Slide was triggered by the 4th person on the slope. He was side-hilling mid-slope when it broke out about 40 feet above him. He was caught and carried another 40 feet. He stayed with his sled, and was only partially buried. Other riders were able to reach him within 15 or 20 seconds from when the debris stopped moving. The rider was not injured.
While investigating another avalanche accident in White Pine Canyon (3/18/16), I came across this avalanche and assumed it was a sympathetic. It became clear though after looking at all the photo documentation that this avalanche was in fact the one that had caught one rider the day before the second avalanche (same slope) caught 4 riders (3/18/16).
Both avalanches had the same weak layer (graupel and facets) as well as steep slope angle (42-44 degrees at the crown) though this one was quite a bit narrower (80' across). The entire slope faces east and was loaded heavily by the westerly winds during the week's storm.