12th Annual Utah Snow & Avalanche Workshop Open and Motorized Sessions November 2.

Avalanche: Superior

Observer Name
Brackelsberg
Observation Date
Thursday, May 23, 2019
Avalanche Date
Thursday, May 23, 2019
Region
Superior
Location Name or Route
Superior
Elevation
10,800'
Aspect
North
Slope Angle
33°
Trigger
Skier
Trigger: additional info
Remotely Triggered
Avalanche Type
Soft Slab
Avalanche Problem
New Snow
Weak Layer
Density Change
Depth
12"
Width
150'
Vertical
600'
Carried
2
Caught
2
Snow Profile Comments
There were several layers of snow with density changes from the storms of the past week.
Comments
As a party of 2, we were planning to ski shots off Cardiac Ridge. As we climbed, we commented on the east winds. We climbed from Alta to Pole Line Pass to Little Superior. From Little Superior, we traversed Cardiac Bowl to the base of Thundrestruck. The snow on the east aspects had been blown off the slopes leaving a firm, but edgable surface that skied well. After skiing Thunderstruck we traversed north to ski another line. We climbed the sub-ridge and commented on how there were some small wind slabs that had formed. There were only ~2" deep and the snow was quite dense under these (it was difficult to sink a pole in). After descending the next wave of the storm moved in socking in Cardiac Bowl in a thick fog. We traversed towards Superior with the plan to ski the south face back to the road. We knew there had been an accident on the south aspect on Wednesday, but other than the small wind slabs from the prior climb, we had not seen any red flags on the north aspects. We ascended Cardiac Bowl with the plan to gain the ridge up the east chute. The snow was dense and supportable. We had not had any cracking or collapsing, and the east winds had not appeared to have loaded the mid-bowl. We were skinning about 20' apart from each other and when we were ~50' from the ridge, we felt the collapse as the slope broke 30' above us 8" deep and propagated 150' wide. 30-40' below the crown, the slide stepped down to the next weak layer forming a smaller, secondary crown. The dense snow knocked both of us over and slowly carried us down with it. We attempted to self-arrest, but the bed surface was too soft for a whippet to stick. Both of us rode the slide down 400' before being able to arrest.
 
Curious to understand what happened and what clues we missed, we ascended the bed surface to evaluate both crowns. Our theory is that the lower bowl did not appear wind loaded since it was an open slope, but the higher slope had rock features that trapped more snow to cause additional loading.
 
A pole and a whippet were lost, the whippet was found during the ascent. 
 
From the ridge, the decision was made to boot/skin back out the ridge to Pole Line Pass as we felt the south aspect would most likely have a similar wind load and the bed surface could likely be a much harder sun layer/crust. We encountered another party of 3 on the ridge who were planning to ski that area. They were able to see the crown as they booted up the ridge and after talking with us, they also decided to retreat back down the ridge.
 
Photo 1: Upper crown
Photo 2: Lower crown the slide stepped down into
Coordinates

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