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We've had a rash of large, human triggered avalanches these past few days. Here is a rundown on all the slides and where they have been occurring by aspect and elevation and a few thoughts about the nature of the beast.
Here, Evelyn and I plotted all the human triggered avalanches since Jan 31. There are 11 human triggered avalanches with 6 people caught, two total burials (with beacon recoveries), one partial burial and one injury. (Add one dot to the graph for late-breaking news, 8,900' NW.)
Notice they fall generally on the aspects where faceted snow both forms and is preserved--the north half of the compass plus southeast. (East is colder than west because it's colder in the morning when the sun comes up and warmer in the evening when the sun goes down.)
All of these avalanches occurred on the faceted snow that formed on the snow surface during the first half of January when we had choking smog in the valleys and sunny skies in the mountains. To get geeky, the faceted snow occurs because of the strong temperature gradients in the upper part of the snowpack because of strong, outgoing, long wave radiation with little long wave radiation coming in.
Then we slammed two large snowstorms down on top of the weak, faceted snow and the last one finished with dense graupel snow and strong wind from the northwest. Just after the storm, the ski resorts and highway avalanche control got quite a few large, deep avalanches with explosives but when we went into the backcountry we were surprised there were so few natural avalanches. My snowpits continued to show easy to moderate propagating fractures from Extended Column Tests, meaning that the snowpack could still avalanche. In other words, the snowpack was not so unstable that it produced avalanches spontaneously, but like a bar room brawl, you had to give it a thump to get things going.
And sure enough, Friday, Saturday and Sunday, we have seen 11 human triggered avalanches, most of them large and dangerous, while at the same time hundreds of people were able to jump onto most every slope without incident. Why did some slide and most did not?
This is the age-old setup that gives avalanche geeks gray hairs and worry lines. Spouses of avalanche geeks roll their eyes and stampede from the room when whenever the subject comes up, which is quite often and invariably in heated arguments. Deep slabs. We hate em.
Deep slab avalanches involve several layers of strong slabs on top of a deeply-buried, persistent weak layer (faceted snow or surface hoar). Mechanically, they are like a thick pane of glass on top of a layer of potato chips. Because the pane of glass is stiff, it "bridges" a person's weight out over a wider area making it more difficult to initiate a collapse in the buried weak layer. But if the collapse ever starts, the entire pane shatters. It's the old low probability - high consequence situation. Difficult to trigger, but if you do, it creates a very large and dangerous avalanche.
Deep slabs are your basic nightmare. Stability tests like snowpits, explosives and cornice drops are unreliable. They seldom produce "alarm signals" such as collapsing or cracking. They often don't fracture with the first person on the slope but, say, with the third, sixth or tenth person. The only way to manage this kind of hazard is avoidance. All the cagy, old avalanche geeks just stick to slopes less than 30 degrees that are not locally connected to steeper terrain. Simple. Or they choose a safer aspect. Also very simple.
As avalanche forecasters, we have always struggled with what danger rating to assign to these beasts. Avalanche hazard (danger) is a product of BOTH probability and consequences. Larger avalanches have larger consequences so even when few people are triggering avalanches you will see our danger ratings bump up a notch or two because of the dangerous consequences. When people see others jumping into all kinds of dangerous avalanche terrain without triggering avalanches, it's easy to conclude that we are crying wolf until they get home and see avalanche accidents from large, dangerous avalanches on the evening news. I'm not sure where we are on the graph below. At first we thought the likelihood was quite low and the size quite large, but as more events show up, many of the avalanches are easier to trigger and more average-sized. So perhaps there is more going on than just lingering, deep slabs. We've had a lot of warming over the weekend and that may be a significant contributor.
Here's another pattern I notice: I believe nearly all of these avalanches occurred on slopes that did not get many tracks during the first half of January when the faceted snow was on the surface. Lots of tracks tend to break up the continuity of the future weak layer and after it's buried, a tracked slope tends to produce less avalanche activity. This is not always the case but the Wasatch is so crowded that it is undeniably a factor.
Finally, the number of avalanches we've seen probably have more to do with the number of triggers (people) than snowpack conditions. Without so many people jumping into all kinds of slopes, we would likely see no avalanche activity and we would all be blissfully unaware of any hazard. As the Wasatch becomes more and more like Chamonix, if there are any instabilities out there, the "volunteer stability tester" as I call them, will certainly find them.
Check out the list of avalanches on our website Detailed Info > Avalanches: http://utahavalanchecenter.org/avalanche-list. Here is a sample from today:
So my advice is: 1) Cool it. As long as there are monsters in the basement, don't mess with them. Avoid steep slopes on the suspicious aspects and elevations. 2) If you absolutely can't help yourself and MUST be on steep terrain on the shady aspects, then go to slopes that had many tracks when the faceted snow was on the surface a couple weeks ago.
Finally, I think almost all avalanche accidents occur because we don't match the danger of the snowpack with the appropriate danger of the terrain. The diagram above is a simple way to keep it straight. Always keep it in the green.