- Location: Ptarmigan Hill, Vail Pass
- State: Colorado
- Date: 2018/02/08
- Summary Description: 1 snowboarder caught and injured
- Primary Activity: Hybrid Rider
- Primary Travel Mode: Snowboard
- Location Setting: Backcountry
- Caught: 1
- Partially Buried, Non-Critical: 0
- Partially Buried, Critical: 0
- Fully Buried: 0
- Injured: 1
- Killed: 0
- Type: HS
- Trigger: AR - Snowboarder
- Trigger (subcode): u - An unintentional release
- Size - Relative to Path: R3
- Size - Destructive Force: D2.5
- Sliding Surface: G - At Ground/Ice/Firm
- Slope Aspect: N
- Site Elevation: 11710 ft
- Slope Angle: 35 °
- Slope Characteristic: Convex Slope
The avalanche was triggered by a snowboarder on a north-facing, near-treeline slope. It was medium-sized relative to the path and had the destructive force to bury, injure or kill a person (HS-AR-R3-D2.5-O/G). The avalanche failed on a layer of faceted crystals, above a melt-freeze crust, and stepped down to depth hoar at the ground entraining the entire season's snowpack. The crown face averaged 3 feet, with a maximum thickness of 6 feet. The width of the avalanche was 230 feet at the top, widening to 400 feet, as it propagated into an adjacent avalanche path.
Snowfall data is from the Copper Mountain Snotel site located at an elevation of 10,550 ft and 4 miles east of the accident site. Wind data is from Vail Mountain's China Bowl station at an elevation of 11031 ft and located about 7 miles northwest of the accident site.
The Snotel site recorded below-average amounts of snow for the months of October, November, December 2017 and January of 2018. On January 31, 2018, accumulated snow-water-equivalent was 80% of the long-term record. Several multiple-day periods of clear weather with no precipitation followed small snowfall events during these four months. An unsettled, west to northwest weather pattern began to affect the area on February 1, 2018. Small amounts of snow fell through February 5, 2018 with westerly winds averaging 15 to 20 mph. From 6 AM February 5th to 6 AM February 6th there was a 5" increase in snow height. Vail Mountain ski area reported 11" of new snow in the last 24 hours on the morning of February 6th. Westerly winds increased during the early-morning hours of February 5th with averages of 25 mph and gusts to 45 mph out of the west. In the 24 hours prior to the accident westerly winds averaged 10 to 20 mph with gusts to 30 mph. The air temperature at 12,000 ft at the time of the accident was 28 °F.
Below average snowfall during the 2017/2018 winter with multiple periods of clear and dry weather led to the development of large-grained depth hoar near the ground on shady slopes throughout the zone. During early-season dry periods, temperatures were warm enough to create a thin, melt-freeze crust on shady slopes above the depth hoar layer. Westerly winds during the first week of February formed wind-drifted slabs on top of this snowpack structure.
Events Leading to the Avalanche
A group of six snowboarders left the Vail Pass trailhead on five snowmobiles on the morning of February 8th to access the slopes around Ptarmigan Hill for snowboarding. Members of the group were in the area on the day prior and snowboarded northerly slopes to the west of the accident site and the southeast slopes of Ptarmigan Hill. The group snowmobiled to the bottom of the north-facing slopes of Ptarmigan Hill where they dropped off three of the five snowmobiles. The group used the two remaining snowmobiles to shuttle up to the top of an area known as Cupcakes. They dropped off four snowboarders, who rode down without incident. The four snowboarders met up with the two snowmobile drivers at the bottom of the run. The group switched riders and again shuttled up towards the top of Ptarmigan Hill. The group was moving east in the terrain as they tracked out areas to the west. They had established they would not be going into Avalanche Bowl and that was their boundary to the east. They did not observe any obvious signs of instability in terrain they rode previously. They dropped off four snowboarders on the right side (skier's right) of their previous run. Riders 1 and 2 stayed closer to the group's first run while Riders 3 and 4 went further right towards a steep, treed chute. This chute is just to the skier's left, or west, of what is known as Avalanche Bowl.
Rider 3 descended first. After a few turns and descending over an 8-foot cliff, Rider 3 felt uncomfortable with the slope. He pulled off to the left and waited for Rider 4 to descend. Rider 4 began his descent and after a few turns triggered the avalanche. Rider 3 yelled "avalanche". Rider 2, on the adjacent slope heard the sound of the avalanche and the scream and stopped. Rider 4 was caught in the avalanche and swept off his feet. He was able to deploy his airbag pack. Rider 4 was carried head-first and on his back in the avalanche about 300 feet before he hit a tree, stopping his downward progression. The avalanche debris washed over Rider 4 and continued down the path. When the debris came to rest he was not buried but had suffered multiple injuries from impacting rocks and trees.
Rider 3 was able to communicate to Rider 2 that he was not involved in the avalanche and that Rider 4 was hurt. Rider 2 was able to communicate with Rider 1, who was still up on the ridge, that everyone in the group was accounted for. Riders 1, 2, and 3 moved to Rider 4's position. Rider 4 thought his shoulder was broken but that he would be able to scoot down the slope to the snowmobiles below. He made his way down the hill with help from the rest of the group. Near the bottom of the slope, slightly uphill of the avalanche debris, Rider 4 put his snowboard on and rode to the snowmobiles below. Rider 4 traveled by snowmobile to the trailhead and then in the group's personal vehicle to the hospital.
Figure 8: A crown profile of the avalanche.