This avalanche was the culmination of several human factors: a highly experienced group of skiers, a larger group of skiers/friends converging in the same zone, not everybody knew each other, euphoria from improved visibility, unfamiliarity with the zone, incomplete plans, tracks to a similar area from another party, powder fever, etc. etc.
We were skiing in a popular snowmobile zone. There were snowmobile tracks on several aspects in bowl features. On our approach on mostly northerly aspects, we did not observe (or communicate) any signs of instability. At the trailhead, it was warm and wet. As we climbed higher into the zone, we found boot-top soft snow. The forecast was (3) Considerable ATL, NTL for storm slabs. The day before at Snoqualmie Pass with a similar forecast and avalanche problem, we did not observe any avalanche activity, which I mistakenly extrapolated to a different zone (I know better than to do this, but I admit that I did.)
Because of the logistics of snowmobile accessed skiing, the partner I came with was going to descend with the snowmobile to the bottom of the run and I got partnered up with someone I didn't know. The new partner and I spent roughly 30 minutes chatting with a mutual friend at the top of the run while waiting for the rest of our friends to arrive with snowmobiles. The exact plan of who was skiing what, where and when was unclear. When everyone arrived to the top of the run, there was a sense of urgency to get down the run so that the next person could ski, get the snowmobile back up to the top the run, etc. I had skied the zone once before years ago and I knew generally where I was going, but not precisely; I also knew that the rolling terrain would funnel us to a bowl feature where we would link with the snowmobiles to return to the top of the run.
A member of a different party had communicated to my new-to-me partner that they had triggered a D1 avalanche. This information did not get passed to me. My partner and I both have professional level avalanche training. We skied one at a time through rolling terrain steep enough to produce an avalanche on a similar aspect, but did not observe any signs of instability. As we descended, visibility improved considerably. We decided on the fly to seek fresh tracks nearby where others had skied. We did not communicate precisely where we would start and stop, and instead skied leapfrog style one at a time. It was my turn to lead the third 35> degree rolling-hill feature. Three turns in I started to notice the snow moving with me. I looked over my shoulder and saw that a slab had released 20 feet away from me and was quickly picking up momentum. I tried to ski off the slab, but got pushed into some trees. The force of the snow rushing over my skis plucked me out of the trees and flushed me 200 feet down from where the slab released. Both skis released and I was deposited on top of the debris pile on a bench feature beneath a 45 degree slope. When I recognized that I was being caught and carried, I didn't know exactly where my partner was above me but I was able to yell "SLIDE" repeatedly and was heard. He skied the bed surface down to me and we were able to radio to the rest of the party what had happened.
While we were trying to locate my missing skis and poles, we observed a rain crust layer from Christmas time and a layer of graupel above the rain crust. We also observed the aforementioned D1 near the toe of the avalanche I triggered.
In our debrief, we concluded that we should have slowed down to get on the same page and formulate a better plan. Even though we skied smaller slopes steep enough to slide, we did not perform a ski cut or any sort of snowpack tests before dropping into steeper and more exposed terrain. My partner had an airbag but didn't have the handle accessible at the time of the avalanche. If haste makes waste, in this case it produced an avalanche. We're lucky that it was a free lesson with the only victim being a missing ski. Like I described above, we checked several of the human factor boxes. Taking the time to ride safely and be in clear communication is not an annoying formality, but a necessity.