CAIC

Avalanche Details

Location: Horse Creek, Wyoming Range
State: Wyoming
Date: 2018/12/22
Time: 10:00 AM (Estimated)
Summary Description: 1 snowmobiler caught, buried, and killed
Primary Activity: Snowmobiler
Primary Travel Mode: Snowmobile
Location Setting: Backcountry

Number

Caught: 1
Partially Buried, Non-Critical: 0
Partially Buried, Critical: 0
Fully Buried: 1
Injured: 0
Killed: 1

Avalanche

Type: HS
Trigger: AM - Snowmobile
Trigger (subcode): --
Size - Relative to Path: R1
Size - Destructive Force: D1.5
Sliding Surface: --

Site

Slope Aspect: NE
Site Elevation: 8950 ft
Slope Angle: --
Slope Characteristic: --

Avalanche Comments

The avalanche occurred on a short, steep slope at an elevation of 8,950 feet. This steep slope was in a small, concave terrain feature. It was on a northeast aspect. The slide was estimated to be 150 feet wide. The distance from the crown to the toe of the debris was also estimated to be 150 feet.

The crown of the avalanche was measured to be 22 inches deep. It was a hard slab avalanche. Due to the shape of the terrain feature (quick flattening of the slope) this hard slab avalanche did not flow very far past the avalanche stauchwall. Due to the short run out distance and an abrupt slope change the avalanche debris primarily experienced laminar flow and for that reason was mostly intact but fractured. This slide was classified as a 1.5 on the destructive size scale. The avalanche debris was several feet deep. Without the complication of being pinned beneath a snowmachine, it is unlikely that the debris would have fully buried a person. The slab failed on a persistent weak layer of faceted snow. Figure 3 is a photograph of the slide that was taken on December 23.

Snowpack Summary

This accident occurred in an area where the Bridger-Teton National Forest Avalanche Center provides daily avalanche hazard forecast. Daily forecasts for this area for this winter season began on November 2. Specialized weather stations operated by the Bridger-Teton National Forest are located near this accident site. The Blind Bull precipitation station (9,000 feet) is situated approximately 2 miles northwest of the site. The Deadman Mountain (10,350 feet) and Mt Coffin (10,870 feet) wind stations are located approximately 6 miles northwest and 20 miles south of the site, respectively. Daily data from these weather stations and field excursions conducted by the avalanche center’s avalanche specialists on December 12, 13 and 20 were resources that were used to determine the weather and snowpack history that preceded this event.

In this area the snowpack was 2 feet deep and mostly faceted on December 10. The Blind Bull weather station received 9 inches of new snow from December 11 and 13 and 6 inches of new snow from December 18 to December 21. Strong westerly winds occurred December 19 and 21. The Mt. Coffin weather station recorded hourly wind speed averages of 47 with wind gusts to 120 miles per hour on December 19 and hourly wind speed averages of 35 with gusts to 80 miles per hour on December 21. These strong winds created stiff wind slabs on leeward aspects.

A crown profile was conducted at the accident site on December 23 (Figure 6). The total snow depth at the crown was 43 inches. The snow in the area beneath the crown where the rider was when the slide was triggered was 2 to 3 feet deep. At the crown a stiff slab had formed on top of several layers of faceted snow. This hard wind slab failed on a weak layer of faceted snow at a height of 21 inches above the ground surface. The slab dug down into deeper faceted layers in the immediate vicinity of where the rider was when the slide released. Figure 7 is a photograph of an avalanche specialist from the avalanche center doing an investigation of the crown face.

On December 22 the general avalanche hazard rating for this area was moderate. This avalanche hazard advisory noted that shallow pockets of persistent slab could be found on leeward aspects and in unusual locations and that a human could trigger these small slabs on steep wind loaded aspects.

Accident Summary

At about 10 AM on December 22, a 31 year old male snowmobiler triggered a small slab avalanche and was pinned beneath his sled. His riding partner was inexperienced, concerned about the avalanche hazard and unable to make it to the accident site on his sled or by wading through the snow.

Rescue Summary

At 11 AM another party of snowmobilers arrived on the scene. A person from this party was able to ride to the accident site. The 31 year old male was found under his sled, face up with his head downhill. His face was a foot beneath the snow surface. This party determined that the rider was not alive.

Tip Top Search & Rescue volunteers were notified of the accident at 11:20 AM. They mobilized and reached the site at 2 PM. They removed the man’s body from beneath the sled and transported it to the trailhead. The Sublette County coroner determined the cause of death was asphyxia.