Negatives - Skier triggered and carried by wind slab avalanche

AVALANCHE OBSERVATION
Negatives

Drainage:

Submission Info
Josh Feinberg
Forecaster
Saturday, March 20, 2021 - 12:00pm
Red Flags: 
Recent loading by new snow, wind, or rain
Avalanche Type: 
Dry
Slab
Slope: 
39degrees
Trigger type: 
Skier
Crown Height: 
1 ft
Aspect: 
Northeast
Weak Layer: 
Storm Snow
Avalanche Width: 
100ft.
Terrain: 
Above Treeline
Elevation: 
11 000ft.
Bed Surface: 
Storm Snow
Avalanche Length: 
700ft.
Number of similar avalanches: 
1
Number of partial burials: 
0
Number of full burials: 
0
More detailed information about the avalanche: 

About noon on Saturday March 20th a skier triggered and was carried by a wind slab avalanche near the top of one of the furthest west of the northeast facing chutes in the Negatives behind June Mtn.  This first crown was about 50’ wide and spread across half the chute, 6-12” tall, and ran ~700’ (R2-D2 avalanche).  Fortunately, the skier was able to get to his feet and ski to the side after being swept ~500’ without injury or lost gear.  His partner, attempting to ski the bed-surface as a safe descent option, triggered another avalanche while approaching the crown that was also ~50ft wide spanning the remainder of the chute, and 12+” tall.  He was not caught in this slide.  See attached video taken on Sunday of the crown, showing a 1finger minus slab of snow on top of a weaker 4finger minus layer on top of a 1finger/1finger minus bed surface.      

Bottom Line for Saturday: “Up to 1.5ft of new snow in the last 36 hours and strong SW winds have built fresh dangerous WIND SLABS leading to CONSIDERABLE avalanche danger near and above treeline, and MODERATE danger below.  Human triggered avalanches are likely and naturals are possible that could easily be large enough to bury you on leeward NW-NE-SE-S facing terrain.”  June Mountain snow study plot at 9148’ recorded 8” of snow at 0.65”SWE for this period.   

*Following Details obtained from conversation with one of the two brothers that rode adjacent chute that did not slide.  Many thanks to this individual for being forthcoming with this information that we can all learn from.

Two separate parties went to June to ski off the backside and head toward the negatives on the morning of Saturday March 20th.  One party was a group of 4 (2 split-boarders and 2 skiers), and the other party was 2 brothers (one split-boarder and one skier).  The parties happened to run into each other at the top of June, and 2 of the people were friends and frequent backcountry partners.  Since both groups had similar ideas for the day to head up the hourglass and assess conditions, and decide whether or not to ride the chutes or head back down the hourglass, they joined forces and got on the same radio channel. All the people were in their late 20s / early 30s.  Some were avid backcountry travelers, others had less experience.  The two brothers had taken an intensive 10-day backcountry course with Outward Bound, and the other’s formal education is unknown, but at least some, including the person who triggered the first slide, was a very frequent back country traveler and seemed very knowledgeable about issues surrounding snow stability.  All were aware of the CONSIDERABLE avalanche danger rating for wind slabs near and above tree line for the day, and they spoke frequently about the SW winds blowing snow, and the potential wind loading.  They noticed 2-3” of ~1finger hard wind slab present ascending the hourglass.  At the top of the Negatives they checked out the first chute and noticed a firm wind slab surface near the top, and 2 recent tracks down lower.  2 riders, including one lesser experienced female (3rd day in the backcountry) decided they would ride the less-steep entrance into this chute and waited while the other 4 made their way to further chutes.  The brothers found one they liked just before the chute that ended up sliding, and they dug a quick pit at the top and did an ECT test.  The test failed roughly in the recent wind loaded snow with hard force, but did not propagate (ECTN27). They felt good about riding this chute.  The other two skiers decided on their chute and waited at the top.  The first and more experienced brother dropped his line, found excellent riding conditions, but did have a decent amount of loose slough release at one point, which he radioed to the rest to be aware of.  The second brother dropped the same line without incident.  The first skier in the chute next to them then made a ski cut across the top, then began turns.  On his 2nd or 3rd turn, he triggered an avalanche at the apex of the crown seen in the picture that broke at his feet and swept him off his feet.  The avalanche broke from the apex to the skiers left, and also pulled out further down slope to the skiers right of the big rock.  He was taken ~500’ downslope, but managed to mostly keep his feet downhill of him, with much of the avalanching snow below him.  After being taken thru the first choke, he managed to get back to his feet and make it to the side without losing any gear.  Seeing the slide, the first brother was preparing to put his skins on and head back up and search, thought about activating his emergency satellite communicator, when he was relieved to see the skier on his feet and ok.  The caught skier skied down to the brothers, and the second skier decided that skiing the bed surface down was his safest option.  He slowly skied down to the crown, and ended up triggering the right-hand half of the chute before he was able to get to it.  He was going slowly and cautiously when this avalanche broke at his feet, and fortunately was not caught.  After he skied down to the others, the two remaining snowboarders on top rode down that first chute without incident.  

Learning points shared:  It is important to be conservative.  It is quite hard sometimes to make the conservative decision especially when the riding conditions appear to be so good.  This incident highlighted how variable stability can be across a slope from one chute to the next.  Just because one test is performed that showed relatively stable results doesn’t mean that all the snow across that aspect is safe.  

An after-thought they had: Should they have insisted that the riders left on-top after the first slide descend their up-track down the more conservative hourglass rather than continuing down the face chutes?  

What they did well: Exposed one person at a time, good use of radios for communication, checked the avalanche advisory, were aware of the wind slab hazard and discussed it, performed a stability test in one chute.

Avalanche Photos: 
Avalanche observation video: 
Number of People Caught: 
1
37° 43' 44.8068" N, 119° 6' 1.6488" W
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