Caught and Carried… a reminder small slab avalanches can have consequences

I have triggered a lot of small avalanches over the 10 years I’ve been an avalanche educator. They have all been intentional, small, and inconsequential. Yesterday morning I triggered one that bordered on the line of consequential as it carried me about 20 feet down slope towards some uninviting looking trees. It was a fairly small avalanche with a crown that was estimated to be about 25 meters across the slope and 30 centimeters deep at its apex. I was high on the slab when it failed and able to self arrest with my ski poles in the bed surface quite quickly. While there were no injuries there is certainly something to learn from the experience so I’ll present the following account with that desire in mind.

Earlier this season a new client hired me for a Backcountry Ski Skills Course. After getting a resume of ski descents and experience from him we conducted a skills course into the bottom of Hillman’s Highway and had a very successful day. A few weeks later he was back to take a 3-Day AIARE Avalanche Course and toured with me in the Gulf of Slides. Having vetted his ski ability and fitness we made a plan to ski from the summit of Mount Washington.

Mother Nature however decided our plan to travel above tree-line on our day together would be unrealistic.

avalanche mount washington
Higher Summits Forecast for 2/19/19

After conveying that the summit was out for our day together he let me know he was still stoked to see how far we could go so we made a plan to head into Tuckerman Ravine and see if we could ski some of one of the renowned gullies located within.

The Mount Washington Avalanche Center forecasted “Moderate” avalanche danger with the following “Bottom Line”:

Low density snow yesterday has been affected by NW wind overnight which has produced relatively small new slabs that are possible to human trigger.

I had experienced this low density snow the day before during a tour in the Gulf of Slides on the last day of teaching an AIARE 1 Avalanche Course with Northeast Mountaineering. Some footage from that tour…

 

The next day we made excellent time up the Tuckerman Ravine trail reaching Hermit Lake in one hour and 15 minutes. Fausto is a fitness coach so it was easy for us to both stay warm in with the current conditions:

avalanche mount washington

After a quick re-fuel stop we made out way up the Little Headwall and observed some signs of instability on the slopes just above.

avalanche mount washington
Shooting cracks observed in thin fresh wind slab just above the Little Headwall

Here the winds were full value and we tucked our heads down and pushed on to Connection Cache. We moved off the trail to the right to find a brief reprieve from the wind and don face masks and goggles before deciding to push a little further into the ravine. The winds were a little less brutal as we set a skin track up to the right of the Lobster Claw run out. We had observed enough active wind loading to rule out entering any of the major avalanche paths and a few hundred feet up the far right side of the ravine we decided it was time to transition and head back down.

We dropped on some stiff wind board low angle terrain and I brought us down into the Cutler drainage then out to skier’s right aiming for the high exit that would connect us down to the far side of the Lower Snowfields. Here we traversed across a small steep slope one at a time before getting to the top of the chute that leads down to the flats then connects back out to the Cutler.

The snow was a little punchy here but we could make a few good jump turns down about 50 feet. I posted up under a little tree island and had Fausto stop just above the island. A slightly lower angle open slope to skiers right looked like it would offer a few more turns before heading back into the drainage but it looked size-able enough to warrant caution. I told Fausto to stay put while I traversed over and onto this slope trying to stay high with the intention of ski cutting it and posting up on the far side.

About halfway across the slope I saw it fail around me. There was no noticeable collapse or whumph but the cracks I saw everywhere in sight made it clear what was happening. I yelled “avalanche” as I started moving down hill and noted the crown was only 10 feet above me. I dug my ski poles into the bed surface and the little amount of debris that I was riding on continued downhill… I came to a stop about 20 feet below where I had triggered it. For a few seconds I watched the debris go down slope for about 100 feet through some small trees that I was glad I wasn’t meeting soon.

Looking back uphill I could see Fausto was still in the spot I told him to stay and I instructed him to traverse over to me as there was no significant hang-fire left and I wanted him to join me on the bed surface. I shot some video and measured the slope angle, aspect, elevation, and position with the convenient Theodolite app. We then made our way down to the bottom of the run out for a couple more pics of the slide.

 

avalanche mount washington
Theodolite app

avalanche mount washington

So what happened?

The 13 cm of super low density (3.7%) snow that fell during an almost windless day on February 18th got introduced to severe winds over the evening hours and into the morning. As mentioned in the bulletin today “winds often allows us to  exceed the 1:3-5 ratio of new snow to wind slab”. This new slab felt like it was probably 1-Finger to Pencil on the Hand Hardness Scale. I estimated the crown to be about 30cm deep at its apex. This small test slope is directly lee to the west winds that were howling all night and observed while we were in the area. In the video you can see active loading occurring directly following the avalanche.

While this was my first “unintentional” triggered avalanche I don’t feel that I was completely caught off guard. Signs of instability along with expert opinion in the form of the avalanche bulletin guided our terrain choices and we stayed under 30 degrees and outside of major avalanche paths for good reason. We moved one at a time across suspect slopes and stopped where we could watch each other. I went onto the slope that failed half expecting to perhaps trigger a small slab and assumed I would be able to stay above it. I ended up taking a short some-what controlled ride 20 feet downslope while Fausto watched from a good vantage.

With the benefit of hind-sight a more conservative choice would have been to descend skier’s left of the Little Headwall, basically down our skin track where we had already accessed slope stability. Assessing “top down” terrain is obviously harder to do. In speaking with one of the snow rangers my terrain choice was somewhat validated as a reasonable choice given the conditions. Still, I’ll be thinking about this day for quite awhile. Unintentional but somewhat expected is a strange way to think about triggering an avalanche, but that’s where I’m at right now.

I hope sharing these details and thought processes with the community is beneficial. It doesn’t happen with out acute knowledge that my choices can be judged with different levels of objectivity. As an avalanche educator though I strongly believe we should learn from every single avalanche involvement and being able to share your experience in a clear and transparent fashion can only benefit the greater avalanche community.

 

5 thoughts on “Caught and Carried… a reminder small slab avalanches can have consequences

  1. Hard to tell from the picture given how differently the terrain looks now, and unfortunately I didn’t bother taking a picture yesterday, but I think this is the same location as that little slide from a couple days ago or so?

    Like

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s