Two of us skied up Rescue Ck up to the top of Mt Penrose and descended the north gully to Skiumah Ck. Most of the open terrain had some level of wind effect, from softly textured dense powder to stiff breakable wind slab. Quite a bit of work to ski but still fun. Recent cornice fall of various sizes had naturally controlled most of the north and east aspects of Penrose. Some large cornices still exist. Towards the bottom of our descent we trended right (east) into a small tree island to find non-wind affected snow. The first skier who trended farthest right, beyond the tree island, triggered a 6-12 inch slab of recently wind transported snow. The skier who triggered the slab stopped above it and yelled to alert the second skier below him. Second skier cut right to avoid the running debris, which ran for 200 vertical ft before collecting on a lower angle bench. Definitely caught both of us off guard, since we were nearing the bottom of a bigger slope and had not experienced any signs of surface instability. See Avalanche Details and photos for more info.
The soft slab released on an isolated pocket of surface hoar, likely from the clear spell on the previous Wednesday, 3/7/18. Minimal new snow in the basin, but SW winds appears to have moved a decent amount of new snow onto leeward features such as the small headwall upon which the slab released. The slab released just to the leeward (east) side of a rib of trees on an otherwise open slope. Skier who skied the tree rib found soft wind loaded snow, but the skier who skied just to the leeward side of the tree rib found denser wind impacted snow. A small, steeper (38-40) rollover provided the trigger and the slab released quickly with energy but toed into lower angle (28-30) degree terrain 200 vertical ft below. Both the tree rib and the adjacent headwall funneled into the same shallow gully. We were skiing the bottom of the slope individually in several shorter pitches in an attempt to find softer snow. By yelling, the upper skier who triggered the slab alerted the lower skier, who skied to the west to get out of any possible terrain trap. The slide wasn't huge but definitely caught us by surprise. No other surface instabilities had been noticed, we were nearing the bottom of a big run, we cut a cornice at the top of the run to test for instabilities, and noticed plenty of recent debris from fresh cornice fall. We were aware, however, that pockets of sensitive windslab could exist.
