Surface hoar is a common persistent grain. Timothy Rogers photo
The last two weeks have seen a string of avalanche fatalities, several of which involved a Persistent Slab problem. [*Please note this blog was originally published on March 13th, 2018.] This type of avalanche is often associated with the thin, cold snowpacks of interior mountain ranges. But our regional accident record shows that they are very much a part of our Pacific Northwest winter mountain landscape.
True to their name, last week’s problems have not gone away. While the likelihood of triggering one of these avalanches may continue to diminish, our eagerness and ability to explore may outpace their rate of repair. This blog is meant to refresh your knowledge of persistent problems, and to revisit the tools and techniques we have to identify, test, and manage these problems.
What is a Persistent Weak Layer and how is it formed?
A persistent avalanche problem begins with a Persistent Weak Layer, or PWL. Most simply, a PWL is a thin layer in the snowpack that fails to gain strength on the timescale of storm and wind slabs. Surface hoar, depth hoar and faceted snow are all termed persistent grain types. Because of their shape, they don’t tend to gain strength quickly, and in fact may get weaker with time. Cold temperatures and thinner snowpacks tend to favor the development of these layers (and that is why they are more common in continental and intermountain snowpacks).
A slab formed over a Persistent Weak Layer is termed a Persistent Slab–one of the nine avalanche problem types. If that layer is deeply buried (at least more than a meter) then it is termed a Deep Persistent Slab, another one of the nine avalanche problems. Depending on conditions, the layer can continue to facet, or it could begin to heal. But as the name suggests, it can take a long time even when things are trending toward stability, with some layers lingering for the length of a season.
As they become more deeply buried, these weak layers become harder to trigger–but the expected size of the avalanche can increase. We frequently arrive at a low-likelihood/high-consequence situation, which can challenge our patience and decision-making abilities.
PWL’s often activate with loading from a storm event. Matt Primomo photo
What are we waiting for?
As with most persistent problems in our lives—nagging injuries come to mind–persistent avalanche problems require patience. But what are we waiting for? The simple answer is that we are waiting for the layer itself to get stronger and make bonds to the grains around it.
We gauge it’s strength and structure using observational tools like snow profiles and snowpack tests. The challenge, however, is spatial variability, and how to qualify our test results. Layers and slabs form with a wide range of properties across our region. Each storm plays out a little differently in each drainage and on each slope; a few degrees’ shift in slope aspect can mean the difference between a thick sun crust and none at all. So the measurements we make of snowpack properties at one location can only be extrapolated so far.
At NWAC, we’re working with Professional Observers, ski patrollers, and avalanche forecasters to track the current PWLs as widely as possible. The Persistent Slab problem is likely to remain, but you can help us track its distribution and development, beginning with a careful reading of the forecast discussion and recent observations.
Faceted grains that have begun to round. Jeremy Allyn photo
Can I observe these layers myself?
You can, and you should. Tracking these layers and observing characteristics of their related avalanches can lead to some of the more informative lessons in your snow career. It can also help improve our forecast!
Before going on, it’s important to be very clear: snowpack tests shouldn’t result in a “go” or “no go” decision, they are simply a small data point in your larger assessment. In other words, it’s dangerous to conclude that a problem is absent or unreactive based on a test. What you find in a pit is a very small window into the reality of the snowpack. PWLs make for tricky decision making and terrain management, and even experts can be caught off guard.
• Begin by making notes on the distribution of the weak layer in your area, using the Forecast Discussion and Recent Observations page to gain a sense of where to find it and what to look for. NWAC observers have been tracking a variety of faceted layers in several areas.
• Take notes on the depth at which it’s been observed, as well as the aspect and elevation of areas where it’s likely present.
• Next, find a safe, undisturbed place to dig a profile at a similar aspect and elevation.
• Observe the pit wall for overall structure. Weak layers may be found near crusts or other hard interfaces. Grains in these layers will be looser, bounce on a crystal card, and exhibit sharp edges under magnification.
• Apply compression and propagation tests to the best of your ability and training. These tests should be repeated when possible in order to verify results.
• If your training includes the use of a snowpack checklist such as the “Yellow Flags” or “Lemons,” take the opportunity to apply it to your profile. These checklists help alert you to conditions that are often observed in accidents.
• Keep interpretation simple where possible: qualifying results in terms of good, fair, or poor can be helpful. This can help you be more efficient in summarizing results and tracking changes over time and distance.
• In addition to profiles and snowpack tests, make detailed observations of any avalanches related to a PWL, these are valuable opportunities to learn about the likelihood and consequences of triggering such an event.
Submit observations when and where you find these layers, or even if you don’t. This can help our forecasters and other recreationalists get a grasp on the distribution and sensitivity of these layers over our very large forecast area.
Relentless digging and testing is required to track these layers. Timothy Rogers photo
How do I manage the problem?
Simply put, Persistent Slab avalanches are a high consequence, low probability event. Overall caution is advised when dealing with Persistent Slab avalanches as they are difficult to forecast and often exist in isolated or specific terrain features. Continuous and wide-reaching snowpack assessment is key to determining a weak layer’s distribution and reactivity. We mitigate the hazard by relentlessly tracking it, and avoiding the terrain where we know or suspect it to be present.
• There is no mitigating this hazard with ski cuts or slough management. These avalanches can break far above you and much more deeply than is manageable.
• Don’t expect the problem to announce its presence. Shooting cracks and recent avalanche activity are generally not observed, although large audible whoomphs can indicate the presence and collapse of a weak layer.
• Put a sufficiently large physical margin between you and any suspect slopes. Terrain is your friend, enjoy the soft snow on lower angle slopes with no overhead hazard.
• In busy backcountry areas, make sure to exercise clear communication to avoid skiing or riding on top of one another. Some parties will have different risk tolerance, understanding, and local knowledge than others, and people could be traveling in terrain that you ruled out.
• PWL’s often awaken during periods of intense loading–think storms that drop over an inch of water within 24 hours. Be sure to give suspect terrain time to digest this new load.
• Warming or even strong solar radiation can trigger Persistent and Deep Persistent Slabs, or render them more sensitive.
As they continue to be buried and our cold temperatures moderate, our PWL’s will likely become more stubborn and the associated grains will begin to heal, but as stated, this process takes time and patience must be exercised. With plenty of safe and lower angle terrain to choose from, this season’s persistent problems shouldn’t prohibit you from enjoying the wild adventure Washington has to offer.
Deep Persistent Slabs that fail near the ground are catastrophic and unsurvivable events, like this avalanche in New Zealand. Timothy Rogers Photo