Clouds and flat light pushed 3 rd skier on slope towards small convexity close to rocks on a gully side wall that was a little more East facing. Snow broke on steeper sidewall and entrained more snow which pulled down skier for maybe 20 ft. Skier was unhurt and visible at all times but lost a ski pole and was able to ski out of the moving snow. Crown estimated around 1.5ft, maybe 30ft wide by 50ft long, bed surface was a moist MF crust ( based only on rider report traversing bed surface after event). Due to very flat light and snowfall commencing slide was not investigated further. This was a small isolated event on a specific terrain feature,we skied all adjacent slopes. All figures are flat light approximations. Suspect combination of radiating heat from rocks, convexity/increased steepness and more East than North facing feature contributed to instability.