12/14/2013 Avalanche Incident – Picnic Notch, Noisy Basin, Swan Range

12/16/2013

Final report on Picnic Notch Avalanche Incident available
Click here for PDF

Picnic Notch Avalanche Incident Report
Noisy Basin, Swan Range, MT
Date of Avalanche: 14 December 2013
Date of Investigation: 15 December 2013

INCIDENT SYNOPSIS 
On Saturday, December 14, 2013, a party of two skiers was traveling in the Noisy Basin area near Camp Misery in the Swan Range, MT. Around noon, both skiers were ascending a slope they had previously descended. The skier in front (a 34 year old male) triggered an avalanche that released about 20 feet above him. He was caught in the avalanche and was carried to a tree where he came to rest fully buried except for one arm. He was able to extricate himself with this free arm and sustained no injuries. The avalanche occurred on a southwest facing slope that wraps around to more of a northwesterly aspect. A small sub-ridge bisects the slope causing cross loading of wind transported snow. The average slope angle of the starting zone was 39 degrees. The crown face depth averaged 12 inches with a maximum crown depth of 20 inches. The avalanche was approximately 250 feet wide and ran approximately 200 vertical feet. The crown was located at approximately 6700 feet in elevation and the toe of the debris was around 6620 feet. The caught skier was carried approximately 50 vertical feet. The United States classification of the avalanche is SS-ASu-R4-D2-I (Greene et al. , 2010).

WEATHER AND SNOWPACK
Weather data are from the Noisy Basin SNOTEL site (48.15˚N, 113.95˚W; 6040 ft.). Total Snow Water Equivalence (SWE) for this site was hovering around 140% of the 30-year median value. This station is located approximately 0.5 miles southwest of the incident site.

Beginning on Tuesday, December 10, 2013, an entrenched arctic air mass started moving east and a more westerly flow brought more seasonal temperatures (19-32 °F for Kalispell). During that arctic outbreak mountain temperatures reached below -20 °F. As the air mass moved east, a storm on December 9-10 deposited fairly low density (<10%) snow to the region. Snowfall amounts at Noisy Basin totaled about 16-20 inches (40-50 cm) with about 0.9 inches (22 mm) of snow water equivalent (SWE) (Figure 1). Since December 10, snowfall tapered and predominantly southwest winds increased on December 13 around the region. On December 14, a few inches accumulated throughout the day.

 

 20131214PicnicNotchTemp.jpg

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Figure 1: Air temperature, snow water equivalent (inches) and snow depth (inches) from Noisy Basin SNOTEL site (elev. 6040 ft.) from December 9 to December 14, 2013. The event took place on December 14, 2013.

 

Avalanche activity (both natural and human triggered) was noted in the region from the onset of the storm on December 11 with a few reports until Thursday, December 12. Observations on the morning of December 14 by FAC staff in nearby Peters Ridge, Swan Range, noted no obvious signs of instability and no recent avalanche activity. The same was noted in the northern Flathead Range, but FAC staff observed substantial winds, wind loading, and “thick” wind slabs forming on leeward slopes. These slabs were not as “sensitive” in this location as expected.

 

FAC staff received a report of a skier triggered avalanche on the evening of Saturday, December 14, 2013. Staff spoke with the reporting party and received detailed information regarding the location and events. FAC staff visited the incident site on Sunday, December 15 (the day after the accident), and completed a fracture line profile on a southwest aspect and found a wind slab consisting of a variety of snow layers sitting on a layer of small faceted crystals over a 0.8 inch (2 cm) thick melt-freeze crust (from the warm, sunny period of late November) (Figure 2). 

 20131214_PicnicNotchAvalancheProfile

 Figure 2: Crown profile of the avalanche with failure layer denoted.

The bed surface of the avalanche was the melt-freeze crust at 12 inches (140 cm) below the surface. Above this crust was a cohesive wind slab ranging in depth from 12 inches (30 cm) to 20 inches (50 cm) across the crown. We were able to safely access the northern half of the crown, but a chute above the remainder of the crown that had not released posed a safety concern. FAC staff noted that the portion of the crown where the avalanche was triggered was much shallower and unsupportable. Ski penetration in this localized area was nearly to the ground.

 

Stability test results on the crown were mixed. The Extended Column Tests (ECTs) performed on the crown had no results (ECTX x2). Compression tests failed on the small facets with hard force (CTH 23, Q3). We noted cross loading from southwest winds while on site. The day of the investigation we did not see any new avalanche activity aside from the avalanche investigated nor did we experience cracking or collapsing of the snowpack along our ascent or descent. Other folks recreating in the area also mentioned that they had not seen any other avalanche activity that day.

 

We noticed that based on the reporting party’s description the avalanche was triggered from the skier’s right side (northern part) of the crown (Figure 3-5). The area where the avalanche was triggered was shallow relative to the rest of avalanche area. Thus, given this information it is probable that the skier initiated a fracture on a weak layer in this shallow area and the fracture propagated along a weak layer underneath a wind slab through the remainder of the slope. The debris ran through and over a small trap (small depression) at the bottom of the slope and continued a bit further downslope. The total snow depth at the crown was 78 inches (199 cm), and the measured debris was about 3 feet deep. The total snow depth in the deposition zone (debris plus existing snowpack was 116 inches (295cm). The avalanche ran approximately 200 vertical feet and about 400 linear feet.

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Figure 3: Looking up at the crown of the avalanche. The skier triggered the slide on the left side of the image. The tracks were not evident both from substantial wind deposition overnight and the avalanche erased the tracks as well.

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Figure 4: Looking up at the crown of the avalanche. Substantial wind between the avalanche event and the site investigation transported snow and filled in some of the site, including the skier’s ascent and descent tracks. It was difficult to determine, but the tree in foreground of this image may be the tree that the skier was buried near. 

