How an avalanche forecast is produced

Avalanche forecasts exist in most mountainous countries with winter tourism and, even though they may look slightly different, the presentation method is basically the same in all countries. To be able to come up with predictions that are as accurate as possible, we use local observers who provide us with qualified information.

We are also happy to accept additional information from public who are out in the mountains. This information might be about the weather, avalanche activity and what the snow or the skiing or snowmobiling is like. Take a picture on your phone, upload the image on our Facebook page Lavinprognoser or on our Instagram, specify the date and the place where the picture was taken and what it shows. Our hashtag is #lavinprognoser.

Forecasting

The forecasts are issued by the Swedish Environmental Protection Agency office in Östersund. In order to produce our forecasts, we use various types of information that are collected and analysed. Local avalanche experts in the mountain region carry out observation tours to assess the avalanche danger and weather data is collected from automatic stations, forecasting models and directly from SMHI meteorologists.

Every day in the winter season the avalanche forecasters analyse and filter the collected data in order to write an avalanche forecast. The forecast contain information about the avalanche danger the upcoming 48 hours. Swedish avalanche forecasts use the International avalanche danger scale, ranging from Low to Extreme to communicate danger levels.

The forecast also contain information about the primary avalanche problem, including avalanche character, aspects and elevation bands that the problem concerns and the estimated likelihood/ consequence of triggering avalanches. Finally the forecast give you the forecasters' comments, risk management and terrain travel advice, detailed information about snowpack and mountain weather
In the end, the avalanche forecast is the best possible prediction of how the avalanche danger will develop in the next 48 hours.

Avalanche activity

Recent avalanches are the most obvious sign that the snowpack is unstable and that avalanche danger exists in at least some places of the terrain. Other obvious clues of avalanche danger and unstable snow are whumpf sounds, also called collapses, and shooting cracks forming in the upper part of the snowpack. These are good signs of avalanche danger that are easy to interpret and should be taken seriously.

The snowpack


Slab avalanches occur when the structure of the snowpack is unstable. The stability/ instability of the snow will always be the main question for predicting avalanches. Unfortunately, it is very difficult to give a certain answer. There will always remain some uncertainty. This is primarily due to the fact that the snowpack is not exactly the same over the entire terrain, not even on a single slope.

The spatial variability of the snowpack creates difficulties for avalanche forecasters and mountain travellers alike. A great deal of training and methodical work is therefore required to interpret the information and to come up with conclusions about stability and avalanche danger. Avalanche professionals dig down in the snow, test the stability and look for weak layers under the snow that form slabs.

Weather

Weather information is often regarded as the least reliable sign of avalanche danger. Even so, everyone working with avalanche safety monitors their weather stations and weather forecasts intensely during the winter – so why do we do that?
First of all, it is because some weather patterns lead to an obvious increase of avalanche danger and the majority of all natural avalanches occur at such times. Perhaps even more important is the fact that the majority of human triggered avalanches occur the days after Storms, heavy snowfall or rain combined with strong winds will bring significant load on the existing snowpack. Especially on lee slopes where the wind often loads up to 5 times more than the accumulated snowfall and creates wind slabs.



Secondly, the weather is the creator of the snowpack and the snowpack structure and will determine how the avalanche danger will vary in different places in the terrain. The current snowpack is the result of the historic weather up until now throughout the winter.

However, keeping track of the weather and what it means for the snowpack for the entire winter is a very difficult task for most people – even for computer software. For that reason, we cannot solely rely on the weather data and models. It needs to be combined with gathering and processing of actual data from the mountains. Weather observations and snow observations complement each other.

Last but not least, the weather forecast is our primary way of predicting the future. Tomorrow's snowpack and avalanche danger depends on today's snowpack plus the changes that the weather brings for tomorrow. Without good weather forecasts it is difficult to produce good avalanche forecasts.

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