2013/04/18 - Colorado - Ptarmigan Hill, near Vail Pass

Published 2013/04/30 by Tim Brown - Forecaster, CAIC

Below average snowfall at the beginning of the 2012-13 season produced a shallow and variable snowpack throughout the Vail and Summit County forecast zone. Wind-drifted slabs of varying thickness formed over widespread weak layers of depth hoar and facets. Even modest storms produced avalanche cycles on these persistent weak layers for most of the season. Avalanche conditions improved at the end of March and early April until strong storms with significant snowfall reactivated deeply buried weak layers in mid-April.

The majority of the snow that fell in October melted away, leaving discontinuous patches of snow on shaded slopes and in wind-drifted areas near and above treeline. Small amounts of snow fell in November and quickly formed a more continuous layer of depth hoar. In early December, several days of warm weather produced a thin crust at the top of the depth hoar, even on northerly aspects above treeline. As it began to snow again in mid-December, small facets formed above this crust and many avalanches ran as larger storms built slabs on top of the weak, faceted layer and underlying crust. By the end of the month, prevailing westerly winds had drifted snow into thicker slabs above the persistent weak layers and several large, natural and human-triggered slides were reported on northeast and east aspects near and above treeline.

Only a couple storms passed through the area in January, so the shallow snowpack continued to facet and more crusts formed on sun-exposed slopes. During the last few days of January and the first few days of February, 18" of snow (1.1" SWE) fell with strong west-northwesterly winds near Vail Pass. This incremental loading period reactivated buried weak layers and produced a significant avalanche cycle that included numerous natural, remotely-triggered, and sympathetic avalanches in the Vail and Summit County zone. Intermittent cycles of large avalanches continued throughout February and most of March as modest storms, some with only 6" of snow and moderate winds, overloaded buried weak layers.

Almost two weeks passed with warm days, cold nights and no significant snowfall at the end of March and the beginning of April. By this time, the depth hoar was buried by at least 3 feet of snow in most places. Deep persistent slabs became less likely and were mostly dormant except for one avalanche incident involving a full burial near Loveland Pass on March 29. Layers in the middle of the snowpack gained strength, but the lower snowpack remained weak and retained a thin crust between the depth hoar and an overlying faceted layer, especially on northerly and easterly aspects near and above treeline.

When it finally began to snow again on April 5, the weak layers became reactive again. The rapid, heavy load from April storms initiated a deep-slab avalanche cycle that began in the adjacent Sawatch forecast zone and then continued in the Vail and Summit County and Front Range zones. Skiers triggered large avalanches near Independence Pass and Buckeye Peak in the Sawatch Range on April 6 and April 8, respectively. Then, riders were caught and partially buried in slides involving deeply buried weak layers near Montezuma on April 11 and in East Vail on April 12. On the same day as this accident, large natural avalanches ran on nearby Jacque Peak, in Straight Creek near the Eisenhower Tunnel, and near Boreas Pass. Two days later, 5 people were killed by an avalanche on a similar, north-facing slope near treeline in Sheep Creek near Loveland Pass, less than 25 miles away.  

The snowpack varied greatly on the north side of Ptarmigan Hill at the time of the avalanche. The crown face was 12 feet deep and contained very hard slabs (Hand Hardness Index values of K) on its west (skier's left) side while a snow profile near the east side of the crown was less than five feet deep and contained much softer slabs (Hand Hardness Index of 1F and 4F). On the east side, the snow was weak and faceted at the intersection of the bed surface and the crown face (see Figure 9). Snowpack tests in this area revealed a high likelihood for propagation in a well-preserved layer of facets above a very thin crust at the top of the depth hoar (see Figures 10 and 11). 

On the day of the accident, the CAIC's backcountry avalanche forecast for the Vail and Summit County zone stated: "Deep persistent slabs are still a lingering concern. The new snow load could be enough to affect deeply buried weak layers that have been intermittently reactive for the last couple months. The slopes where these layers remain the weakest are north through northeast to east aspects near and above treeline where the snowpack is still cold and winter-like." The forecast highlighted two large avalanches on northeast and east aspects near treeline less than 20 miles away in the previous week and noted that one ran naturally and the other was reportedly triggered by the fifth rider on the slope. Pictures of both of these slides were included the previous day. Finally, the forecast warned, "You are most likely to trigger large avalanches from areas of shallow snow along the margins of thicker hard slabs."

