Recent avalanches showcase a trigger-sensitive snowpack, at least in certain regions. Two dangers threaten currently:
- Fresh snowdrifts
- Trigger-sensitive old snowpack regionally
Freshly generated snowdrifts are always easily triggered when unbonded fresh snow is transported at low temperatures. Both coordinates are currently fulfilled. It is very cold. There were strong-velocity winds in the foehn lanes on 13.12. That is the moment when even minimum additional loading is sufficient to trigger the snowdrifts in steep terrain. (Powder and decomposed snow crystals form a weak layer. The latter form will incrementally bond with the drifted masses in the coming few days.)
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| Particularly along the Main Alpine Ridge, huge snow transport was observed today. Zillertal Alps.(photo: 13.02.2018) |
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| Avalanche accident below the Spannagel descent in Hintertux Glacier ski area on 13.12.2018. The north-facing slope is very steep at 2500 m. Immediately prior to the avalanche, strong winds transported lots of snow onto this slope. Snowpack analysis carried out today, 13.12, showed that the fresh snowdrifts were prone to triggering. The primary weak layer was powder snow covered with fresh snow. (Secondarily, the layers of faceted snow crystals referred to above surely fractured from place to place.) The avalanche is about 100 m long and 200 m wide. |
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| The police helicopter transports the dog guide down to the valley after the accident (photo: 13.12.2018) |
Weak layers more deeply embedded inside the snowpack are currently a risk, particularly in the regions along the Main Alpine Ridge between about 2300 and 2900 m, especially on shady slopes. In the vicinity of (rain)crusts there are frequently loose, faceted snow crystals. These weak layers are not evident area-wide as a rule.
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| Small-scale, covered, naturally triggered slab avalanches in the Stubai Alps at about 2400 m (center of photo) indicate heightened trigger-sensitivity of the snowpack (photo: 12.12.2018) |
Below the Pfaffenbichl, a slab was released while there were people on the descent. The fracture was located at 2300 m on an extremely steep north-facing slope. The fracture depth varied between 20 cm and 160 cm. Snowdrift accumulations from the weekend deposited atop faceted weak layers were triggered.
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| Avalanche accident from 12.12.2018 below the Pfaffenbichl (photo: 13.12.2018) |
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| View from upper zone of the avalanche fracture towards avalanche deposit. Pfaffenbichl (photo: 13.12.2018) |
Snow profile of the upper avalanche can be viewed - as usual - here.
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There is also positive news:
Apart from the Main Alpine Ridge, older snowdrift accumulations have consolidated well in general. The main problem there currently: the predominantly small-sized, fresh snowdrift accumulations which are frequently found in shady terrain adjacent to ridgelines.
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| Predominantly good conditions on the Arlberg (photo: 13.12.2018) |
Wherever snowfall has been heavy, we observe increasingly frequent glide-snow releases. These avalanches are usually small-sized, sometimes medium-sized.
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| Numerous glide-snow releases in Lechtal (photo: 13.12.2018) |
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| Glide cracks indicate potential threats. We recommend avoiding all spots below such cracks. (photo: 12.12.2018) |
Outlook for the weekend:
The strong winds today (13.12) are expected to taper off during the night tonight. Next to no further snowdrift accumulation is anticipated. However, the snowdrift accumulations which formed today are expected to remain prone to triggering for several days due to the low temperatures. The old snow problem which occurs regionally between about 2300 and 2900 m in the above-mentioned regions seems to be the major problem. Weather change is expected on Sunday, 16.12.
(What we need to keep an eye on: Danger Pattern 4 (cold on warm) particularly on sun-bathed slopes far removed from the regions where there has been the most snowfall.)
