Nearing the top of a NW aspect couloir at 5000' we had been feeling a soft, non-cohesive slab structure around 5" deep on a 40ish° slope with obvious signs of wind effect. We had only seen isolated shooting cracks of a few feet at the corners of our skintrack turns. We felt it was safe to cautiously continue upwards booting rather than skinning, as the slab did not seem likely to cohere and release all at once. The first skier down tested for cracking on a lower-angle slope near the top, observed none, then ski cut the rollover before the start zone with no results. The first turn made onto the steeper slop triggered the D2 slab. We had ensured all nearby parties were in safe areas and the triggering skier was not caught. No burials or injuries.