It was overcast with heavy drifting snow in Vassdalen on that fateful March day in 1986, almost exactly 40 years ago. The temperature was around minus ten degrees Celsius. 31 engineer soldiers were digging their way forward through deep, loose snow to help tracked vehicles through the narrow valley. At 13:00, a massive slab avalanche released from the mountain Store Balak. It buried the entire platoon and three tracked vehicles. 16 soldiers died. Vassdalen became the Armed Forces’ worst peacetime accident.

The knowledge about the danger was there. Avalanche experts, including an NGI researcher, had tried to issue warnings. The experts had assessed the conditions and issued a clear warning: they must not enter the valley. But the message was lost in the system. Called in to a telephone exchange, forwarded by fax, and stopped there.

“The knowledge base was correct. They didn’t make any professional errors. But the messages didn’t reach the right person in time,” says Kjersti Gisnås, avalanche researcher at NGI.

The Maps That Followed

The commission of inquiry that delivered its report in 1987 demanded concrete measures: better training of officers, revised exercise maps, strengthened avalanche expertise during exercises, more systematic safety work, and improved reporting routines. Already the following year, the Armed Forces commissioned NGI to map military exercise areas across the country. Experts went into the field, identified potential avalanche paths, and delivered maps the Armed Forces could use in their planning. It was manual, time-consuming, and expensive – and it did not provide nationwide coverage. But it was the start of a systematic approach to avalanche hazard that did not exist in the Norwegian Armed Forces before 1986.

In parallel, NGI continued the long-term research that would lay the foundation for everything that followed. In 1973, the institute established the research station Fonnbu at Strynefjellet. Shortly after came Ryggfonn, a full-scale test site where researchers trigger controlled avalanches to measure velocity, impact forces, and run-out distances. This type of infrastructure exists in very few places worldwide.

In 2021, one of the largest avalanches ever triggered at Ryggfonn was captured on film from a drone on the opposite mountainside, from several angles simultaneously.

“You get a sense of the tremendous forces involved. Nobody wants to be out skiing when an avalanche like that comes, because then you’re done,” says Gisnås.

The data from Ryggfonn is not theory. It is reality, measured and repeated at full scale for more than 50 years. And it is precisely this data that ensures the calculations in the next generation of avalanche maps can be trusted.

Three and a Half Million Avalanches in a Single Run

For the new avalanche susceptibility maps produced by NGI on commission from NVE (the Norwegian Water Resources and Energy Directorate) in 2023, NGI developed a new model they call NAKSIN – New Susceptibility Maps for Snow Avalanches in Norway. NAKSIN is also used as a decision-support tool in land-use planning and building permit processing in Norwegian municipalities.

“In the model, we build in all the logic you apply in a standard avalanche assessment. We identify all potential areas where avalanches can release, divide them into individual release zones, run a climate analysis using a so-called Monte Carlo simulation, and calculate the probability that an avalanche releases and the associated fracture height. Then we run a fully dynamic run-out model for a total of three and a half million avalanches,” explains Dieter Issler, snow avalanche researcher at NGI and lead developer of the NAKSIN model.

Monte Carlo simulation is fundamentally a simple method: instead of calculating one “average” outcome, you feed the computer all known variables (temperature, precipitation, wind conditions, snow density) and let it simulate thousands of different combinations.

“The result is not a single answer, but a probability picture: how often is this slope unstable, and under what conditions does it release?” Issler explains.

Based on 65 years of climate data, various realistic combinations of the aforementioned variables are assembled across approximately 2.5 million synthetic days. These reflect everything from mild winters with little snow to extreme years with heavy, layered snowpacks. The stability of each release zone is then tested in NAKSIN to determine the probability of avalanche release.

What makes the model particularly valuable is its flexibility, allowing it to calculate susceptibility zones for less rare events than those used for building development in Norway.

“This functionality is useful for the Armed Forces. Where the old hazard maps for the Armed Forces were drawn manually by experts who went out in the field, you can now adapt the NAKSIN model and get realistic hazard maps covering the entire country,” says Issler.

The old maps were also static, explains Kjersti Gisnås:

“With the old susceptibility maps, you could never make any estimate of future climate, because climate was not taken into account. Now we can run our logic with climate projections instead of historical climate data, and see how we expect the snow avalanche hazard to change in the future,” says Gisnås.

The maps can be delivered in two versions – with and without forest effect – so that avalanche hazard can be assessed both in the current situation and under changed forest cover.

GPS on the Snowmobile

The NAKSIN maps were used operationally by the Armed Forces for the first time during the 2024 winter exercise Nordic Response. In 2026, new maps are being calculated that account for forest cover and also include parts of Finland and Sweden, so that all participants can share the same basis for decision-making. The contrast to the situation in Vassdalen on 5 March 1986 is stark.

“They can have the maps on a GPS on the snowmobile and see in real time where they are in relation to avalanche-prone terrain, and combine this with clear, predefined plans about where they can move under different weather conditions,” says Gisnås.

But it is not just the maps that have changed. During major winter exercises, the Armed Forces now deploy dedicated avalanche teams that assess conditions in the exercise areas. The Armed Forces Winter School drives out every morning to dig snow profiles, test stability, and measure the snowpack at various locations. The results are reported before the day’s activities begin, and the exercise command adjusts plans according to conditions.

“This is an operational safety culture that did not exist in 1986, and it captures local variations that supplement the regional forecasts from Varsom.no, and is used in combination with the avalanche hazard maps,” explains Staff Sergeant Torgeir Storflor at the Armed Forces Winter School.

From Support Element to Professional Sparring Partner

The collaboration between NGI and the Armed Forces has not been limited to maps and technology, but also to roles. One of the key lessons from the Vassdalen disaster was precisely to improve the integration between the Armed Forces and civilian specialist communities as independent expertise.

“Previously, we assisted at the Cold Response exercises. We did that perhaps up until five years ago. Now the Armed Forces’ avalanche team is just as well-trained as we are on the practical side out in the field. They have extensive experience in making field assessments and carrying out practical risk evaluations,” says Gisnås.

But it is not just the tools that have changed since 1986; the culture has changed as well. Where a fax message disappeared in the system that March day in Vassdalen, there are today established structures for risk assessment, daily field observations, and clear decision-making lines that are built into the way the Armed Forces operate in the winter mountains.

“Today, we have good tools and routines that enable us to plan safely and make good decisions in the field. The NAKSIN maps give us a foundation we have never had before, and the collaboration with NGI has been crucial to getting us to where we are today. But it is not just about technology. It is about the fact that we today have a safety culture where the right information reaches the right person at the right time. Our soldiers must be able to trust that their safety is taken care of when they are out in the winter mountains,” states Torgeir Storflor at the Armed Forces Winter School.