Avalanche Hazard Bulletin (AHB)

ByEduardo Azcona

Just when it seemed that the winter was coming to an end with hardly any significant snowfall, the storm of the past few days has once again left thicknesses that we had not seen in many moments of the winter, now a meteorological spring.

And of course, this new snow accumulated on top of the existing snow, the moderate southerly winds and a changing snow level which has caused the snowpack to become wetter, has led to an increase in the danger of avalanches. In fact, on Friday 11 March there were already areas of the Pyrenees with a grade 4 avalanche danger.

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How to read the Avalanche Hazard Bulletin (AHB)

Although this is not the first time we have talked about the avalanche danger bulletin, not long ago we published an article on weather warnings, alerts and GAPsAt that time, we were only looking at the final number. Well, knowing the grade is important, but if we only look at that, we are ignoring a lot of very valuable information.

To understand this, let's look at the BPA published by AEMET of this past Sunday 13th March for the Navarre and Aragonese Pyrenees.

As can be seen, it consists of a graphic part and four sections for reading.

  • Mantle condition and recent observations
  • Evolution of the mantle and avalanche danger for the 20th.
  • Weather forecast for the day XX.
  • Developments for the 20th day.

Let's take a closer look at the sections that make up the graphic part of the bulletin:

Massive:

This first section does not need much explanation, it refers to the area for which the avalanche hazard is described. Here we have to take into account that very large areas are evaluated and that for a particular area this may not correspond. It is our duty as mountaineers to judge the actual danger once on site, if it is greater than the predicted one.

Danger of avalanches:

Through the first symbol the avalanche danger is assessed using a five-level scale commonly used in all European countries.

  • Green = Level 1 (Weak)
  • Yellow = Level 2 (Limited)
  • Orange = Level 3 (Merit)
  • Red = Level 4 (Strong)
  • Red/Black = Level 5 (Very strong)
European Avalanche Danger Scale.

With the second symbol the degree of avalanche danger is indicated depending on the altitude. In the BPA of the example, in the case of the Gállego, the avalanche danger is strong (grade 4) above 2,200 metres and notable below this altitude.

Avalanche problem, elevation, orientation and size:

Looking again at the example bulletin, we see that there are two problems: blowing snow and fresh snow. A detailed description of these problems can be found in the document "Snow and snow problems".Typical avalanche problems"published by AEMET and approved by EAWS.

This is followed by a description of how the hazard is distributed, the elevation at which it appears, the orientation and the size of the avalanche defined by its capacity to cause damage.

Looking at the example bulletin for the case of the Gállego, we have that the danger of blowing snow is expected above 2,200 metres on North, East and South slopes, with an avalanche size of type 3. In the case of the danger of fresh snow, this can occur at any elevation, in any orientation, with an avalanche size of type 2.

Finally, the snow cover on the north and south face is defined, 1400 metres in both cases for the example of the Gállego.

Where to consult the Avalanche Danger Bulletin (BPA) for Spain and the Pyrenees: