Monday, March 25, 2019
Sunday, March 24, 2019
Location Name or Route
Trigger: additional info
Buried - Partly
Accident and Rescue Summary
A party of six was caught and carried in an avalanche in Cardiac Bowl below Mt. Superior in Big Cottonwood Canyon. There were two partial burials, and one injury. The party was able to move from below the bowl to safer terrain in the drainage bottom, where a rescue involving patrollers from Cottonwood resorts moved the entire party out of the canyon by 4 pm.
Cardiac Bowl is a steep, north-facing bowl on the north side of Mount Superior (11,000') in Big Cottonwood Canyon. (Link to Wasatch Backcountry skiing map.)
Weather Conditions and History
A period of storminess with generally light winds moved into the region Thursday March 21. By late Saturday afternoon, the Cardiac Bowl area had received approximately 12" of new snow, with occasional periods of warming from sun as well as greenhousing. A colder system moved into the region overnight on Saturday, and persisted through the day on Sunday March 24. Beginning at approximately 10 am on Sunday March 24, the snowfall rate increased, with precipitation intensity reaching 2"/hr at times. Weather stations at Brighton resort reported 9" new snow, and Alta reported 10" during the day on Sunday March 24. Recorded weather data from the weather station at nearby Cardiff Peak showing temperatures and wind speeds.
[Forecaster Comment - Thanks to the party involved for contacting the UAC, and for discussing the details of the occurrence. Although there was one injury involved, the situation could have been much worse and we are thankful they were all able to safely return from the mountains.]
A party of 6 went into Cardiff Fork, with their first run into the drainage from Little Superior Buttress (LSB). They reported generating sluffing as they descended the north-east facing LSB (photo below taken the day after the accident), and cracking in the storm snow as they ascended into Cardiac Bowl, especially during periods of higher precipitation intensity. From the bottom of the Cardiff drainage, the party then began ascending towards Cardiac Bowl. They reported widespread cracking in the storm snow, propagating up to 2' wide from their skis, as they worked their ways up the bowl using the standard skin track on the east side of the bowl. Skier A was breaking trail, and approximately 300' below the summit of Superior, triggered a sluff of storm snow. As the sluff began to move, it sympathetically triggered a much larger storm slab avalanche that broke 200' above Skier A, and breaking out across the entirety of Cardiac Bowl. All 6 skiers were carried in the slide. Skier A was carried an estimated 600', and was buried up to their neck. Skier B was carried and buried to their chest, and sustained a possibly torn ACL. Skier C was carried a short distance, and sustained a partially-dislocated shoulder. The other three skiers in the party were carried shorter distances and were able to extract themselves from the debris. (SS-ASu-R4-D2.5)
Once the party was able to extricate all members from the debris, they called Alta Central, and requested a rescue as the injured skier was unable to move on their own. The party was able to construct a rescue sled by attaching skis together, where they were able to get down lower in the canyon and out from underneath the risk of additional avalanching. Patrollers from Alta and Snowbird ascended from Little Cottonwood Canyon, and met the party. Snowmobiles from Solitude resort approached from Cardiff Fork in Big Cottonwood Canyon, and were able to get the injured skier out. By 4 pm the party was safely out of the canyon. Forecaster note: the preferred and fastest way to get help in an emergency is to dial 911. State "This is a backcountry emergency, involving an avalanche accident." and then continue with other information and the details of the help you are requesting. We have a recently update Backcounty Emergencies page here.
Analysis: As the party was ascending Cardiac Bowl, Skier A reported finding a density inversion down about 6", with lower-density snow underneath a denser slab on top. The denser snow was most likely from a period of warming that occurred late morning, and the heavy snowfall quickly overloaded the weaker, low-density snow. They also reported the initial sluff was failing in the top 6", but the larger, sympathetically-triggered slide was 12-14" deep, possibly breaking out in an area that had more wind-loading. Several natural and human-triggered avalanches occurred during the day on Sunday, especially during the period of greatest precipitation intensity around mid-day. Reports included the same density inversion the party encountered in Cardiac Bowl.
The photo below shows the approximate outline of the crown of the avalanche. Th 'X' is the location where Skier A triggered the avalanche. This photo was taken on Monday March 25, after additional snowfall had obscured some of the crown.
A UAC forecaster was in Cardiac Bowl on Saturday March 23 and reported stable avalanche conditions. However, Sunday's storm rapidly changed conditions, and there were three red flags that should have alerted the party that the avalanche hazard was rising: (1) the sluffing on LSB; (2) widespread cracking in the storm snow; (3) the heavy rate of snowfall. They also had identified the weaker layer with the density inversion. Cardiac Bowl is very difficult terrain to manage with heightened avalanche conditions, with essentially "no place to hide". The party did spread out on the skin track, and this possibly made a difference by somewhat limiting people's exposure to the avalanche risk where some members were carried shorter distances and were able to get out of the debris without aid and assist those with deeper burials.
Avalanches conditions can change very rapidly, especially during periods of wind-loading, warming, or heavy snowfall. It is important to always reassess conditions, especially during a period of rapid change, and modify plans based upon the changing conditions.