2015/01/01 - Colorado - McFarlane's Bowl - The Happy Face

Published 2015/03/17 by blase reardon - Forecaster, CAIC

A layer of near-surface facets developed at the top of the snowpack during the dry, mild weather in early December. When this layer was buried on December 13, the Height of Snow (HS) on shaded slopes in McFarlane's Bowl was roughly 24-32 inches. The underlying snowpack consisted of relatively supportable snow (Hand Hardness of 4-Fingers) that sat on thin layers of depth hoar and basal ice that developed during mild weather in October. 

Late-December storms nearly doubled the height of snow in McFarlane's Bowl to an HS of 46-56 inches by December 26. A 24"-thick slab had accumulated above the December 13 interface. The slab had a Hand Hardness of 4 Fingers and the underlying near-surface facets had a Hand Hardness of Fist-plus. Another 6 inches of new snow accumulated at Aspen Mountain between December and the day of the accident (January 1).

The near-surface facets buried December 13 were the failure plane for a series of avalanches that occurred in and near McFarlane's Bowl during Christmas week. These slides included natural and remotely-triggered slides on December 22 on slopes above McFarlane's, and a skier-triggered slide further north on Richmond Ridge on December 23. On December 24, the third skier in a Powder Tours guided party triggered a slide in McFarlane's but was not caught or carried in the debris. This last skier-triggered avalanche occurred on a very similar aspect as The Happy Face, though on a steeper slope. No avalanches were reported after December 24 on the east side of Richmond Ridge, and many nearby slopes received heavy ski and snowboard traffic.

The group was composed of two guides (Lead Guide and Tail Guide) and three clients. Additionally, a snow safety team of two (Snow Safety 1 and Snow Safety 2) was conducting a regularly-scheduled, bi-weekly visit to Powder Tours operational area to collect snow data. All four Powder Tours personnel carried avalanche rescue gear (beacon, shovel, probe). The clients carried just beacons.

Prior to the accident, the guides, clients and Snow Safety 2 had skied five runs elsewhere in McFarlane's Bowl without incident or signs of instability. Snow Safety 1 had skied the last three of those runs. For their sixth run, the group chose the skiers' left side of the Happy Face.

The Lead Guide and three clients skied the slope one at a time with no incident. The Tail Guide was 2-3 turns into his run when the avalanche released. Snow Safety 1 and 2 were on top of the run and witnessed the slope fracture. They were able to see the Tail Guide as he was carried downhill on the surface of the slow-moving avalanche until he came to a stop. He was strained through a small stand of trees, which spun him around and sent him headfirst downhill with only the bottoms of his skis visible. The Lead Guide watched the avalanche from a safe zone at the bottom and had a clear, face-on view of the incident. The clients had already started across the traverse from the base of the run to the snowcat pickup point.

Snow Safety 2 immediately radioed Aspen Mountain Ski Patrol dispatch notifying them of the incident. Once the avalanche debris stopped moving, Snow Safety 1 made his way down the bed surface to the Tail Guide, who was partially buried and injured. He was on his right side, head downhill, with one arm and part of one leg under the debris. His chief complaint was “a hurt leg”. The ski on the injured leg had released.

The Lead Guide started sidestepping towards the Tail Guide but stopped once he established verbal communication with Snow Safety 1 and the injured Tail Guide. The Lead Guide turned and accompanied the clients as they finished the traverse to the pickup point. The snow cat carried the Lead Guide and clients back to the top of the run. From there the Lead Guide skied a toboggan and other medical and rescue equipment down to Snow Safety 2, who in turn skied it down to Snow Safety 1 and the injured guide with the Lead Guide following.

Snow Safety 1, 2, and the Lead Guide loaded the injured Tail Guide into the toboggan and began pulling him to the snowcat pickup. By this point, ski patrollers from the top of Aspen Mountain had arrived at the top of the bowl, where they stayed to act as a secondary rescue party in the event of an additional avalanche. Snow Safety 1 used skins to make pulling the toboggan out on slightly uphill traverse easier. Once the toboggan arrived at the snowcat, the rescuers loaded the injured Tail Guide into the snowcat, which carried him to the top of the mountain. Ski patrol then transported him to an ambulance at the base of the mountain.

This accident is a reminder of both the unpredictable nature of Persistent Slab avalanches and the importance of maintaining safe travel protocols in order to mitigate that unpredictability.

On January 3, a CAIC Forecaster and a two-person Powder Tours snow safety team (including Snow Safety 1) conducted an on-site investigation of the January 1 accident. Based on snow profiles at several points in the crown and flank of the slide, they concluded it broke on the facets buried December 13. Both small and large column tests in and near the crown and skiers' left flank near the trigger point produced propagation on the this persistent weak layer with moderate force. They observed no other signs of instability on nearby slopes.

The Powder Tours personnel present on January 1 assumed that this weak layer was present when they made the decision to ski the slope. In making that decision, they considered a number of observations from nearby slopes with similar snow structures, including those they had skied earlier that day. Among them were the following:

• It had been eight days since the last reported avalanche in McFarlane's Bowl or other slopes on the east side of Richmond Ridge.
• Ski cuts and traverses from December 29 were present on the slope, including across the line they planned to ski.
• The weather in the 3-day interval had not markedly changed conditions on the slope. The party reported no freshly-formed wind slab on the slope prior to the slide and there was no evidence of wind slab in the crown and flanks of the slide during the on-site investigation January 3.

A factor they do not appear to have considered was a natural avalanche that occurred on the slope in mid-November. That early season slide left the snowpack shallower and likely more prone to faceting during the prolonged early December dry spell. While the snow structure in the crown and flank of the January 1 slide did not show notable differences with other slopes, it is possible that patches of weaker facets had formed above the bed surface of the mid-November slide. The slide may have been triggered at one of these points. None of the Powder Tours personnel present on January 1 were aware of the mid-November natural avalanche.

The decision to ski or ride a given slope when a Persistent Slab structure is present typically involves inferences based on observations from nearby slopes with similar snow structures, as was done in this case. However, an unknown degree of uncertainty - and thus risk - is inherent in these decisions, because the exact structure and its critical properties are unknown for all points on the slope. A snow profile and tests conducted on a slope prior to skiing or riding it cannot always resolve this uncertainty. Similarly, the presence of previous tracks on a slope is not a reliable indicator of stability, since those tracks may have missed trigger points or conditions on the slope may have changed in the interval. Moreover, the instability associated with a Persistent Slab avalanche problem may last for weeks, not just days.