Natural avalanche observed on Pump South Face

We were hiking on the Mount Seymour Trail when we observed a naturally occuring avalanche in front of us. The avalanche occured at 2:52PM on the South face of Pump Peak between elevations 1300m and 1340m on a slope of 31 degrees. The avalanche started with a loud rumbling sound "VRRRRROOOMM" that lasted between 1.5 to 2.5 seconds, and we saw some of the debris sliding down the open slope for 10 to 20 seconds. The debris then reached a patch of trees and slowed down. From there, sluff snow started dripping down the nearby slopes, which lasted over two minutes. When the avalanche occured, we were at elevation 1250m and we had just left Simple terrain. we were testing the stability of the snow on a small test slope in inconsequential terrain. On the solar irradiated South aspect, pinwheels and sluff were easily generated from the top 5-10cm of snow and the snow was moist. On the West aspect, there was no pinwheeling. Off of the hiking trail, the snow was wet, loose, and granular and it was easy to sink into the snow (post-holing) from 20-60cm deep.