Let's talk about avalanches, hazards and snowbiking

[North]

You can not play around with a buried Persistent Weak Layer (i.e. faceted grains or surface hoar). These types of weak layers can "persist" and cause avalanches time and time again over a long duration. They are unpredictable. Your only sane choice when dealing with these buried weak layers is to stay off of and out from underneath slopes that are 30˚ or steeper. This stuff will bite you long after you think there's nothing going on. This is the case this year here in Utah and things will continue to be dangerous for some time to come.

I wanted to re-iterate and emphasize Brett's message on PWL avalanche problems. Anecdotally, I hear and read "we didn't observe any signs of instability" often in accident reports and close calls. I think our educational message is sometimes confusing as we preach observation and looking for warning signs. This approach is very useful for other types of avalanches but not PWLs, as they will produce avalanches even when the usual indicators are not present. This season we have PWL in many snow climates that do not see them often, and I'm concerned that users in those areas are applying their typical mitigation strategies for an atypical problem. Even in CO where our users see PWLs every season, it seems like the avoidance message is not reaching everybody.

A lot of hill climbs we all play on see traffic all year and I would think that slide potential is greatly reduced when a hill is tracked up but is that always the case?

In particular, persistent weak layer avalanches are notorious for releasing unpredictably, even after the slope has been covered with tracks. This is why avalanche professionals like Brett recommend avoiding avalanche terrain when PWL problems are present. In the heuristics acronym FACETSS, the T is for tracks - don't rely on them as a sign of stability.

 

[needpowder]

Obviously tracks in new snow are not a sign of stability with a layer buried deep. But we have slopes that we have been hitting since very early in the season and destroying the persistent weak layer. I feel like that is much different. I’m not talking about huge slopes just small mini golf slopes and aspects that are 30 to 40° that we have hit and packed every single snowfall. And yes, we hit them again this weekend in high avalanche danger and felt totally fine about it. It’s a lot like the ski resort. Were we watching each other while we did it? Of course.

 

[ProAlpine]

Like both tribal and simple have said, tracks don't necessarily equate stability. Skier compaction is something we talk about when accessing stability at ski resorts. If skiers have repeatedly skied up the weak layers before they get buried by a new storm, then it can help with stability. But, if the weak layer is below the reach of skiers (and/or sledders) you are not compacting the weak layer.

For example, I ski patrolled for 7 years and in 2009 did a ski patrol exchange at Kirkwood in Tahoe. We arrived 2 days after a 9' storm. The patrol had done all their control work (hand charges and howitzer) and opened the slopes to the public. Everything got skied hard for 2 days because it was a weekend. We arrived Sunday night at dark and could see the grooming cats working the hill. The next morning we woke to see a massive avalanche had slid on the face of the resort, in bounds. The winch cat hat hooked up for his first pass on the run and coming from the top triggered a weak layer underneath 6-8' of moguls. The whole slope slid. Luckily the groomer was not caught in the avalanche, but was perched on top of the crown, winched to the top of the mountain. That, for me, was a real good indicator of how tracks (compaction) may not equate stability...