Red Slate - Skier triggered Avalanche - Partial Burial

Skier triggered wind-slab – One skier caught, partially buried, torn ACL, lost skis and poles.  Self evacuated with assistance from partners.

Skier triggered wind-slab avalanche about 12,600’ near the top of the North Couloir on Red Slate Mountain around 12:15pm on Friday March 31st, 2017.  Crown was variable between 2ft and 8inches deep, began approximately 25’ wide, and quickly entrained snow the entire width of the couloir and ran to the apron, approximately 1300-1500 vertical ft.

Days events:  A group of 5 highly experienced, skilled, expert back-country skiers from Salt Lake City and Tahoe set out from the Convict Lake parking lot at 7am on March 31st to ski the North Couloir on Red Slate Mountain.  The group was all male with at least 2 in the group having advanced AIRE level 2 avalanche training.  The weather was overcast and breezy.  The group knew that there was very strong Westerly winds the day before with gusts over 100mph over ridgetops, and that winds were forecasted to be moderate out of the North for today.  They also were aware that light snowfall was forecasted the previous night, but there was no new snow at the trailhead.  The crew made it to the base of Red Slate Mtn about 9:30am.  The sky was still socked in with clouds, there was moderate northerly winds blowing, and very notably there was about 6” of new snow from the previous day / night.  They discussed and debated their objective for about an hour at the base as they waited for conditions to clear.  As it did clear, they rationalized that the northerly winds would likely be stripping the north couloir and therefore not creating fresh wind-slabs where they wanted to ski.  They proceeded up the easterly ridge-line to the summit, where north winds were still blowing moderately.  They dropped in the north face shortly after noon, descended a few hundred feet, and did the exposed traverse to the west to gain entrance into the North Couloir proper.  The skier who triggered and got caught in the slide (skier 5) was taking pictures of the rest of his crew traversing across the north face, and was the last to enter the couloir.  Skiers 1 and 2 entered the couloir, made some turns and posted up on the skiers Left side of the couloir near where the sneaker entrance from the west face is.  Skiers 3 and 4 then entered the couloir, descended a short distance and anchored up on the skier’s Right side of the couloir.  Skier 4 took off his pack and took out his camera to take pictures as skier 5 entered the couloir, Skier 5 followed the tracks of Skier 3 and 4, and when he was about 3ft from them, the slope fractured, propagated approximately 2 meters above him, and took him violently down the entire couloir and left him buried ¾ of the way up to his thighs at the apron.  He was under the snow most of the way, until the slide began to fan out at the bottom and he was able to fight his way to the surface.  The avalanche propagated just below the skis of skier’s 3 and 4, and luckily they were not swept down as well, however skier 4’s backpack was swept away.  Skier 5 lost both skis and poles, and was able to dig himself out before his partners made it down to him.  When he tried to stand, his knee buckled and he realized he damaged his knee in the fall, likely before his skis released.  He reports going over a rocky area.  Of note, his dynafit bindings were fully locked out.  They splinted his knee as best they could with skins and shovel handles, and his partners were able to let him use 2 of their skis, and he was able to slowly evacuate himself the 6+miles back out to convict lake before night fall.  This required 2 of the other skiers to make their way out with one ski each.  Skier 5’s knee was later diagnosed with a torn ACL and potential meniscus tear, and will undergo surgery soon.          

Other pertinent details:  The avalanche was a very hard dry wind slab, and obviously very stubborn as it was the 5th skier to enter the couloir, and 3rd track over that particular wind slab for it to trigger.  The wind slab took them all by surprise, no hollow sound, and the surface was actually being etched away and stripped by the current north winds.  The wind slab was likely deposited during the very strong westerly winds the day before which cross -loaded the slope with snow from the west face.  The party speculated that perhaps the weight of the 3 skiers in close proximity is what caused the stubbornly sensitive slope to be triggered.  It could also have been the subtly different force that skier 5 had on the slope when he skied that triggered the sweet spot.  

March 31st ESAC Avalanche advisory (published 6:58am):

• Bottom line:  “With up to 6+ inches of new snow in some areas (such as around Mammoth) in the last 24 hours and moderate N winds today (shifted from very strong out of the SW yesterday morning), isolated areas of new wind slab sensitive to human triggering will be the greatest avalanche concern today in areas that received more than a few inches of new snow.  If the sun comes out this afternoon, southerly and westerly facing slopes with new snow could have the potential for small loose-wet natural slides.  Evaluate snow and terrain carefully and identify areas of concern.  Beware of firm conditions in areas that did not receive much new snow where a fall could result in a slide-for-life.”
• Avalanche Problem 1 - Wind Slab: Likely small to large at mid and upper elevations.  “Strong SW winds blew yesterday morning at all elevations, with gusts well over 100mph at upper elevations.  These winds lightened to moderate levels and shifted out of the North late yesterday morning, and will continue that way today.  In areas that received more than a few inches of new precipitation since yesterday morning (such as around Mammoth), new wind slabs sensitive to human triggering likely have formed on the leeward side of ridges, across slopes and gullies, and around other features that promote drifting.  Be aware that the winds shifted directions dramatically yesterday afternoon, so all aspects could be suspect.  Be on the lookout for areas with new denser snow and watch for signs such as cracks shooting from your skis to let you know that steep slopes should be approached with caution and wind-loaded slopes avoided.

Discussion:  This group was very in tune with the weather and aware of the potential avalanche dangers, even though their morning departure time was too early for them to actually read the daily avalanche advisory.  It sounds like they communicated well, and were open with each other about their concerns for the day.  In hind site they realized there were many times during the day where they rationalized their way into believing that they could ski their objective safely, when they actually should have pulled the plug.  Some human factors which may have effected their decision making process (which have effected many back country skiers and snowboarders at one time or another, including the author of this report):  

• Scarcity of time and goal driven:  They had one week to ski in the Eastern Sierra, and they wanted to make the most out of every day.  They had many big objectives they wanted to accomplish.
• They were all very accomplished strong skiers, and perhaps were overly confident in their ability to recognize a dangerous slope or condition, as well as being able to ski out of a dangerous situation.  
• Perhaps there were some unspoken concerns, and perhaps no one wanted to be the one to put the foot down and be the party pooper and call the day off amongst such a highly skilled and driven group.