Nock Mountains

Combined wet and persistent weak layer problem

Combined wet and persistent weak layer problem

Above the tree line, the avalanche risk is moderate. Below this, the danger is low in the morning and quickly increases to level 2, moderate, during daytime changes. As the snowpack softens, spontaneous and human-triggered wet snow avalanches can occur in all aspects, which usually remain medium in size but can occasionally accumulate large amounts of wet snow. In addition, medium to large dry slab avalanches can be triggered by individuals in a few places, especially on shady slopes above the tree line. Danger areas are difficult to recognise away from obvious pillows of wind drifted snow, which is why caution and a defensive choice of route is still advised. This applies in particular in the area of the tree line and in forest aisles. Loose snow slides can easily tear through the persistent weak layer and reach dangerous sizes. There is little sliding snow activity.

A stable melt-freeze crust forms overnight, at least on the steep sunny slopes, but this softens quickly and only allows a short firn phase. The upper part of the snowpack is then formed by several slabted layers of felt and wind slab of varying hardness (and therefore prone to triggering), which in turn lies on an unfavourable base of faceted crystals and deep rime. Fractures primarily occur between the last snow packs, but can also break through to the old snow foundation, which is usually completely transformed. In places with little snow and in deep or sunny slopes, the snowpack becomes isothermal during the day changes. The snow depth has now reached average values in some places, but is already losing mass again.

Saturday night will be clear and windless, with zero-degree levels at around 2800 metres. On Saturday, the sun will once again shine frequently, with a few thin, high patches of cloud moving across the sky at times and not disturbing visibility. The wind will blow weakly to moderately from different directions, reaching a maximum of 40 to 50 km/h at high altitudes. It will remain very mild, reaching around 6 degrees at 2000 metres and 1 degree at 3000 metres.

The persistent weak layer problem in combination with moisture penetrating into ever higher and shadier areas continues to create a tense situation that is difficult to calculate in the terrain. Outgoing longwave radiation on Sunday night is severely limited.