Snowpack forfeits firmness due to water seepage
As a result of above-average temperatures and solar radiation, a progressive wetness of the snowpack can be observed. This applies particularly to sunny slopes below 2600 m, shady slopes below 1800 m.
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| Indicative of the current situation: steadily rising temperatures over the last few days. Snowpack-surface temperature is subject to daytime cycle, reaching 0 degrees by late morning. Intensive solar radiation. |
The water seeping into the snowpack reaches the ground to an increasing degree. The snowpack is then isotherm, thus, the temperature from ground level to the surface = 0°C.
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| Snow profile Jöchlspitze, Ausserfern: isotherm snowpack, almost 1700 m in 6° steep south-facing terrain |
The lower the altitude, also the steeper and more sun-exposed the slope, the more massive the process of wetness. The situation is correspondingly better at higher altitudes.
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| Yesterday, 21 February, the snowpack in the Karwendel, 38° gradient south-facing slope at 2680 m only superficially wet. There are still reserves of cold there, thus, the snowpack will hold out longer before becoming destabilized. |
Glide-snow avalanches can release at any time. Exposed transportation routes, hiking trails, forest roads at risk
More water at the interface between ground and snowpack means a lesser amount of friction with the smooth ground, thus, heightened likelihood of the entire snowpack gliding over the ground. For that reason we have seen increasingly frequent glide-snow avalanches throughout the land since yesterday (20.02). What these avalanches have in common: the gliding process at ground level continues even during the nocturnal hours when the snowpack surface freezes so that it can bear loads. That means, glide-snow avalanches can release even in the middle of the night! In regions where snowfall has been heaviest, e.g. East Tirol, these avalanches can grow to large size and place not only exposed transportation routes at risk, but also hiking trails and forest roads.
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| Glide-snow avalanche near an ascent track. Defereggental (photo: 21.02.2021) |
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| Endangerment of ascents and descents in southern East Tirol due to glide-snow starting zones, not visible from the valley. Extremely treacherous! (photo: 21.02.2021) |
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| Fresh glide-snow avalanche due to inceased water seepage into the snowpack, Schmirntal, western Tux Alps. (photo: 22.02.2021) |
A thoroughly wet snowpack also furthers slab avalanche releases. Heed daytime cycle.
During the last few days, increasingly frequent wet loose-snow avalanches have been observed in sunny, extremely steep terrain. Below the Brandjoch in the Northern Massif, for example, a winter sports enthusiast was overtaken by a loose-snow avalanche, swept along and injured. The danger of wet naturally triggered loose-snow avalanches has subsided in the interim. Artificial triggering by skiers is still conceivable, however, if the snowpack surface is sufficiently wet.
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| Loosely-packed slides visible where the arrows point. These were triggered naturally. One person was swept along. Brandjoch, SE, extremely steep (photo: 20.02.2021) |
Only isolated danger zones remaining for dry-snow slab avalanches
As mentioned in the
last blog, there are only few isolated danger zones remaining for dry-snow slab avalanches. These can be triggered most easily in extremely steep, shady terrain wherever the snow is shallow or in transitions from shallow to deep snow. Instructive for this situation is an avalanche release in Wildlahnertal, near Hohe Warte in the western Tux Alps where a descending skier was caught in a slab avalanche, swept along and partly buried (uninjured).
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| Avalanche release in rocky, extremely steep terrain. Hohe Warte (photo: 21.02.2021) |
Summary:
Apart from the glide-snow problem, favourable conditions reign widespread in the early morning hours after nights of clear skies. During the daytime, danger of wet-snow and glide-snow avalanches increases. Caution: avalanches can plummet all the way to the valley floor.
