Detailed Accident Report

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Date: 1999-11-26
Submitted By: Doug Chabot GNFAC
Place: Lone Mountain
State: MT
Country: USA
Fatalities: 1
Summary: 2 skiers caught, 1 buried and killed



November 26th, 1999

On Friday, November 26th, five backcountry skiers were hiking up a slope in an

unopened, uncontrolled area in the bowl of Lone Mountain when two of them were

caught in a slide that resulted in one death. One partially buried person with

his hand exposed suffered minor injuries. The other person was buried 6 1/2

feet deep from the avalanche carrying him into a terrain trap created by a rock

glacier. The avalanche released on a north facing slope, at an elevation of

10,500'. The slide was 16" to 24" deep, 150 feet across and ran approximately

300 vertical feet. The slope angles ranged from 37 degrees at the triggering

location to 44 degrees at the crown. The US Classification of the slide is


Southwest Montana has had a relatively dry fall. For October and most of

November the only skiing that could be found was in pockets or gullies on north

facing slopes at higher elevations. Wednesday evening, November 24th, was the

start of our most significant snowfall to date. Approximately 12-16" of 16-22%

snow fell during the 24th, 25th and 26th with winds from the west averaging

10-15 m.p.h. gusting into the 30's. This new snow more than doubled the

snowpack depth. The avalanche happened at approximately 1245 and current

conditions according to National Resource Conservation Service Snotel sites and

the Big Sky datalogger show temperatures in the high 20's, snowing, and winds

averaging 10 m.p.h. with gusts in the 30's from a west-northwest direction.

After talking to a witness and spending time at the avalanche site Ron Johnson,

Karl Birkeland and I were able to gleam the following information. On Friday,

5 skiers hiked up to a north facing slope above the triple chair in the bowl on

Lone Mountain. This area was uncontrolled and unopened to skiers and therefore

had backcountry skiing conditions. Some of them had skied this slope within

the past few days before the recent snow load. Picking their way up the slope

by linking small rock outcroppings they got to a spot where they had to hike

across a 100-150 foot slope to another rock island. Visibility was compromised

since it was snowing quite hard. As the fist person was almost across, a

second person started the traverse and on his fourth step the slope avalanched

with the crown about 75 vertical feet above him. Both people had their packs

on with their skis still attached. They slid about 250 vertical feet to the

bottom where there was unfortunately a terrain trap collecting all of the snow.

The debris piled up into a steep gully at the edge of the rock glacier

completely burying one person, with the other one having just his hand exposed.

This slide was witnessed by two other people who then helped in the initial

rescue. The person with his hand out was unburied quickly and suffered minor

injuries. A transceiver search located the other person who was buried 6 1/2

feet down. They were able to dig his head out within 5-8 minutes of his burial

and started resuscitation efforts. An off duty pro patroller arrived about

this time and coordinated the rescue and CPR efforts. Unfortunately the

resuscitation was unsuccessful.

It's impossible to know exactly what happened within the snowpack that led up to

the skiers triggering this avalanche but after digging a few snowpits we can

make some educated guesses. This steep north facing slope was at 10,500 feet

and there was a 10 cm layer of well developed facets sitting either on the

ground or on top of some older snow. The slope was variable in its depth and

layering, but with the addition of all the new snow that was added over the

last 36 hours the snowpack was under increasing stress. One or both of the

skiers most likely collapsed this weak layer of facets which caused failure of

the slope. At their crossing point the avalanche ran on the ground, but in

spots near the crown and flanks the slab was sitting on the old snow surface

which was a dirty ice crust.

It's possible that the party may have underestimated the avalanche danger due to

the shallow snowpack and the fact that they skied this slope a few days prior

to this recent load. However, the basal layers of the snowpack were quite weak

and they just received a substantial snow load. Further, even though this

slide was not very large, the terrain trap at the bottom made the consequences

of a avalanche very serious.

If people have any question or comments they can contact myself and Ron Johnson

at 406-587-6984. Karl Birkeland can be reached at 406-587-6954.

Doug Chabot

Avalanche Specialist

Gallatin National Forest

November 28th, 1999