The front page of the avalanche forecast gives you an overview of the avalanche hazard for the current day and the expected trend for the next two days. Under the headline, there are three tabs—Danger ratings, Problems, and Details. We will explore these more in the following pages.
The danger ratings are based on the five point North American Public Avalanche Danger Scale. Each level is defined by the likelihood of avalanches and their expected size and distribution. Check out this graphic to see how each rating is defined and hover over or tap the icons for some advice on managing avalanche hazard under each rating.
Extreme avalanche danger is used rarely in avalanche forecasts. It is employed when the likelihood and size of avalanches could present conditions that are outside the realm of many people’s experience. Conditions such as avalanches running beyond the existing bounds of their path, avalanches running into forested terrain, avalanches running across valleys and up the other side, or avalanches impacting simple terrain are possibilities under extreme danger. Decision making under considerable danger can be challenging. While conditions are dangerous, avalanches may be less widespread, smaller, or less likely than under high avalanche danger, potentially making the danger less obvious. Many slopes should be avoided when avalanche danger is rated considerable. Use the Avaluator Trip Planner and Slope Evaluation Tools to help decide on appropriate areas to travel. Historically, the highest number of avalanche fatalities have occurred when the danger was rated considerable. Under high danger, all avalanche terrain should be avoided. Use extra caution when travelling in simple terrain and stick to very mellow slopes or dense trees that are free of overhead hazard. While many people hold the attitude of “green means go” when the danger is low, one still needs to be cautious and exercise smart travel habits when entering steep slopes. Watch for any signs of instability and avoid obvious trigger points such as shallow rocky start zones. Decision-making under moderate avalanche danger can be challenging, since avalanche danger is present, but is limited in distribution, likelihood, and/or destructive potential. It can apply to a variety of avalanche problem scenarios, including low-probability/high-consequence situations, when sporadic yet very large avalanches are possible. Pay careful attention to the avalanche problems in the forecast to avoid slopes where avalanches may be triggered.
Danger ratings are applied to three elevation bands: alpine, treeline, and below treeline. Hover over or tap the icons to learn more about the elevation bands.
The alpine is the treeless area at the upper reaches of the mountains. More snow falls, winds are stronger, and the sun’s rays are the most intense. Terrain irregularities, such as ridges, bowls, and gullies, are fully exposed to wind and sun, promoting the creation of unstable snow layers. This is where the most avalanche accidents occur. Treeline is the transition zone, where trees are smaller and less dense than those in the valley, yet there is more vegetation than in the alpine. You don’t get the protective effects of the forest, but also not the full exposure to the elements as in the alpine. Avalanche accidents happen at treeline almost as often as in the alpine. There is enough wind at treeline to build slabs and the sparse trees can make things worse by acting as natural snow fences that create significant drifts. The complexity of this zone and the variety of factors at play means it’s easy to underestimate the hazards that exist at treeline. Below treeline encompasses the valley bottom and up the mountainside to the point where the forest begins to thin out. Here you will find less snow and the thick trees tend to act as anchors within the snowpack, making avalanches less likely. Dense trees also mitigate the effects of wind and sun, further reducing the chance for unstable snow to form. Don’t overestimate the effect of tree cover. Avalanches can happen in large clearings, or even anywhere it’s open enough to ride. Raven Eye Photography