Two snowboarders met in the parking lot at Vail Pass shortly after 9am, rode their snowmobiles about four miles into the backcountry, and took turns shuttling one another to the top of Ptarmigan Hill. They made two to three laps on the north side of Ptarmigan Hill, in an area known locally as Cupcakes. This area is just to the west of the well-defined avalanche path on the north side of Ptarmigan Hill, locally known as Avy Bowl, where the fatal avalanche occurred later that day (see Figure 1). Around 11am, a third person joined them and the group made several more laps, including multiple runs in Avy Bowl. They avoided the large cornice/wind roll at the top of the bowl, instead choosing to enter the bowl from its right and left sides before working their way to the middle farther down the slope.

This group was very familiar with the area. Each of them had ridden there for multiple years, including many days this season. Riders from another group that was riding Avy Bowl that day said they had seen three groups do three to four laps each in Avy Bowl before the avalanche. Some of these riders jumped off the large rock on the upper skier's left (west) side of the avalanche path. One of the victim's partners reports that the victim entered the bowl from the skier's right (east) side at least twice earlier that day.

At approximately 1pm, the three members of the group involved in this avalanche left the top of Ptarmigan Hill. One of them rode a snowmobile along a road to the bottom and waited for the two riders. Rider 1 went to the north and entered the avalanche path from the skier's right (east) side while Rider 2 went northwest above the top of the bowl to enter from the skier's left (west) side. Rider 1 waited at the top of his line for Rider 2 to come into view and remembers last seeing him near the large rock (see Figure 12) on the skier's left (west) side of the bowl.

Rider 1 planned to drop off a rock below and describes seeing the snow slide off the rock as he approached it, which indicates the avalanche was moving at this time. As he landed, he remembers the snow "looking like an ocean" all around him and he was knocked down by snow that came from behind him. He fought to stand up, keep his board on the surface, and gain speed, but could not move faster than the moving debris. The speed of the flow decreased as it hit a bench part way down the avalanche path on the skier's right (east) side, and the avalanche released him into the trees below (see Figure 13).

Rider 2's location at the time of the slide is unknown, but he was caught in the slide and carried down the slope and through thick vegetation. He came to rest on the surface of the snow against an 8" diameter tree near the bottom of the debris.

Rider 1 rode to the bottom of the slide path, hoping to meet up with Rider 2 where the snowmobiles were parked. He met up with the third member of the group, and the two of them rode one snowmobile back up to the bottom of the debris. Upon reaching the bottom of the debris, the third member of the group turned his avalanche beacon to search and began moving uphill. Within a minute, they saw Rider 2 on the surface of the snow with several broken trees across his back. They removed him from the tree and noticed that he had sustained significant trauma. The third member of the group began CPR as Rider 1 rode to the top of Ptarmigan Hill to attain phone service and call for help.
After placing a call to emergency services at 1:30pm, Rider 1 told another party that was at the top of Ptarmigan Hill about the accident. Two more people showed up about the same time. Two people from each of these groups worked their way down the slide path, using their beacons to search the debris for additional victims.

Two US Forest Service snow rangers responded to the avalanche, led resuscitation efforts for the victim, and organized a search for additional people that were unaccounted for. Members of the Eagle County Sheriff's office, Summit County Sheriff's office, Vail Mountain Rescue, and Summit County Rescue Group also aided in the recovery effort.

There are several things we can learn from this accident. First, persistent weak layers can go from reactive to dormant to reactive again several times within a winter. In this case, deep persistent slabs had become difficult to trigger during a prolonged period of warm, dry weather for nearly two weeks prior to the accident. Despite heavy traffic in this area, buried layers of faceted snow remained weak on shaded, high-elevation slopes. Intense snowfall and wind reactivated the layers over four months after they formed.

Deep slab avalanches are difficult to forecast and manage. When dealing with them, we should expect the unexpected. These riders and many others rode this terrain countless times without incident before the fatal avalanche. Deep slabs are often triggered from areas where the snow is shallow and weak. It is difficult for us to affect weak layers when they are under thick, hard slabs, but the cracks we start in thin areas can propagate into deeper snow, producing very large avalanches. Shallow areas are often difficult to recognize. Monitoring avalanche trends in surrounding areas can be effective ways to help anticipate these types of events.

When faced with uncertainty, make conservative decisions. Choose your terrain carefully, move one at a time through areas where you are exposed to avalanches, maintain contact, and regroup out of harm's way. This will minimize the likelihood of triggering a slide onto another person in your group or having more than one person caught. Finally, this avalanche reminds us that even if you remain on the surface, getting carried into the trees can have fatal consequences.