kjb
Active VIP Member
Good blog post on the CAC website.
Post Storm Risk Management Strategy - Karl Klassen, November 23 2011
I think it’s pretty clear things are touchy right now. Size 3s hitting valley bottom in the Columbias, 3.5s logging mature timber and extending trimlines in the Lizard Range, lots of snow, rain, wind, and warm temperatures. The season is starting with a bang. And, while I’m sure everyone is hunkered down sitting this one out or staying out of avalanche terrain—right? I know we’re all excited about getting out there and hitting it when the storm ends and the weather clears. Before you do that though, I’d like to talk a bit about how to approach the mountains after a big storm like the one we’re in now.
After the storm ends, there are a number of weather scenarios that could play out and how they affect stability will be different depending on the character of the snowpack. Please keep in mind that what follows is generalization. It’s intended to educate and inform you about how things work and help you develop big picture strategies rather than give you a recipe for deciding which slope to ride when.
Everyone recognizes that big storms are usually associated with big avalanche cycles. The question is how to get out there after the storm ends and maintain adequate levels of risk. How long instability will persist after a storm has to do primarily with temperatures and settlement. The effects of both are different in the short term than the long term. Warm temperatures promote settling, which is a good thing—it leads to a more uniform, strong snowpack with fewer layers and better bonding between the layers. This takes some time. In the short term, warmer temperatures mean weaker bonds between snow grains and snow layers (the snow is less frozen, so to speak). Cold temperatures promote stronger bonds between grains and layers (the snow is essentially more frozen) which is a good thing. This is a short term good thing however because cold temperatures inhibit settling so it takes longer to get a nice, solid, uniform snowpack if indeed you ever get there. Following are some scenarios to illustrate my point and hopefully help you when you go out into the mountains after a storm ends.
A warm (say -2, -3 degrees C) storm followed by cold weather (-6ish to -10 or so): You’ll see rapid stabilization of the warm snowpack and a fairly quick end to natural avalanche activity. Human triggering persists for a short while then becomes progressively more difficult. This is often referred to as “tightening” as in “the snowpack is tightening up.” If the cold deepens (say to -20) and lasts, all avalanche activity is likely to cease. However, weak layers in the snowpack are not settling and bonding. In fact, they may be getting weaker, and everything is held in place only by the strong, frozen bonds in the snow above the weak layer. When the weather changes, say it warms and the frozen bonds in that upper layer weaken again or it snows some more and adds load, these weak layers sometimes “reactivate” and you see another round of avalanche activity before settlement and bonding finally does away with the weak layer.
In this scenario, don’t get fooled by the initial rapid cessation of avalanche activity. Wait for a while until you are sure things have stabilized. This generally takes two or three days depending on how warm the initial conditions were, how cold the subsequent cold spell is, and the nature of the weak layers in the snowpack. Be wary of large or steep slopes with high consequences when you start to push out. Most importantly, be prepared to pull back in when the weather breaks so you don’t get caught by surprise if a lingering weak layer reactivates.
A cold storm followed by warm weather: You’ll likely see avalanche activity continue or even increase after the storm ends. This is often accompanied by “upside down” conditions where warmer, more settled, and more cohesive snow lies over colder, less dense layers making for difficult trail-breaking. In relatively short order (say 36-48 hours) avalanches in the upper part of the snowpack will generally stop. If there are weak layers deeper in the snowpack, they will continue to be a problem for longer but if conditions stay warm without more storm activity, deeper layers settle and bond eventually. How long this takes depends on various factors such as the characteristics of the deep weak layer and how far below the surface lies.
In this scenario, wait 48-60 hours to let the storm snow instabilities settle out then cautiously move into steeper, more aggressive terrain, starting with smaller, lower consequence slopes. Ensure there are no lingering deep instabilities before venturing into larger, steeper, more complex terrain features.
Cold storm followed by cold weather: More than likely activity will taper off slowly regardless of whether it’s a storm instability or a deeper layer at play. Plan to wait longer than normal before starting to poke out into more aggressive terrain; several days or a week for storm related instabilities and longer, of course, if a deeper more persistent instability is the player.
Warm storm followed by warm weather: This is a typical coastal situation. Intense avalanche activity during the storm to essentially bombproof conditions in 12-24 hours or so. (Barring deep instabilities of course). Wait 24-36 hours then check things out by starting on smaller low consequence slopes.
These are, of course, simplistic scenarios and real situations are often much more complex and involve characteristics of more than one of these snowpack, storm, or post-storm scenarios. Hopefully this brief overview helps improve your ability to understand CAC warning service products and aids you in managing your risk in the mountains.
You can send comments or questions about this article directly to me.
