Avalanche Details
- Location: Wounded Buck Creek, northwest of Wildcat Lake
- State: Montana
- Date: 2021/02/06
- Time:
1:00 PM
(Estimated)
- Summary Description: 5 snowmobilers caught, 1 partially buried-critical and killed
- Primary Activity: Snowmobiler
- Primary Travel Mode: Snowmobile
- Location Setting: Backcountry
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Number
- Caught: 5
- Partially Buried, Non-Critical: 3
- Partially Buried, Critical: 1
- Fully Buried: 0
- Injured: 0
- Killed: 1
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Avalanche
- Type: SS
- Trigger: AM - Snowmobile
- Trigger (subcode): u - An unintentional release
- Size - Relative to Path: R3
- Size - Destructive Force: D2.5
- Sliding Surface: O - Within Old Snow
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Site
- Slope Aspect: N
- Site Elevation: 6300 ft
- Slope Angle: 35 °
- Slope Characteristic: Sparse Trees
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Avalanche Comments
This was a large or very large avalanche unintentionally triggered by a snowmobiler. According to reports from the party, it was large or very large relative to the path and produced at least enough destructive force to bury, injure, or kill a person. The avalanche likely involved a soft slab that broke into old snow layers and failed on a layer of faceted snow and/ or surface hoar (SS-AMu-R3/4-D2.5/3-O). The positions of the victims and survivors at the time of the accident suggest they were caught in a slide that broke at least 1500 vertical feet wide and ran 300 to 500 vertical feet.
While the slab likely included new and drifted snow, the avalanche character would be Persistent Slab.
Backcountry Avalanche Forecast
On the morning of Feb. 6, the Flathead Avalanche Center (FAC) continued an Avalanche Warning for the Swan Range first issued on Thursday, Feb. 4 (attached). The warning noted:
* IMPACTS...New and drifted snow are overloading fragile, reactive weak layers. It will be easy to trigger large, wide, and deadly avalanches. Natural avalanches may run long distances.
* PRECAUTIONARY / PREPAREDNESS ACTIONS...Avoid riding or traveling in avalanche terrain.
The FAC’s forecast for the Swan Range for Feb. 6 rated the avalanche danger HIGH (level 4 of 5) for mid and upper elevations (above 5000 feet) and CONSIDERABLE for low elevations (below 5000 feet). The forecast listed three avalanche problems. The primary problem was Persistent Slab avalanches (all aspects above 5000 feet), with a Likelihood of Very Likely and a potential size of large to very large (up to D3). The forecast also listed two secondary problems, Wind Slab avalanches (all aspects at upper elevations; northwest through northeast to southeast at mid elevations, Likely, small to large) and Dry Loose avalanches (all aspects, mid and lower elevations, Likely, small to large).
The forecast bottom line stated:
Conditions are downright treacherous at mid and upper elevations. The danger will rise through the day as more snow accumulates. You can trigger surprisingly large avalanches way too easily with way too little warning. You're unlikely to survive being caught in one of these slides. Large and very large natural avalanches are likely. Stay off of, and out from under, slopes steeper than 30 degrees.
Weather Summary
The accident occurred at the end of a five-day period of frequently intense snowfall and moderate to strong winds. The Noisy Basin SNOTEL (3.3 miles south-southeast of the accident site; elevation 6040 feet) recorded 4.9 inches of Snow Water Equivalent (SWE) from Feb. 2 to Feb. 6. Most of that water (3.1 inches) fell on the last two days of the period, during sustained intervals of precipitation intensity (0.1-0.2 inches of SWE per hour). The height of snow at the station increased by over 20 inches during the 5-day interval, from 66 to 89 inches.
Winds during the snowfall were moderate to strong from west-southwest and then west. Sensors at the Aeneas Ridge weather station (3.8 miles south-southeast of the accident site; elevation 7186 feet) measured average speeds of 15 mph on the three days prior to the accident, with gusts to 47 mph. Wind speeds dropped on the day of the accident (9 gusting to 33 mph).
Air temperatures declined over the five days preceding the accident. Maximum temperatures on Feb. 1 and 2 were above freezing then dropped to the mid 20s Fahrenheit (F). Minimum temperatures fell from the upper 20s F to the low 20s F. At the time of the accident, the Noisy Basin SNOTEL reported an air temperature of 26 degrees; Aeneas Ridge showed 20 degrees F. These are typical values for this area in early February.
Snowpack Summary
On Jan. 13, an atmospheric river event ended with rain and above freezing temperatures to the crest of the Swan Range. The wet snow rapidly refroze, leaving a pencil- to knife-hard crust or ice layer (depending on location) at the snow surface. Avalanche concerns in the subsequent three weeks centered around weak layers and slabs that developed above the Jan. 13 crust or ice layers, because they significantly limited the potential for human-triggered avalanches on layers deeper in the snowpack.
