The avalanche occurred on a sparsely treed, northerly-facing slope north of Lookout Peak in the West Mountains. It was unintentionally triggered by a snowbiker. It was medium-sized relative to the path and produced enough force to bury, injure, or kill a person and break a few trees. The avalanche broke under a hard slab formed by the wind on a layer of faceted crystals above a crust (HS-AMu-R3-D2-O). The avalanche broke 2–3 feet deep and 240 feet wide. It ran 120 vertical feet into trees at the bottom of the slope. It occurred on a sparsely-treed slope at 7550 feet, which is exposed to the wind. The runout transitioned abruptly from the steep slope to a bench that piled debris deeply relative to the size of the avalanche.

While the accident did not occur within an Avalanche Center forecast area boundary, the Payette Avalanche Center (PAC) forecasts for an area less than 10 miles to the north. While no PAC forecast was issued for the day of the accident, the forecast for the previous day was rated as Considerable (level 3 of 5) at mid and upper elevations and Moderate (level 2 of 5) at lower elevations. The primary problem was Storm Slab avalanches on all aspects and elevations. The likelihood was likely, and the expected size was small to large (up to D2).

The Bottom Line read: Over a foot of new snow is forecasted in the mountains by this evening. Breezy south winds and heavy snowfall will create dangerous avalanche conditions by this afternoon. Storm slabs will increase in size and sensitivity throughout the day. Conditions will be most dangerous above 7000' near and just below ridgelines.

Pertinent Weather & Snowpack

The West Central Mountains of Idaho received above-normal precipitation in November. A prolonged period of high pressure brought dry conditions and valley inversions for 11 days from late November into early December. Temperatures associated with this system reached over 40 F in the first six days of December at elevations around 6500’. During this period, surface hoar and near-surface faceting were observed at multiple locations throughout the PAC forecast area. A small storm on December 8 buried this weak layer. A series of storms began to impact the region beginning on December 12. Temperatures rose on December 13, and light rain capped these facetted crystals with a thin crust. In the two days prior to the accident, the Puhi Flat SNOTEL, about 18 miles northwest of the accident site at 6240’(1200’ lower than the accident site), recorded an increase of 1.3 inches of Snow Water Equivalent (SWE). Unfortunately, there is no high-elevation wind data available along the West Mountain Range.

The seasonal snowpack began accumulating in early November. Consistent snow fell from November until early December. December saw small storms punctuated by periods of high pressure with warm temperatures, forming multiple cohesive layers of snow separated by melt-freeze crusts or faceted layers. In places, winds drifted more than 3 feet of snow over these weak layers. The avalanche released on a layer of facets above a thin rain crust. The slab was made up of 1–2 feet of recently wind-drifted snow.

Accident Summary Riders 1 and 2 met at the Anderson Creek Trailhead at around 1115 AM to start their day. They climbed up to a shortcut that took them off from the groomed trail and up the east slopes of West Mountain to Lookout Peak. The pair were riding without a specific objective and had agreed not to go into "sketchy terrain,” as Rider 1 forgot the battery for his avalanche airbag. They were experienced snowbikers familiar with the area, wearing avalanche transceivers, airbags, and carrying rescue equipment. They had not read the avalanche forecast from the Payette Avalanche Center (nearest to the accident site) or discussed the avalanche conditions, although Rider 2 had noted the avalanche warning (from the Sawtooth Avalanche Center to the south) on his phone weather app.

Prior to the accident, the group had been riding low-angled, easterly-facing slopes. They had not seen any signs of instability while riding that morning.

At approximately 12:00 PM, Riders 1 and 2 began traversing down and across a steep, wind-loaded, north-facing slope when the avalanche broke above them. Rider 2 was in front and recalled the avalanche feeling “very liquid,” and he was knocked from his bike immediately. As he was being swept away, Rider 2 was swept into a small tree, which kept him on the snow surface when the debris stopped. Rider 2 did not deploy his airbag. Rider 1 was swept away in the avalanche.

Rescue Summary

Rider 2 began searching the debris; both snowbikes were almost completely buried, with just part of a ski visible. There were no clues to Rider 1’s location on the snow surface. Rider 2 turned his transceiver to search mode and received a signal. After 6 minutes of searching, Rider 2 had a positive probe strike, finding Rider 1 buried about 6 feet deep in a benched terrain feature. Rider 1’s airbag was not deployed. Two snowmobilers (group A) arrived within minutes after the avalanche occurred. At approximately 12:20 PM, one member of group A called 911. Rider 2 and group A were removing Rider 1 from the snow when another group of two snowmobilers (group B) arrived to help. While conducting CPR, two other snowmobilers arrived to help (group C). Four individuals alternated performing CPR for about an hour. The local fire chief, who was snowmobiling nearby, was alerted of the incident and arrived on the scene around 1:15 PM. After consultation with medical advisors, CPR was stopped. Cascade Fire and Valley County Search and Rescue responders transported Rider 1 from the accident site at 4:00 PM.

Comments

All the fatal avalanche accidents we investigate are tragic events. We do our best to describe each accident to help the people involved, and the community as a whole better understand them. We offer the following comments in the hope that they will help people avoid future avalanche accidents.

Many riders don’t recognize the elevated level of risk posed by traveling above terrain traps like gullies, streambeds, or trees. Any terrain feature that causes debris to pile up deeply can lead to an unsurvivable avalanche burial. Even with skilled rescuers, the statistical chances of a live recovery diminish with increasing depth of avalanche burial. Additionally, having two riders on the slope simultaneously increased risk and could easily have resulted in two fatalities. Careful assessment of stopping points is critical to ensure that no more than one person at a time is exposed to the threat from avalanches.

This was an experienced group carrying appropriate rescue equipment. Each member of the group had safety tools such as airbags, avalanche transceivers, probes, and shovels. Although the group was equipped with avalanche airbags, neither rider deployed them during the avalanche. While practicing rescue skills is crucial, it's equally important to familiarize yourself with your airbag. Make it a habit to ride with your trigger accessible and practice deploying the airbag as frequently as you practice with your beacon. Regular practice pulling your airbag trigger will improve your response time if you are caught in an avalanche.
During their parking lot check, Rider 1 recognized he did not have the battery for his airbag. Regular inspection and maintenance of safety equipment ensures it is ready for use in an emergency.

The riders did not check the avalanche forecast, leaving them unaware of the current conditions. Checking the avalanche forecast is an essential part of the trip planning process, as it informs users how to recognize and avoid dangerous areas. Although the PAC does not issue a forecast for the accident area, users can extrapolate information from the closest avalanche center forecast across the slopes they plan to travel.