While ascending the north ridge of Pioneer above the Dog Lake Chutes, and standing at the top of the Healing Shot (the east facing ramp into Pioneer Bowl), we witnessed the Pioneer Main Chute natural. Visibility was poor at the time, but we got a pretty good look at it running from above the cliff lines until it was out of our sight behind the ridge. Estimates would put it at an R3, D3. Unfortunately we were unable to see the crown due to limited visibility. It was reported to the Brighton Snow Safety Team and apparently there was no one in the runout at the time of the slide.