A
avyfire
Member
Great questions Scott that have been topics for doctorate dissertations at MSU and other schools. Buried surface hoar is our biggest problem in western MT. It's easily destroyed on the snow surface. Ski or sled compaction, sun, warm temperatures and wind help prevent later problems. Once buried, it is insulated from the effects of sun, wind, moderate temperatures and in some cases it can't be stabilized by heavy ski or sled traffic as demonstrated by the large avalanches next to Bridger Bowl this year. Check out the Saddle Peak Q & A on the Gallatin NF Avy Center's video archive for an excellent and informative talk on the close call near the ski area. http://www.mtavalanche.com/video/youtube
As far as the layer we're concerned about in our area, I am certain it is the very same layer you looked at when you took the students to Lolo Pass. At that time, it wasn't buried very deeply and was starting to show signs of strengthening, but then it quit snowing and we had several episodes of cold clear nights which allowed these already faceted crystals to be exposed to a huge temperature difference from the buried layer to the surface which allowed the crystals to continue getting bigger and weaker over a period of several days. Then we started getting some snow. Continued snowfall and settling over time also helps destroy or at least strengthen these buried surface hoar layers. In this case it was not enough to make a difference in the temperature profile of our snowpack or to crush the crystals into a more rounded condition. Around Presidents Day weekend we started hearing about and seeing significant avalanche activity involving this layer that continue to this week. It kind of snuck up on us. Lots of close calls involving this layer.
So the quick answer is that during normal years with decent snowfall, these layers gain strength more quickly than they do during years when we have shallow snow and infrequent storms like this year. Unusual snow and weather conditions often give us unusual avalanche conditions. Most locations are moderately stable now but whenever conditions change quickly, like rapid warming or a heavy load of snow (like right now) on any slope that has this persistent weak layer, it's time for a high level of situational awareness and carefully evaluate the slopes we want to ride on. Time and timing is everything.
Hope this helps. This is a complex topic that has taken me years to wrap my head around. This year just made it even more complex. No snow is no good!
As far as the layer we're concerned about in our area, I am certain it is the very same layer you looked at when you took the students to Lolo Pass. At that time, it wasn't buried very deeply and was starting to show signs of strengthening, but then it quit snowing and we had several episodes of cold clear nights which allowed these already faceted crystals to be exposed to a huge temperature difference from the buried layer to the surface which allowed the crystals to continue getting bigger and weaker over a period of several days. Then we started getting some snow. Continued snowfall and settling over time also helps destroy or at least strengthen these buried surface hoar layers. In this case it was not enough to make a difference in the temperature profile of our snowpack or to crush the crystals into a more rounded condition. Around Presidents Day weekend we started hearing about and seeing significant avalanche activity involving this layer that continue to this week. It kind of snuck up on us. Lots of close calls involving this layer.
So the quick answer is that during normal years with decent snowfall, these layers gain strength more quickly than they do during years when we have shallow snow and infrequent storms like this year. Unusual snow and weather conditions often give us unusual avalanche conditions. Most locations are moderately stable now but whenever conditions change quickly, like rapid warming or a heavy load of snow (like right now) on any slope that has this persistent weak layer, it's time for a high level of situational awareness and carefully evaluate the slopes we want to ride on. Time and timing is everything.
Hope this helps. This is a complex topic that has taken me years to wrap my head around. This year just made it even more complex. No snow is no good!
