Detailed Accident Report

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Date: 2003-03-09
Submitted By: GNFAC; Doug Chabot
Place: Mount Abundance,10 miles northwest of Cooke City
State: MT
Country: USA
Fatalities: 1
Summary: 3 snowmobilers caught, 2 buried, 1 killed

On Sunday, March 9, 2003 three snowmobilers were caught in an avalanche burying two and killing one. This slide was on the southwest flank of Mount Abundance located about 10 miles northwest of Cooke City, Montana.

The avalanche occurred at 11:15 am at an elevation of 9,200 feet (2800m) below a small ridge on an east-facing slope (110 degrees). A party of four was riding together when one person went up on the slope and got stuck. The victim and another member of the party went up to assist when they triggered the slope. All three people were caught, but one managed to stay on top; another was buried, but was able to swipe his face clear of snow; and the victim was completely buried. All three snowmobiles were also buried, two adjacent to the victim and another uphill against a tree. Everyone had avalanche rescue gear.

After visually locating and extricating the partially buried rider they all did a beacon search, which showed they were within 12 meters of the victim. He was pinpointed and dug up within 15 minutes (estimated by party). His was in a semi-sitting position with his head 2 feet from the snow surface, there was no obvious trauma and he wasn?t breathing. CPR was administered immediately and continued for over an hour with no results. Someone in the group rode to Daisy Pass and relayed the need for a rescue. Cooke City SAR responded and evacuated the victim.

I investigated the avalanche site the next morning. It was a small slide measuring 2 ? feet deep (80 cm), 200 feet (60m) wide and 250 feet (76m) vertical distance. The crown was near the ridgetop and the slope was windloaded. The slide was triggered in the bottom third of the path at a slope angle of 30-33 degrees. The slope had an average angle of 37 degrees and steepened near the ridgetop into the low 40s. The alpha angle was 26 degrees. The avalanche debris was measured at 6 ? to 7 feet (2m) at its deepest. The US Classification is HS-3-AV-O; the Canadian classification would be Class 2. For March 9th, the avalanche danger in this area was rated ?high? on all windloaded slopes and ?considerable? on non-windloaded slopes.

On February 27th, the mountains around Cooke City had warm sunny weather with temperatures at the 8000-foot (2400m) level reaching 40?F (4?C). This created a melt-freeze ice crust, which became capped with near surface facets (>.5mm). Small storms buried this layer under a few inches of snow on March 1. Between March 5th through 9th over 5? (13cm) of snow water equivalency fell totaling over 60 inches (152cm) of snow. During this period the winds blew 20-40 mph out of the west with gusts exceeding 50 mph. At the time of the avalanche it was blowing and snowing hard with a temperature of 26?F (-3?C). The slope adjacent to the slide path naturally avalanched earlier that morning with debris visible.

My snowpack investigation revealed that the slope avalanched on the layer of facets sitting on the 3cm thick melt-freeze crust. The slab on this weak layer was 1 finger hardness. Earlier in the day I was able to get a 27 degree slope to collapse, fracture and slide two inches on this layer on a similar slope. Stability tests at the crown gave me two results of Stuffblock 20, Quality 1. The stress of three riders probably triggered the slope from the low angle of 30-33 degrees, which propagated uphill to the ridgetop catching all three. A fourth rider was in a safe zone and witnessed the slide. Although all riders carried rescue gear, dug out the victim quickly and performed CPR, the burial had fatal consequences for the 52-year-old male.

If you have any questions or comments you can contact me at 406-587-6984, or email me at dchabot@fs.fed.us.

Doug Chabot, Director

Gallatin National Forest Avalanche Center

March 11, 2003