- Location: Straight Creek, north of Eisenhower Tunnel
- State: Colorado
- Date: 2019/02/10
- Summary Description: 2 skiers caught
- Primary Activity: Backcountry Tourer
- Primary Travel Mode: Ski
- Location Setting: Backcountry
- Caught: 2
- Partially Buried, Non-Critical: 0
- Partially Buried, Critical: 0
- Fully Buried: 0
- Injured: 0
- Killed: 0
- Type: HS
- Trigger: AS - Skier
- Trigger (subcode): u - An unintentional release
- Size - Relative to Path: R2
- Size - Destructive Force: D2
- Sliding Surface: O - Within Old Snow
- Slope Aspect: --
- Site Elevation: 12200 ft
- Slope Angle: 36 °
- Slope Characteristic: Convex Slope
The avalanche was a hard slab avalanche triggered by skiers ascending a slope. The avalanche was medium sized relative to the path and had the destructive force to bury, injure, or kill a person (HS-ASu-R2-D2-O). The avalanche failed on a layer of 1 mm faceted crystals that were buried from 2 to 3 feet below the snowpack surface. This weak layer sat on top of a hard melt-freeze crust. The crown face was 32 inches deep at its maximum and averaged 24 inches deep. The width of the avalanche was estimated at 450 feet with a vertical fall of about 300 feet. The avalanche released on a south-facing slope above treeline. Although the crown broke on a part of the slope where the slope angle was 30 degrees, the slope was convex with a maximum slope angle of 36 degrees.
Several large storms in November and early December brought above average snowfall to the Vail and Summit County forecast zone. In mid-December the weather pattern changed and the area received near or below average snowfall through the end of January. This period was characterized by short stints of settled weather separated by small snow storms. One such period of clear weather was around January 20. A series of small storms with moderate to strong northwest winds followed from January 23 to 30. Temperatures climbed to near-freezing before another set of small storms dropped about 10 inches of snow over several days. Strong westerly and southwesterly winds drifted the new snow on February 7 and 8.
Dry periods in December led to the development of depth hoar near the ground throughout the zone. Small storms piled snow on top of the weak basal structure. The additional snow began to weaken under steep temperature gradients. Several small but windy storms deposited harder layers of wind drifted snow on top of this weak snowpack structure. In between wind events, warm temperatures around January 20 and then again around February 1 formed melt-freeze crusts on sunny slopes. The snow above and below these crusts faceted and was subsequently buried by wind-drifted snow.
Events Leading to the Avalanche
Three backcountry skiers ascended Straight Creek, starting from a trailhead just west of the Eisenhower Johnson Memorial Tunnel and north of Interstate 70. From the creek they traveled northwest and gained a ridge at a point northeast of Coon Hill. On the way up they observed slabs of wind-drifted snow and variable surface crusts. On the ridge they determined that much of their intended steeper terrain was best avoided. The areas of wind-drifted snow concerned the skiers. They toured northeast along the ridge, coming to a slope with both ski and skin tracks. It looked less steep than the adjacent terrain (investigators measured it at 30 degrees). All three skiers descended the slope without incident.
At the base of the slope, they agreed upon another lap and decided to skin up and link to the existing skin track. The skiers set their own skin track and did not wind up linking to the older track. Their skin track was further west (looker's left) of the slope they previously skied. Skier 1 described putting the skin track into steeper terrain than they intended.
Approximately 100 feet below the ridge Skier 1 noticed a crack shoot out in front of him. Skier 2 and 3 were in close proximity to each other and about 50 feet below. They felt a collapse and heard a whumpf. Skier 1 turned downhill and saw Skier 2 and Skier 3 as the avalanche broke. Skiers 2 and 3 immediately turned into the fall line. Skier 3 successfully deployed his airbag. Skier 2 pulled the trigger on his airbag but the airbag did not deploy. The avalanche carried both skiers to the bottom of the slope. Both skiers were visible to Skier 1 for the duration of the slide. Neither skier was buried, and both confirmed “OK” by radio.
Skier 1 was concerned he might trigger a second avalanche. He waited for his friends to clear the debris field. He then removed his skis and descended on foot to the crown. Once on the bed surface, he skied to the avalanche debris. Skier 1 confirmed with his partners that neither were injured. All equipment was accounted for except for a single ski pole.
Skier 2’s airbag did not deploy. He pulled the trigger, heard the air canister discharge, but the airbag did not inflate. Skier 2 frequently checked the connections and cylinder pressure, and has practiced deploying the airbag at the end of every season. This was the first time that the airbag did not deploy.
The skiers deliberately chose what they thought was the safest terrain in the area compared to the many steeper slopes. While most of the slope was lower-angled terrain, there was a short section of the slope that was 36 degrees in steepness. With a Persistent Slab avalanche problem, slopes that are connected to steeper terrain can be just as dangerous as large and obviously steep terrain.
Figure 8: Crown profile from an avalanche on a south-facing above treeline slope that caught and carried two skiers on February 10, 2019.