Natural and human triggered avalanches on buried surface hoar

We observed multiple natural and 2 rider triggered avalanches in the North Bowl of Castle Peak today.  All of these appeared to fail on large (12mm) buried surface hoar on top of a rain crust 8-10" below the surface.  

We planned a tour to look at terrain and make obs in the North bowl of Castle today for the final day of our Rec 2 course.  Given the new snowfall and avalanche hazard, we planned on making numerous snowpack assessments before committing to any avalanche terrain.  At the trailhead we observed 6-7" of dense new snow.  There were signs of wind transport as we made our way into higher terrain but no signs of instability.  We crossed paths with another party of 2 who we knew and synced radio channels with them.   At the ridgetop, we observed a D1 natural avalanche in the north bowl.  Curious as to what this had failed on, we dug some quick test pits and discovered large (12mm) surface hoar between the new snow and the previous surface/rain crust.  This layer varied in depth from 15-25 cms down from the surface.  At this time, we watched the group of our 2 friends descend one of the gullies off the ridgeline without incident.  We radioed them to tell them about the buried surface hoar we'd found.  A few minutes later, a single snowboarder descended the next gully to the west.  On his 3rd or 4th turn, he triggered an avalanche.  It pulled the gully 6-8" deep, size D1.5.  He managed to avoid being caught and rode on the bed surface to the bottom.   Our friends radioed and said the snowboarder was OK and that they were planning to exit the area avoiding avalanche terrain.  We continued our investigation of the surface hoar, which had variable reactivity in snowpit tests, ECTN, and PST 33/100 end.  The variation seemed to be due to whether there was enough overlying snow to form a slab.  We spoke with the snowboarder as he skinned out past us and discussed our snowpit results.  Shortly after that we watched him attempt to descend the slope nearest our test pits.  He triggered this slope on his first turn, a small D1 avalanche that failed on the same surface hoar layer.  We were able to easily access the crown of this avalanche to investigate it.  This was a small slope that we had considered skiing prior to observing the natural avalanche.  The snowboarder who triggered the avalanche stated he didn't think it was steep enough to slide, and we measured the slope to be 32 degrees at the steepest point.  Our friends later radioed to us that they also observed 3 or 4 more similar natural avalanches on their skin out, within the north bowl area.     

We learned quite a few lessons from our day out today:  It was a reminder that the snowpack can be highly variable throughout the forecast area, drainage, and slope scales.  Even on a moderate hazard day, specific geographic areas with more reactive avalanche problems may exist, particularly problems like surface hoar that may be very specific to a particular basin or elevation band.  Natural avalanche activity should always be a huge red flag.  Signs of poor snowpack structure, and weak layers like surface hoar, are also important red flags, even if they're not reactive in snowpack tests.  We also had a powerful reminder that we don't often get good feedback about the environment and our decision making process.  No amount of experience insulates us from making mistakes.   Any time we observe avalanches, we should take this opportunity to learn, to critically assess our own decision making process, and to think about the pitfalls we all make as human beings without being judgemental of others.