A skier wrote the following email with the picture. The red line marks the crown. Wind-loading was the likely trigger.
"From the ridge between the east and north face on Mount Blackmore we observed a crown line on the north ridge of Blackmore. The crown line was approximately 150' long and 1'-2' deep. We skied the SE terrain on the skiers left side of the east face of Blackmore, so we didn't get close to the crown line to inspect the snow. There were no tracks visible near the avalanche so it was likely naturally triggered by wind-loading blowing over the ridge from the west." Photo: T. Chingas
Forecast link: GNFAC Avalanche Forecast for Tue Dec 19, 2017