Storms on Jan. 15 and 17 deposited 1.5 inches of SWE above this crust. Dry, cold weather prevailed during the following 12 days, with Noisy Basin SNOTEL recording only 0.6 inches more SWE by Jan 29. The height of snow at the station decreased (settled) by two inches during this time.
The snow above the crust weakened. Observations from Jan. 23, 25, and 26 described 15 to 20 cm of faceted snow above the crust; each also noted a layer of surface hoar capping the faceted snow. These observations were made on the west side of the Swan Range, though the observation on Jan. 26 was from the ridge above Wildcat Lake within a mile of the eventual accident site.
Wet weather returned to the region at the end of the month. It was immediately accompanied by reports of shooting cracks, whumpfing collapses, propagating snowpack tests, and remotely and unintentionally-triggered avalanches. One of these was a near-miss, and another large slide caught and carried a skier. The reports consistently described slabs breaking just above the Jan. 13 crust, on weak facets or preserved surface hoar. The reported activity occurred almost exclusively on north, northeast, and east facing slopes, in the Whitefish and Flathead zones.
When snowfall started in earnest in the Swan Range on Feb. 2, so too did evidence of similarly reactive slabs. That day, FAC staff remotely triggered three avalanches on northerly, mid elevation slopes. These had crowns about a foot thick and failed on preserved surface hoar above the Jan. 13 crust and ice. The forecast team was on the east side of the range, several drainages north of the eventual accident site.
On Feb. 3, an observer in Noisy Basin reported extended column tests propagating with moderate force on surface hoar six to eight inches above the Jan. 13 crust. The next day, an FAC forecaster in the same basin observed high snowfall rates, moderate blowing snow, small slides on test slopes, and decreasing stability through the day. Reports from elsewhere in the region describe thickening but still reactive slabs. By Friday, Feb. 5, FAC forecasters in the Flathead Zone were observing remotely-triggered and natural avalanches that were 20 inches or more thick and very large relative to the path, all failing on northerly and easterly slopes on preserved surface hoar above the Jan. 13 crust.
Events Leading to the Avalanche
On the morning of Feb. 6, five riders left a trailhead at the Hungry Horse Dam planning to ride in Wounded Buck Creek, on the east side of the Swan Range. They had decided on the destination at breakfast that morning, after discussing the avalanche forecast. All were carrying a full set of avalanche rescue gear (beacon, shovel, and probe) and at least two riders (Riders 1 and 4) had avalanche airbag packs. The group conducted a transceiver check before leaving the trailhead. While all of the group had ridden with Rider 1 previously, not all of the group had ridden with each other prior to Feb. 6.
The ride up Wounded Buck Creek travels through the runouts of numerous large avalanche paths with mid-elevation start zones that face southeast or northwest. The group did not see much evidence of avalanche activity, even on slopes where Rider 1 had previously seen avalanches. It was snowing, however, preventing a complete survey of the surrounding terrain.
Rider 4 thought the plan was to play in the trees because of the avalanche danger. At some point, the plan changed to riding a loop. It would entail climbing to the ridge overlooking Wildcat Lake, then heading north to Strawberry Lake and out a drainage further north. Rider 1 had done some version of this loop many times before.
The climb out of Wounded Buck Creek starts at about 4300 feet. For the next 1.2 miles, the slopes are sheltered, treed, and less than 30 degrees, though there are steep, open slopes on climbers’ right. At about 5300 feet, the route steepens. It turns right up a tributary stream on a treed rib below a very steep, open slope that generally faces northeast. The ground cover on this slope consists of talus and scree, much of it fallen from a cliff at the top. The slope is over 2000 feet wide. At the south end of the slope, riders can gain a saddle above Wildcat Lake via a steep, north-facing slope. This slope is roughly 250 vertical feet high, with a short section where the slope angle is over 35 degrees.
The five riders climbed to below this exit and parked at about 6000 feet elevation on the treed rib below the steep, open slope. They were about 200 yards below the exit climb. Rider 1 went ahead. The parked riders did not have a clear line of sight to him because of trees.
Accident Summary
At about 1300, an avalanche hit the group with little warning. Rider 4 reported having no idea it was coming until it was about 20 yards away. The slide knocked all four riders off their parked snowmobiles.
Rescue Summary
Rider 4 reported that none of the four parked riders were trapped in the snow. Their machines, however, were tipped over or partially buried and had to be dug out. The group got no response from Rider 1 when attempting to contact him via radio. They didn’t go look for him on foot because it seemed slower than getting their snowmobiles unstuck.