Karl Klassen, Manager
Public Avalanche Warning Service
Canadian Avalanche Centre
kklassen@avalanche.ca
Post Storm Risk Management Strategy - Karl Klassen, November 23 2011
I think it’s pretty clear things are touchy right now. Size 3s hitting valley bottom in the Columbias, 3.5s logging mature timber and extending trimlines in the Lizard Range, lots of snow, rain, wind, and warm temperatures. The season is starting with a bang. And, while I’m sure everyone is hunkered down sitting this one out or staying out of avalanche terrain—right? I know we’re all excited about getting out there and hitting it when the storm ends and the weather clears. Before you do that though, I’d like to talk a bit about how to approach the mountains after a big storm like the one we’re in now.
After the storm ends, there are a number of weather scenarios that could play out and how they affect stability will be different depending on the character of the snowpack. Please keep in mind that what follows is generalization. It’s intended to educate and inform you about how things work and help you develop big picture strategies rather than give you a recipe for deciding which slope to ride when.
Everyone recognizes that big storms are usually associated with big avalanche cycles. The question is how to get out there after the storm ends and maintain adequate levels of risk. How long instability will persist after a storm has to do primarily with temperatures and settlement. The effects of both are different in the short term than the long term. Warm temperatures promote settling, which is a good thing—it leads to a more uniform, strong snowpack with fewer layers and better bonding between the layers. This takes some time. In the short term, warmer temperatures mean weaker bonds between snow grains and snow layers (the snow is less frozen, so to speak). Cold temperatures promote stronger bonds between grains and layers (the snow is essentially more frozen) which is a good thing. This is a short term good thing however because cold temperatures inhibit settling so it takes longer to get a nice, solid, uniform snowpack if indeed you ever get there. Following are some scenarios to illustrate my point and hopefully help you when you go out into the mountains after a storm ends.
A warm (say -2, -3 degrees C) storm followed by cold weather (-6ish to -10 or so): You’ll see rapid stabilization of the warm snowpack and a fairly quick end to natural avalanche activity. Human triggering persists for a short while then becomes progressively more difficult. This is often referred to as “tightening” as in “the snowpack is tightening up.” If the cold deepens (say to -20) and lasts, all avalanche activity is likely to cease. However, weak layers in the snowpack are not settling and bonding. In fact, they may be getting weaker, and everything is held in place only by the strong, frozen bonds in the snow above the weak layer. When the weather changes, say it warms and the frozen bonds in that upper layer weaken again or it snows some more and adds load, these weak layers sometimes “reactivate” and you see another round of avalanche activity before settlement and bonding finally does away with the weak layer.
In this scenario, don’t get fooled by the initial rapid cessation of avalanche activity. Wait for a while until you are sure things have stabilized. This generally takes two or three days depending on how warm the initial conditions were, how cold the subsequent cold spell is, and the nature of the weak layers in the snowpack. Be wary of large or steep slopes with high consequences when you start to push out. Most importantly, be prepared to pull back in when the weather breaks so you don’t get caught by surprise if a lingering weak layer reactivates.
A cold storm followed by warm weather: You’ll likely see avalanche activity continue or even increase after the storm ends. This is often accompanied by “upside down” conditions where warmer, more settled, and more cohesive snow lies over colder, less dense layers making for difficult trail-breaking. In relatively short order (say 36-48 hours) avalanches in the upper part of the snowpack will generally stop. If there are weak layers deeper in the snowpack, they will continue to be a problem for longer but if conditions stay warm without more storm activity, deeper layers settle and bond eventually. How long this takes depends on various factors such as the characteristics of the deep weak layer and how far below the surface lies.
In this scenario, wait 48-60 hours to let the storm snow instabilities settle out then cautiously move into steeper, more aggressive terrain, starting with smaller, lower consequence slopes. Ensure there are no lingering deep instabilities before venturing into larger, steeper, more complex terrain features.
Cold storm followed by cold weather: More than likely activity will taper off slowly regardless of whether it’s a storm instability or a deeper layer at play. Plan to wait longer than normal before starting to poke out into more aggressive terrain; several days or a week for storm related instabilities and longer, of course, if a deeper more persistent instability is the player.
Warm storm followed by warm weather: This is a typical coastal situation. Intense avalanche activity during the storm to essentially bombproof conditions in 12-24 hours or so. (Barring deep instabilities of course). Wait 24-36 hours then check things out by starting on smaller low consequence slopes.
These are, of course, simplistic scenarios and real situations are often much more complex and involve characteristics of more than one of these snowpack, storm, or post-storm scenarios. Hopefully this brief overview helps improve your ability to understand CAC warning service products and aids you in managing your risk in the mountains.
You can send comments or questions about this article directly to me.
Karl Klassen, Manager
Public Avalanche Warning Service
Canadian Avalanche Centre
kklassen@avalanche.ca
