skinned to Hermit Lake to inform the USFS Snow Rangers and promptly submitted an observation before 12:00 PM to share reactive snow conditions with the community. 

Avalanche Statistics 

Type: Soft Slab

Trigger: Skier, unintentional

Aspect: East

Slope Angle: 40 degrees

Size: R3, D2

Elevation: 4700 Feet

Sliding Surface: New/Old Interface

Width: 500 Feet

Discussion

One of the main reasons we share details about avalanche incidents is for education and awareness for the greater backcountry community with the hopes of becoming collectively safer. In return for sharing this information, we ask that you take the time to read and reflect on lessons learned in a non-judgmental and constructive manner. 

In many ways this group did many things right, but they also self-admittally missed some clear red flags and warning signs. They each had all avalanche rescue gear of a beacon, shovel, and probe, are daily readers of the MWAC forecast, and limited their exposure by descending one at a time. They alerted our staff and shared information about this avalanche as well as submitted an observation promptly with the hopes of informing the backcountry community of reactive conditions observed. 

While we could go through this pair’s decision making, the skiers themselves have shared some thoughtful reflections that we feel are valuable. The themes of confirmation bias and spatial variability are well written to and this combination has caught many professionals and experienced recreators by surprise.

MWAC would like to thank all the skiers involved for their openness and honesty about their avalanche with the hope of public awareness and education. This information is incredibly valuable to our forecasting team and backcountry community as a whole.

Some takeaways, lessons learned, and thoughtful words shared from the skiers: 

1. “Confidence in our data, desire for good turns, and confirmation bias played a big role in terrain selection, snow assessment, decision making, and communication that day. I allowed positive test results to override gut feelings and visual observations. Upon performing tests and getting our skis in the snow I started thinking, “Things are looking better than expected. Maybe we will get some good turns today after all!” I extrapolated that conditions across the apron would be fairly similar to the test slope. Maybe this was a subconscious effort to justify pushing it a little higher to gain 10 more turns. But I believe another part of my subconscious was making the argument to turn back, only I didn’t listen. It wasn’t what I wanted to hear. When suggesting a test slope I remember looking up at the deeper areas of wind loaded snow to the right and thinking, “I don’t want to go over there, that doesn’t look safe. Seems kind of deep and exposed.” I suggested the area below Pinnacle gully because it seemed more isolated and free from overhead hazard but also deep and wind loaded. A good spot to try to trigger a small avalanche with a ski cut in a controlled setting. In the end, the slope I ski cut was the only part of Central Gully that was not ripped out by the avalanche.”
2. “This experience has hammered into my head that spatial variability on Mount Washington exists not just between one gully and the next but between micro terrain features, elevations, and aspects within each gully. The southwest wind that day was coiling over the top of Pinnacle buttress, creating a relatively sheltered zone beneath the cliffs that did not show obvious signs of instability. The wind had been funneled down the upper part of Central Gully and was hammering the section above the choke as well as the skiers left side of the apron and depositing a pile of spindrift against the rocks skiers left. By the time we reached the dense and reactive wind slabs, we had already climbed too high. I felt the snow become increasingly cohesive as I trended left on the descent but it was too late. In my defense, the spindrift had partially obscured how planar and obviously slabby the slope was, but I should have taken a minute to sit in silence at the top of the fan and watch where the wind was blowing. Tune into my gut feelings. Think about mountain hazards beyond the avalanche hazard. These visual observations and gut feelings are not to be ignored – they should be verbalized. And should have made me say, “Things are looking good over here, but it could be a totally different story over there. And these slopes are connected. And there are a bunch of boulders below us. Let’s stop while we’re ahead.””
3. “My perspective on backcountry skiing has shifted. I’ve gone out skiing a few days since the avalanche and had some of the most fun I’ve had all season even though most of the turns have been nothing to write home about. I seem more fulfilled just by breathing in the fresh air, chatting with friends, and enjoying the setting together. I want to make good soft turns and ski classic lines in midwinter conditions just as much as I always have, but I don’t feel any rush. I feel less jealous when I see other people have been getting out and skiing the steeps. The mountains are always talking – we are the ones who need to slow down and listen. And if it doesn’t come together with partners, work schedules, or weather, that’s alright with me.”
   

   Central Gully from a previous year to show the path of the large avalanche