Vail & Summit County
AvalanchePatrick Gephart · May 1, 2019 · Vail & Summit County
Weather
Calm below 11,500 feet. High winds above 12,000 feet to the base of the couloir. Seemed like the wind was consistently S/SW. Overcast to broken cloud cover with short windows of sun.
Snowpack
High SWE in new snow was noticed from the trailhead and throughout the day. New snow ranged from 6 inches to what seemed like 1-2 feet in the couloir proper. Whumpf was noted in the snow pack below treeline in the flats but was concluded to be weak freezing in lower elevations. Wind slab was the key issue we were looking for during the day.
Avalanche
Myself and 1 partner (splitboarder) switched over to crampons in a safe zone from both wind and the slop itself below a large rock face. From the start of the boot pack, it was clear that the snow was deep, but no weak layers were evident, including wind slab. The new snow appeared to be well bonded and had come in entirely right side up. Once we got above 13,000 feet and toward the top of the couloir, we noticed a small slab, but it seemed very well bonded and showed no signs of propagation when breaking through this. Towards the top out this wind slab became thicker but again manageable due to its structure, or so we thought (heuristic in hindsight). A small but notable convexity right before the top-out was noted, which would be key later. We transitioned and set up for the descent. I was dropping first. Upon gaining speed and making the first turn, I noted the wind slab was a bit thicker where I was riding than where we had booted. As I turned by the small convexity, the wind slab broke, flowing skier's left and propagating a bit towards the skier's left, eastern aspect of the apron below the sheltered choke. I was able to turn hard skier's right and drive my hands in the consolidated snow in the bed surface, arresting myself and getting out of the slide path. I would say I was caught for 20-30 feet before exiting the slide (a GoPro video that I will submit later may be helpful here). The avalanche ran to the bottom of the couloir through the apron. The small debris pile and the wind slab portion that slid seemed to be isolated only to where we noted it upon inspection of the flanks. If we had booted up the climber's right, more eastern-facing aspect, I think we would have noted this as more of a weak layer much quicker and turned around. I would classify the avalanche as an R1-2 D1.5. If I had been taken by it, I would have gotten taken over a few rocks but would not have been buried. Heuristics certainly played a big part here. Both my partner and I consider ourselves conservative backcountry snowboarders with advanced snow science knowledge, safe practices, constant discussion, continuing observations, etc. We have safely navigated isolated pockets of wind slab before in isothermic spring snowpacks and managed them safely. We deemed what we found on the climb up "safe" due to its structure and noted resistance to propagation when punching through, although it was still a wind slab. The classic "I've been in these conditions before, and we managed them safely" was at play here. It only took one turn to find the shallow weak spot that propagated to a thicker slab to the skier's left to create a wind slab avalanche. Luckily, I was able to exit it quickly. The spatial variability of the slab's thickness was also a factor, as we were climbing in the thinner part on climber's left, but the thicker and more dangerous part was to our climber's right and out of observation on the way up. This was a good wake-up up and will keep our heads on more of a swivel in the future. Upon reading the avalanche forecast again for 4/30/19, we deemed it spot on and exactly what we encountered. Great reporting and a user error on our end.