The WSL Institute for Snow and Avalanche Research SLF uses the avalanche bulletin and various supporting products to inform the public about the current snow and avalanche situation in the Swiss Alps and in the Jura. The content of the avalanche bulletin is designed as an informed warning. It is published twice daily in winter and primarily contains a forecast of the avalanche danger in the Swiss Alps, Liechtenstein and, when there is sufficient snow cover, the Jura as well.
Additional information about the weather's effects in terms of avalanches and the condition of the snowpack gives users a clearer picture of the current situation and provides a basis for making their own assessment, as the information provided the avalanche bulletin cannot take the place of their own local field assessment. The information contained in the bulletin is too general for this because of the limited data it is based on.
The avalanche bulletin is aimed at all those who are exposed to potential avalanche danger in the mountains in winter, whether in a professional or recreational capacity, and those who are responsible for the safety of others. This includes members of the following groups:
- Avalanche services and committees of the communal authorities and civil engineering offices, as well as the safety services of the mountain railway/cableway operators
- The police and rescue services and the armed forces
- Mountain guides, snow sports instructors and backcountry tour guides
- Residents of mountain villages
- Winter sports enthusiasts away from secured ski runs, such as freeriders, backcountry skiers, snowshoe hikers, mountaineers and ice climbers
When the snow and avalanche situation is relatively favourable, the avalanche bulletin will mainly contain information for winter sports enthusiasts. From danger level 3 (considerable) upwards, information for the avalanche warning services is issued more frequently. At the very high avalanche danger level (level 5), when snow sports are hardly possible anyway, the avalanche bulletin will mainly contain information for the avalanche warning services.
Structure of the avalanche bulletin
In high season the structure of the avalanche bulletin is based on the 'information pyramid', i.e. the most important comes first (danger level), followed by details of the locations where the danger is particularly significant and then the avalanche problem, the danger description and information about the snowpack and the weather. Finally, measured data can also be displayed. The use of standardised terminology aids both understanding and implementation.
Individual parts of the avalanche bulletin
In winter, the avalanche bulletin consists of a zoomable hazard map, including a hazard description and a section headed "Snowpack and weather". A description of this content is given below. Less information is usually provided in low season and when there is a lot of snowfall in summer. In those periods the avalanche bulletin is published in plain text format, with no hazard map and usually no danger levels either.
The flash section is a very brief pointer to the key aspects of the avalanche situation.
The avalanche bulletin's most important component is its avalanche danger forecast. This comprises the following aspects:
The five-level European avalanche danger scale is used. This groups together areas with the same danger level, the same avalanche problems and the same particularly affected aspects and altitudes in cases where a joint danger description is possible. The danger levels and, if possible, also the particularly affected aspects and altitudes are presented on a hazard map.
If the danger level is likely to change during the day, the danger level is normally based on the situation in the morning. In typical springtime conditions, two maps show both the more favourable morning situation (dry-snow avalanches) and the more unfavourable situation in the afternoon (wet-snow avalanches as the day progresses) (double map).
Avalanche prone locations
In most cases, the locations where the danger is particularly significant are described in plain text format and, if possible, also illustrated in graphical form.
Often the avalanche danger is classified as falling under one or more of the five 'typical avalanche problems'. Normally the avalanche bulletin assigns the situation to one or more of these categories. The following problems are distinguished:
- New-fallen snow (considerable snowfall in recent days)
- Snowdrifts (snow transported by the wind)
- Old snow (weak layer(s) prone to triggering within the old snowpack)
- Wet snow, in which case a distinction is made between:
- Wet-snow avalanches
- Wet-snow avalanches as the day progresses
- Gliding avalanches
If there is no significant avalanche problem (often level 1 in the case of low avalanche danger), none will be given.
There is a specific danger description for each danger region marked on the map. Among other aspects, it may describe the likelihood and size of the anticipated avalanches or give information on the bonding of the snowpack. If necessary, remarks or recommendations for individual groups of users will also be provided.
Additional danger: If necessary, the description of the main danger can be followed by an additional danger, for example both dry-snow and wet-snow avalanches being expected. Apart from the absence of a graphical representation of the particularly affected aspects and altitudes, the additional danger is written up in the same way as the main danger. In double maps, each hazard map will only show the main danger.
Remarks: The 'Remarks' field at the end of the danger description can be used to pass on additional information to users, e.g. about an additional source of danger or a particularly uncertain weather outlook.
This part of the avalanche bulletin is only updated in the evening and is made up of the following parts:
The snowpack is the principal determining factor in the formation of avalanches. A general description is given, usually covering both the layering of the snow and the stability. While snow layering is determined by the stratification of the snowpack and the structural properties of its individual layers (grain shape and size, hardness). The snow layering is crucial to the stability. This section also describes the observed avalanche activity if applicable.
The weather affects the snowpack and therefore how the avalanche danger develops. Key factors for avalanche danger such as fresh-fallen snow (or rain), air temperature and wind are set out. The description begins with a review of the weather conditions in the immediately preceding period (covering at least the current day), which is followed by the forecast for the validity period of the avalanche bulletin. The danger assessment is formulated on the basis of this weather forecast. If locally the weather until that time has deviated from that described or the actual weather pattern differs from the forecast, this may also affect the anticipated avalanche danger.
Based on the medium-term weather forecast, the indicative outlook assesses the general pattern of the avalanche danger for the two days immediately after the bulletin's validity period.
In addition to the danger assessment, the avalanche warning service often also issues recommendations. These are generically defined in the danger scale but can also form part of the danger description and be addressed to individual groups of users. These are of course recommendations, not rules. After making his or her own evaluation of the situation on site, the decision as to how to respond to the avalanche danger and what risks he or she is willing to take rests with the individual.
Recommendations for transport routes and settlements are sent to those responsible for the cantonal and communal avalanche warning services, roads, railways, mountain railways/cableways and secured ski runs. 'Safety measures' include avalanche blasting, blocking transport routes, slopes or ski runs, or, in particularly critical conditions, searching for shelters or evacuation routes from individual locations or entire communal areas. The safety measures to be taken in a specific situation vary from case to case and are determined by those responsible for public safety.
Recommendations for individuals outside secured areas are especially aimed at snow sports enthusiasts away from the ski runs, for example freeriders, backcountry skiers, snowshoe hikers, ice climbers and mountaineers. 'Conditions' refers solely to avalanche danger, not to the amount of snow, snow conditions (powder snow, breakable crust) or weather conditions (fog, storm) even if these may sometimes pose a potential danger. 'Experience' always means experience in assessing the avalanche danger. This is best acquired under expert guidance, e.g. on avalanche courses or on guided backcountry tours or off-piste activities.