Rider 2’s sled was the first to be freed, and he rode up to look for Rider 1. Though he started a transceiver search, he saw part of Rider 1’s airbag at the surface. Rider 3 soon followed, then Rider 4.
Rider 2 reportedly found Rider 1 against a tree, with his airbag partially deployed. His face was below the snow surface. He was not responsive. An attempt to resuscitate him failed. It’s not clear whether the airbag was damaged did not have time to fully inflate.
The survivors moved Rider 1 off the slope and down to where they had been parked, where he it would be easier to retrieve him by helicopter. They flagged the site and flagged trees on either side of Rider 1’s snowmobile, which was buried to the handlebars. They rode back to the trailhead and alerted 911 at 15:23.
The Flathead County Sheriff’s Department alerted Search and Rescue teams, then decided to wait for safer conditions to recover the victim. At 09:30 on Sunday, Feb. 7, Two Bear Air was able to retrieve the victim via helicopter.
Comments
Each fatal avalanche accident we investigate is tragic. We do our best to describe the events to help the people involved and the community as a whole better understand factors that may have contributed to the outcome. We offer these comments in the hope that it will help people avoid future avalanche accidents.
This report is based on limited information. Rescuers did not reach the accident site the day of the accident, and the following day, when they recovered the victim via helicopter, visibility was poor and flying conditions were difficult, so they did not get any videos or images of the site. Due to concerns about the avalanche hazard, FAC staff did not investigate the site immediately after the accident. FAC staff did visit the site on March 8, although subsequent storms had obscured the crown, debris, and most other evidence of the slide. Interviews with Flathead County Sherriff’s Department and one of the survivors were instrumental in compiling this report.
This accident involved a party of experienced and expert riders who had discussed the avalanche forecast, carried avalanche rescue gear, and had a plan to ride in what they thought was safe terrain. It appears they changed their plan while riding. In the terrain where the danger was most acute, they used travel practices that limited their exposure and the consequences of a person getting caught in a slide. They may not have recognized unusually dangerous conditions or anticipated the extent and sensitivity of avalanches likely that day.
In northwest Montana, High (Level 4) avalanche danger is often associated with the arrival of powerful storms and difficult travel conditions. In this case, the avalanche danger was rising steadily with incremental loading of the weak layers buried in late January. People reported reactive slabs above these weak layers almost immediately after they were buried. The size of the triggered slabs grew each day during the first week in February; by Friday, February 5, the slabs had thickened from several inches to several feet and were breaking the full width of start zones, often with remote triggers. However, natural avalanches were isolated. Travel conditions remained excellent in the storm snow accumulating above the January 13 melt-freeze crust, allowing riders to travel extensively below 6000 feet with few signs of instability. Above that elevation, particularly on slopes that faced north to northeast to east, steep slopes were primed to avalanche.
That scenario likely existed in Wounded Buck Creek on February 6. The party’s route prior to the accident passed through the runouts of some avalanche paths but not through or near start zones. The slope that avalanched on February 6 was likely the first start zone that threatened the February 6 party. Avalanche activity on similar slopes the week prior suggests that the triggered slab was likely two feet thick on average and released on the surface hoar and faceted snow buried in late January. It appears Rider 1 triggered the slab from below, as he approached the steepest part of the exit slope.
One rider in the party remarked that it was surprising how far the crown extended. Indeed, the fracture would have propagated at least 1600 feet in order for the debris to hit the other
members of the party parked on the treed rib. Because the open slope extends another 1000 feet to the north, the potential for an even wider release exists. A less likely scenario is that more than one avalanche released simultaneously, and it was debris from a sympathetic slide that hit the parked riders in the group. In either case, it appears the group believed they were parked in a spot safe from overhead hazards.
Carrying a full set of avalanche rescue gear allows a group the best chance of a successful companion rescue. Supplementing this gear with airbags can reduce the chances of being fully buried. Airbags don’t, however, protect a rider caught in a moving avalanche from trauma. Rider 1’s airbag was reportedly deflated when the other riders reached him. Two scenarios may account for that. Either the airbag did not have time to fully deploy, or it was damaged while Rider 1 was carried in the debris or. Accounts indicate Rider 1 was partially buried – critical; that is, his airway was below the snow surface.
Although we will never know if the outcome would have been different had Rider 1’s airbag remained inflated or the survivors been able to come to his aid immediately, we do know that companion rescue can be quickest when a party can keep a rider in view from a spot safe from avalanches running down from above. This practice narrows the search area, limits the number of people caught in a slide, and allows a victim’s companions to start searching immediately. It is important to select viewpoints based on the terrain rather than past experience. In this case, Rider 1’s ascent was obscured by snow, trees, and the distance to the remaining members of the party, who were exposed to the steep slope above them. It is fortunate that more riders were not buried in this avalanche.
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