Blog – “Is it a Moderate kind of day?” – 1/7/2014

This blog is an area where the Flathead Avalanche Center avalanche specialists (and other avalanche experts) discuss a certain topic in detail. This blog will allow us to expand upon certain concepts or topics related to avalanches such as backcountry travel, search and rescue, decision-making, and many others. You will find information here that we just can’t quite fit into the advisories, but is still very relevant and important. On to the music….

A friend of mine who previously lived in the Flathead Valley and transplanted to Missoula about four years ago came back for a visit.  During his visit he was treated to fresh, new snow and ventured into the backcountry in Canyon Creek in the southern Whitefish Range. Later that evening he commented to me that “there are a lot of people in the backcountry these days”. The increase in backcountry use is indeed noticeable from many vantage points especially with the viral (pun intended) use of mobile computing technology, social media, and, of course, head cams. 

Skiers took this image of an avalanche triggered by a snowmobiler in the southern Whitefish Range near Canyon Creek immediately after hit happened. Note the previous ski tracks that existed on the slope prior to avalanching. Photo: Michael Voison.

Skiers took this image of an avalanche triggered by a snowmobiler in the southern Whitefish Range near Canyon Creek immediately after it happened. Note the previous ski tracks that existed on the slope prior to avalanching. This is a good example of using tech to get the word out. Photo: Michael Voison.

Then, he mentioned that when he woke up the next morning he felt like it was a Moderate hazard sort of day. He was joking with this off the cuff assessment, of course. Yet, my friend’s comment and a few comments I’ve heard over the past year or so about the perception of avalanche hazard caused me to start pondering, again, avalanche hazard ratings and the public’s perception of these ratings. 

In the United States, avalanche centers that provide avalanche advisories for backcountry recreation use the North American Public Avalanche Danger Scale. This is a five category estimation that is used to communicate backcountry travel advice, likelihood (probability) of avalanches, and the size and distribution of avalanches. Grant Statham (2010) summarizes the hard work and intricacies that went into developing the current public avalanche danger scale. He concludes, 
     “Well, in the end it’s just a scale; nothing more than a table of carefully crafted words, symbols and colors designed with the best of intent to help the public better understand and manage their risk. The scale is simply one more step in the larger process of understanding avalanche risk. Although fleshing out this process was time consuming, it has resulted in a more solid foundation upon which to base our knowledge of avalanche risk and public warnings.”

The scale certainly is a great tool that we can use to help our decision-making process when recreating in the backcountry, but it is just that, a tool. It is one tool among many in our entire toolbox that we can use to make sound decisions in the backcountry. It helps us understand risk and associated travel recommendations.

Along with understanding our own risk and associated hazard ratings, I’ve noticed recently that sometimes we become complacent with certain hazard ratings, particularly the ratings in the middle of the scale. When it’s HIGH or EXTREME, mostly everyone is paying attention and on their toes just waiting for an avalanche to occur.  Many folks see LOW as a green light, but avalanches can and do occur during LOW hazard.  

It appears that MODERATE and CONSIDERABLE avalanche hazard that seem to be a bit more nebulous for many folks recreating in the backcountry. However, most fatal avalanches occur when the hazard is CONSIDERABLE (Greene et. al. 2006). Patt and Schrag (2003) state that when it comes to changes in the middle of a probability scale (like the avalanche danger scale) most people are indifferent, and view the chances of an occurrence (like an avalanche) the same as flipping a coin (50/50). The way people view probability descriptors depends on their experience with the frequency of such occurrences.  For example, they state, “People interpret a ‘slight chance’ of rain in London as meaning a higher numeric probability than a ‘slight chance’ of rain in Madrid.”

Skiers traveling in complex avalanche terrain.

Skiers traveling in complex avalanche terrain. Red arrows point to skiers. Rhetorical qusestion: Is this appropriate terrain to travel in  during Moderate hazard?

While the North American Public Avalanche Danger Scale uses language as opposed to numeric probability, it is important to know what each hazard means. I’ve seen many folks treat MODERATE hazard as a green light; meaning it’s good to go hop on any slope.  This type of behavior can be dangerous, especially when dealing with persistent slab problems which we have been for about a month now (video). MODERATE hazard means that human triggered avalanches are possible. So, if you’re standing at the top of a slope waiting to ski it or sitting at the bottom of the slope waiting to highmark, and someone says, “Yeah, it’s possible to trigger an avalanche on that slope.”, what would you do? If you are armed with enough objective information about the snowpack on that slope, then perhaps it is reasonable to ride or ski it, or perhaps not. When the hazard is MODERATE and a persistent slab exists it is possible to trigger an avalanche, but the consequences may be greater given the depth of the weak layer and the potential size of the avalanche. This can be viewed as a low or medium probability, but high consequence situation.

So, the point of this post is to get folks to think about the North American Public Avalanche Danger Scale more critically, and to really look at the language for each category. It’s also helpful to examine the travel advice for each rating. We are able to base the hazard rating on either the probability, size and distribution, travel advice, or a combination thereof. Thus, it helps to know, for instance, that MODERATE hazard means heightened avalanche conditions, and that you should evaluate snow and terrain carefully. Hopefully, doing so will allow you to add another tool to your decision making toolbox when recreating in the backcountry, and help in making sound decisions. 

Danger Scale


Greene, E., Wiesinger, T., Birkeland, K., Coleou, C., Jones, A., Stathan, G., 2006. Fatal avalanche accidents and forecasted danger levels: Patterns in the United States, Canada, Switzerland, and France. In: Proceedings of the 2006 International Snow Science Workshop. Octrober 1-6, 2006, Telluride, CO. pp. 640-649.

Patt A.G., Schrag D.P., 2003. Using specific language to describe risk and probability. Climatic Change 61 (1-2):17-30. doi: 10.1023/A:1026314523443

Statham, G., Haegeli, P., Birkeland, K.W., Greene, E., Israelson, C., Tremper, B., Stethem, C., McMahon, B., White, B., Kelly, J., 2010. The North American Public Avalanche Danger Scale. In: Proceedings of the 2010 International Snow Science Workshop. October 17-22, 2010, Squaw Valley, CA. pp. 117-123.