The North American Public Avalanche Danger Scale (NAPADS) is a system designed to assess avalanche risk and offer general travel advice. It takes into account factors such as the likelihood, size, and distribution of potential avalanches. This scale consists of five levels, which are as follows:
- Level 1 – Low
- Level 2 – Moderate
- Level 3 – Considerable
- Level 4 – High
- Level 5 – Extreme
Avalanche danger is typically evaluated for three specific elevation bands. It's important to note that while these levels are assigned numerical values, the risk increases significantly as you move up the scale. In other words, the hazard becomes much more severe as you approach higher levels on the scale.
Level 1 - Low
The Low danger level is the lowest on the avalanche danger scale. Under this level, avalanche conditions are generally considered safe, but it's still possible to trigger avalanches on isolated features and in extreme terrain. Natural and human-triggered avalanches are unlikely.
While some may interpret "low" as "green means go," caution is still necessary when entering steep slopes. Be watchful for signs of instability and remain vigilant around terrain traps or in extreme terrain.
Level 2 - Moderate
Moderate is the second level on the avalanche danger scale. Under Moderate danger, there are increased avalanche conditions on specific terrain features. These areas of concern should be identified, and terrain and snow should be evaluated carefully. Natural avalanches are unlikely, but human-triggered avalanches are possible. Small avalanches may occur in specific areas, and large avalanches in isolated regions.
Decision-making under Moderate avalanche danger can be challenging since the danger exists but is limited in distribution, likelihood, and potential damage. Pay attention to specific avalanche problem areas in the forecast to avoid slopes where avalanches could be triggered.
Level 3 - Considerable
Considerable danger is also the second level on the avalanche danger scale. Under Considerable danger, there are heightened avalanche conditions on specific terrain features. These areas of concern should be identified, and terrain and snow should be evaluated carefully. Natural avalanches are unlikely, but human-triggered avalanches are possible. Small avalanches may occur in specific areas, and large avalanches in isolated regions.
Decision-making under Considerable avalanche danger can be challenging since the danger exists but is limited in distribution, likelihood, and potential damage. Pay attention to specific avalanche problem areas in the forecast to avoid slopes where avalanches could be triggered.
Level 4 - High
High is the second-highest level on the avalanche danger scale. During periods of High danger, avalanche conditions are very dangerous, and travel in avalanche terrain is not recommended. Natural avalanches are likely, and human-triggered avalanches are very likely. Large avalanches are expected in many areas, or very large avalanches in specific areas.
Under High danger, a natural avalanche cycle is underway, or conditions are very sensitive to human triggers on either end of a cycle. Avalanche terrain should be avoided. Use extra caution when traveling in simple terrain and stick to very mellow slopes or dense trees that are free of overhead hazards. Remember that you don't have to be on a steep slope to be struck by an avalanche that starts from above you. Small terrain features that may seem benign, such as road cuts or creek beds, could be dangerous.
Level 5 - Extreme
Extreme is the highest level on the avalanche danger scale, indicating extraordinarily dangerous avalanche conditions. Natural and human-triggered avalanches are certain, and very large avalanches are expected in many areas. All avalanche terrain should be avoided.
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 destroying structures, or avalanches running across valleys and up the other side are all possibilities under Extreme danger.