CAIC

Details

Date: 2019/05/01

Observer: Patrick Gephart
Organization: Public

Location

BC Zone: Vail & Summit County
Area Description: N/NW facing couloir off east ridge of Pacific Peak

Weather

Weather Description: Calm below 11,500. High winds above 12,000 ft. to base of couloir. Seemed like wind was consistently S/SW. Overcast to broken cloud cover with short windows of sun.

Snowpack

Snowpack Description: 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 couloir proper. Whumpf was noted in snow pack below treeline in flat but was concluded to be weak freezing in lower elevations. Wind slab was the key issue we were looking for during they day.

Avalanches

Avalanche Description: 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 back 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 came in entirely right side up. Once we got above 13,000 ft. 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 ticker where I was riding then where we 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 (GoPro video that I will submit later may be helpful here). The avalanche ran to the bottom of the couloir through the apron. 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, most 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 a R1-2 D1.5. If I had been taken by it I would have gotten take over a few rocks, but would not have been buried. Heuristics certainly played a big part here. Both myself and partner 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 snow packs, and managed them safely. We deemed what we found on the climb up "safe" due to its structure and noted resistance to propagate 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 skier's left to create a wind slab avalanche. Luckily I was able to exit it quickly. 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 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.

Date

Location/Path

Elev

Asp

Type

Trig

SizeR

SizeD

2019/04/30 ^†

10-mile Range

>TL

D1.5

Media

Images

<b>Figure 1:</b> Crown looking towards skiers left. I triggered skier's right (<a href=javascript:void(0); onClick=win=window.open('https://caic-production.imgix.net/bkwqfyjiprflxpribfpe53jduz71?ixlib=php-3.1.0&s=f8df9242f0a55d54d604adf711a6014d','caic_media','resizable=1,height=820,width=840,scrollbars=yes');win.focus();return false;>see full sized image</a>)

<b>Figure 2:</b> Avalanche propagation, flank and bed surface in first apron before second choke (<a href=javascript:void(0); onClick=win=window.open('https://caic-production.imgix.net/wetd9axjr6k08wqzv9waqvha5tix?ixlib=php-3.1.0&s=869f76895a93db63c6eea953d2a6b893','caic_media','resizable=1,height=820,width=840,scrollbars=yes');win.focus();return false;>see full sized image</a>)

<b>Figure 3:</b> Avalanche propagation and flank (<a href=javascript:void(0); onClick=win=window.open('https://caic-production.imgix.net/v3tyto3j0e5gv6auobc9wzay8968?ixlib=php-3.1.0&s=158bc4e81942a13f957472c2cb6404eb','caic_media','resizable=1,height=820,width=840,scrollbars=yes');win.focus();return false;>see full sized image</a>)

<b>Figure 4:</b> Looking up at avalanche propagation and flank from towards the bottom of the couloir. Note the few rocks that would cause injury if taken over. Couloir goes looker's left past view. (<a href=javascript:void(0); onClick=win=window.open('https://caic-production.imgix.net/cddidiaeondp9zpifvzb4yp6fzal?ixlib=php-3.1.0&s=2063338472238acd05923eab606fb64f','caic_media','resizable=1,height=820,width=840,scrollbars=yes');win.focus();return false;>see full sized image</a>)