A typhoon brought unseasonably cold weather - the mass casualty accident of middle-aged and elderly people on Mt. Tateyama on October 8, 1989

Early October 1989, Mt. Tateyama mountain range, Northern Alps. A party of 10 people became trapped one by one in the intensifying snowstorm. Why did the snowstorm cause so many people to be stranded, even though it was still early October? We will use the Japan Meteorological Agency's JRA-55 (Japan Meteorological Agency's 55-year long-term reanalysis data) to confirm the situation at the time.

Dear YamaKei Online readers, this is Oya, a mountain disaster prevention weather forecaster. In September, the mountains begin to transition to autumn earlier than the foothills, and sometimes they start to show the appearance of winter mountains. Snowfall in September is quite common in the mountains of Hokkaido, and it is not unusual in the Northern Alps either. This period is similar to spring in that there is a large gap in weather between the foothills and the mountains, making it a time when many accidents occur due to judgments based on the weather in the foothills.

This time, it was a mass accident similar to the Tomuraushi accident in July 2009, and it sounded an alarm to the world about the way middle-aged and elderly people hike and the dangers of hypothermia. 1989 Mt. Tateyama mass casualty accident I would like to explain about this.

Summary of the mass casualty incident involving middle-aged and elderly people on Mt. Tateyama on October 8, 1989

First, let's review the outline of this accident.

For details of this accident, see Documentary Weather Disaster It is published in (Author: Osamu Haneda, YamaKei Bunko), so if you are interested, please read it.

In this book, the Mt. Tateyama disaster is titled "Autumn Pacific Coast Low Pressure System Mt. Tateyama - Frozen to Death," but in reality it was a disaster caused by Typhoon 25, which passed to the east of Japan and brought with it unseasonably cold air.

Furthermore, although various media have explained it as an 'accident caused by a winter pressure pattern,' as stated in this column article, it should be noted that the surface weather map does not actually show a typical winter pressure pattern with strong cold air.

A strong cold front that was not predicted by surface weather charts

The main focus of this case is, Why did the weather become so bad? I think it comes down to this. Indeed, it seems that bad weather was forecasted on the day of the accident, but it was clear in the early morning. This undoubtedly led to complacency.

Furthermore, let's analyze what the weather conditions were like on that day using data from the Japan Meteorological Agency.JRA-55We decided to analyze it using the Japan Meteorological Agency's 55-year long-term reanalysis data (JRA-55). JRA-55 is a reanalysis of weather conditions, including the upper atmosphere, from 1958, when weather analysis technology was still insufficient, over a span of 55 years using current analysis techniques. I believe this is probably the first time it has been used to analyze past mountain accidents.

First, the surface weather map. The 6th article was about Mt. Shirouma disaster on October 7, 2006. When compared to the surface weather map at that time, the difference is obvious.

In comparison, in the 2006 Mt. Shirouma disaster, there were six isobars crossing from Kyushu to Kanto, whereas in the 1989 Mt. Tateyama disaster there were only half as many, three.

It is common knowledge among those in the know that 'when a typhoon or southern coastal low-pressure system moves north along the Pacific side, the mountains on the Sea of Japan side become turbulent due to cold air.' However, it is difficult to read the descent of strong cold air from this surface weather map alone.

Furthermore, I am concerned that if Mt. Tateyama disaster is labeled as a "disaster caused by winter atmospheric pressure patterns," people will look only at surface weather charts and think, "It's okay because the isobars are spaced far apart," or "It's okay because the typhoon is moving away."

So what was the weather like around Mount Masago on Mt. Tateyama ?

Next, based on the JRA-55 data, we analyzed the weather conditions at 700 hPa (approximately 3000 meters above sea level). Figure 4 The descent of cold air from the north is clear, and near Masago-dake, the temperature at 15:00 on October 8 was -6°C with a west-northwest wind at 15 m/s.

The perceived temperature drops by 1°C for every 1 m/s of wind speed, so near Masago-dake, the perceived temperature was -21°C (=-6°C-15°C), making it impossible to survive with light hiking gear. Incidentally, the two survivors had cold-weather gear, which made the difference between life and death.

More attention is Figure 5 This shows the temperature changes over time near Masago-dake. Compared to 900 hPa (approximately 1000m above sea level) and 800 hPa (2000m), the temperature at 700 hPa, which corresponds to the elevation near Masago-dake, dropped sharply by more than 10°C from the previous night to 15:00. According to the observation data from the Toyama Local Meteorological Observatory at 15:00 on October 8, the temperature was 13.2°C, with a south wind at 1.9 m/s and cloudy skies, indicating that the weather near Masago-dake was much harsher than could be imagined from the weather at the foothills.

Why did such unseasonably strong cold air come in?

If we stop the analysis here, we will not be able to grasp the true cause. We proceeded with the analysis using JRA-55 data. Figure 6 This is a wide-area analysis chart of the 500hPa (approximately 5500m above ground) contour line (equivalent to the isobaric line on a surface weather map), temperature, and wind at 3 PM on October 8.

There is a Pacific high-pressure system to the east of Japan, and the typhoon strengthened the northward extension of the Pacific high-pressure system. As a result, the meandering of the westerlies around Japan increased, and unseasonably strong cold air from the north moved south... This scenario can be read from this 500hPa weather chart.

I believe that the true cause of the large-scale disaster on Mt. Tateyama was the occurrence of such a large-scale phenomenon. Therefore, I think we should be cautious in the future when a similar atmospheric pressure pattern is expected.

By identifying the true causes of past mountaineering accidents, we hope to reduce the recurrence of similar accidents as much as possible. We intend to continue our research activities in Professor Jun Yoshino's laboratory at Gifu University.