Southern San Juan
AvalancheAnonymous · Apr 30, 2016 · Southern San Juan
Weather
Temp during tour mid 20s F on apron and couloir (12,000; to 12,900'). Gusty SSW winds (20-30+mph?) on ridgeline above couloir. Mix of full sun and snow showers (minimal accumulation of flakes and graupel during tour). Aspect 330.
Snowpack
Within the couloir, approx. 10-15cm of fresh powder (dustless) on top of approx. 15 cm of brown dusty powder from an earlier phase of this week's storm set. This total of 25-30cm of powder rested on an older melt-freeze crust. Handpit testing indicated strong bonding of the dusty powder to the underlying melt-freeze crust.
Avalanche
Two parties (Party 1: three skiers / Party 2: four skiers) simultaneously made a bootpack ascent of the couloir. Party 1 reached a highpoint approximately 20m below the ridgeline (~12,900'), ~20m above the partially covered rockband and just above the steepest (~50degree) part of the couloir, and began to transition for descent. Meanwhile, three of four skiers from Party 2 began to ascend from the widest section of the couloir ('the torso') into the narrow upper section, with the fourth skier of Party 2 (skier 2.4) below in the 'torso' section. At this point, Party 1 indicated they would wait for everyone to ascend before skiing. Party 2 then communicated the offer to stand off to skier's right in the narrow upper couloir section below the partially covered rock band (20m below Party 1) to let Party 1 descend past them. Party 1 agreed. As Party 1 prepared to ski, the fourth skier of Party 2 (skier 2.4) was still ascending the bootpack in the narrow upper couloir section (far climbers left), approx. 10m below the rest of Party 2. At this point Party 1 and probably Party 2 expected the possibility of loose dry sloughing in the middle of the narrow upper couloir feature. The lead skier from Party 1 (Skier 1.1) edged out slowly 1-2m from the couloir wall, at which point a storm slab avalanche released nearly wall to wall from Skier 1 down to the top of the partially covered rock band where the first three members of Party 2 stood( Skiers 2.1, 2.2, 2.3). The initial slab released (Avalanche 1) had a 15cm crown, 3-4m width and 20m length. This soft slab drained below the partially covered rock band, bypassed skiers 2.1, 2.2 and 2.3 but caught skier 2.4. Avalanche 1 began to build downslope, entraining the upper (dustless) part of the storm slab in the central gut of the couloir but did not appear to entrain any snow outside of the runout path. Skier 2.4 was observed to come to rest on top of the thin deposit near the top of the apron (12,150'). At this point, Party 1 was wary of hangfire in the upper couloir and remained in place, so the lower placed skier 2.1 quickly transitioned and began to descend. As soon as skier 2.1 entered the wider 'torso' part of the couloir (approx. 35-40 degrees), nearly all of the remaining upper (dustless) storm slab in the couloir released (Avalanche 2), in particular drawing crowns well up the eastern sidewalls (skiers left) of the couloir. Skier 2.1 immediately descended the scour surface to assist skier 2.4 below. The remaining 5 skiers from both parties then descended the couloir on the scour surface. All seven members of both parties were able to descend to the trailhead on ski from that point. Collectively, the two avalanches ran 850' vertical and were estimated at D2R4, In the case of both avalanches, the slip surface was the interface between the dusty and dustless parts of the storm slab, and the slide did not seem to step below this interface aside from minor scouring.