 20131214PicnicNotch3

Figure 5: Looking up the avalanche path with the crown visible. Note the rocky terrain above the crown as well as the small sub-ridge on the right side of the image. This small terrain feature (ridge) causes snow to be loaded and cross loaded onto this slope. The skier triggered the slide out of the image to the left.

AVALANCHE
The Flathead Avalanche Center avalanche advisory for December 14 read:

“The avalanche hazard is CONSIDERABLE on wind loaded slopes steeper than 30 degrees. The hazard is MODERATE on all other slopes. This means that human triggered avalanches are likely on steep slopes with sensitive wind slabs and possible on other slopes where facets and surface hoar still exist about 1-2 feet below the surface.

 The skier who triggered the avalanche provided valuable information regarding the accident site and all of the events of the day of the incident. Skier A (34 year old male) and Skier B (34 year old female) approached Noisy Basin along the Jewel Basin Rd. via snowmobile and parked at Camp Misery (Figure 6 and Figure 7). They checked the avalanche advisory before departing in the morning, and they each carried a beacon, shovel, and probe. They consider themselves expert skiers with about 14 years of backcountry skiing experience, and had taken “a couple” of Level 1 avalanche courses throughout the years.

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Figure 6: Overview of avalanche site near Picnic Notch, Noisy Basin, Swan Range, MT.

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Figure 7: Close-up view avalanche site near Picnic Notch, Noisy Basin, Swan Range, MT.

During the morning, they headed toward Picnic Notch, ascended to the notch, and safely skied just skier’s left (south) of their uptrack. They then transitioned from downhill to uphill travel mode and began their second ascent toward Picnic Notch. They made progress up the slope via a series of switchbacks. Skier A then decided to go a bit further south to access an area with more potential vertical feet of skiing. This area is beneath a rocky section just south of Picnic Notch. As he continued upward in a southerly direction, he felt the snowpack consistency change and it “felt weird”. He then advised his partner to remain in her location. He took a few more strides and began to notice localized cracking of the snow. Just after this, he looked up to see the slope above him about 20 feet begin to avalanche. Still in uphill travel mode, his skis were not attached to his heels and his climbing risers on his bindings were still in an upright position. He immediately notified his partner that the slope was sliding and tried to put his climbing risers down to become more stable on his feet. He was instantly swept off his feet by the moving snow and transported down the slope in the debris. He made swimming motions and tried to spring out of the moving snow. He eventually hit a tree and remained in the “eddy” of the tree while debris moved around and over him. He became buried with only an arm sticking out of the snow. He mentioned that his mouth was full of snow at this time. He was able to brush the debris away using his arm and eventually free his head and airway. He was able to remove himself entirely from the debris. After assessing for injuries (of which there were none), he and his partner descended and debriefed the situation.

 Skier A mentioned a few points that are common in avalanche incidents and highlight the “human factor”.

  • He mentioned that on this day they did not dig a pit. He acknowledged that they typically dig snowpits in this area and in the past noticed the variability across the slope due to crossloading (i.e. shallow on one side and deep on the other).
  • He acknowledged that he and his partner on this day succumbed to the familiarity notion suggesting that the slope is safe after being safely descended nearby once earlier in the day. From many avalanche accidents, we know that it isn’t just the first person (or first time on the slope that day) that triggers the avalanche.
  • He also discussed that they had “let their guard down” after skiing the slope the first time, and they were headed back up for just one more run before departing.

We appreciate Skier A speaking with us and sharing the details of the event so that they may be used to inform and educate others.

SEARCH AND RESCUE
No search and rescue operations were initiated as the caught skier was able to extricate himself and was uninjured.

Direct any questions regarding this report to fac.admin@flatheadavalanche.org or 406.261.9873.

Report prepared by Erich Peitzsch and Seth Carbonari of the Flathead Avalanche Center.

 

REFERENCES
Greene, Ethan M., D. Atkins, K. Birkeland, K. Elder, C. Landry, B. Lazar, I. McCammon, M. Moore, D. Sharaf, C. Sterbenz, B. Tremper, and K. Williams, 2010. Snow, Weather, and Avalanches: Observational Guidelines for Avalanche Programs in the United States.  American Avalanche Association, Pagosa Springs, CO: Second Printing Fall 2010, 152 pp.


Previous post:
12/15/2013
Update to skier triggered avalanche in Swan Range on 12/14/2013.

A skier triggered avalanche was reported to the Flathead Avalanche Center (FAC) Saturday evening (12/14). FAC staff was able to speak with the skier who triggered the avalanche and was subsequently caught in the avalanche that evening.  The location was just skier’s left (south) of Picnic Notch and north of Crown Bowl in the Swan Range. The skier triggered the avalanche while ascending the slope after previously descending the slope. There were two members of the party and only one was caught.  The skier was caught and carried approximately 200 vertical feet downslope where he came to rest near a tree, fully buried with just one arm sticking out of the debris.  He was able to extricate himself using his free arm and was uninjured. 

FAC staff investigated the avalanche site today (12/15). The crown depth of the avalanche averaged approximately 12 inches and the maximum was 16-18 inches. The flanks were difficult to see due to overnight wind loading, but the width of the crown was estimated at approximately 200 feet. Debris ran approximately 500 vertical feet. It appears that the avalanche was triggered in a shallow part of the slope near rocks and propagated along a layer of small facets underneath a wind slab.

The avalanche was preliminarily classified as SS-AS-R4-D2-I. This was a soft slab avalanche unintentionally triggered by a skier. 

A full incident report will be published tomorrow (12/16/2013) on the FAC website.