Weather review
Variable conditions. That is the best way to describe this last week in North Tirol, particularly in the western and northern regions, which got some fresh snow, which was heaviest on 9 and 10 December, when 20-40 cm, locally as much as 50 cm of fresh snow was registered. Along the Main Alpine Ridge there was about 15 cm, in East Tirol only a bit.
A strong, sometimes stormy wind was blowing from varying directions above the timberline.
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| Galzig weather station: indicative of the western and northern regions of North Tirol where recent snowfall has been heaviest. Upper graph: precipitation and fresh snow. Lower graph: temperatures drifting closer then further apart, and melting points, bear witness to the variable weather conditions. Windy. |
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| Kals weather station: striking in East Tirol, esp. on 10.12. Strong winds. |
Snowpack development
As is well known, weather conditions have immediate impact on the snowpack, and the snowpack determines avalanche danger. What counts in the current situation was the high-pressure zone until the weekend. At low temperatures and during nights of clear skies, the snow crystals near the snowpack surface transformed to loosely-packed, generally decomposed, faceted crystals. This is a serious weak layer for the snowdrifts deposited on top of it. The heightened proneness to triggering of the snowpack in these areas was corroborated by increased glide-cracks and also by generally small-sized avalanches.
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| Glide cracks indicate heightened trigger-sensitivity of the snowpack. High Zillertal (photo: 07.12.2019) |
On sunny slopes, on the other hand, the beginnings of an expansively metamorphosed (faceted) layer beneath the melt-freeze crusts on the surface have been observed above approximately 1900 m, even more above 2200 m. Stability tests currently show a still minor tendency towards fracture propagation. In any case, this is a process which needs to be watched attentively.
Here is a Video from the Arlberg region, recorded on 10.12.2019. During a stability test, a sensitive weak layer beneath a melt-freeze crust was consciously triggered, and subsequently, the fracture propagation observed.
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| Also indicative: at high altitudes (prior to recent snowfall) and also in the regions where there was little precipitation, an old snowpack surface manifesting high wind impact. Stubai Alps (photo: 07.12.2019) |
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| In the interim also a winter portrait: Kitzbühel Alps. But be careful: beneath the shallow snowpack, stones threaten. (photo: 11.12.2019) |
Avalanche danger development and avalanche activity
The longish period of low avalanche danger has passed for the moment.
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| In East Tirol, mostly (very) favourable conditions for winter sports enthusiasts prevailed over the weekend. Karlskopf (photo: 07.12.2019) |
Snowdrift accumulations could be disturbed with startling ease. There were also small, naturally triggered slab avalanches, and remote triggerings, observers have reported.
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| Fresh slab, near a ridgeline on Geigenkamm (photo: 10.12.2019) |
Gliding-snow activity continues to be minor. In the western regions where snowfall amounts are increasing these releases could increase somewhat, especially in case of further snowfall.
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| Ellipse: fresh glide-snow avalanche. Arrow: older glide-snow avalanches and glide cracks. Tux Alps. (photo: 12.12.2019) |
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| Encircled: recently released glide-snow avalanche (photo: 08.12.2019) |
During this last week, three avalanche releases involving persons have been reported. On 6 December a person was injured in shady ridgeline terrain on a descent from Nederkogel in the Ötztal Alps, swept along by a small slab. On the Seegrube above Innsbruck, two slab avalanches were triggered when winter sports enthusiasts descended in east-facing, very steep ridgeline terrain.
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| A slab avalanche remotely triggered by a ski-piste vehicle, in the Northern Massif (photo: 10.12.2019) |
What’s next?
According to ZAMG Weather Service forecasts, the southerly wind will shift to westerly tomorrow (13.12) and intensify to strong-to-storm strength velocity. In the western regions, 10-15 cm of fresh snow is anticipated, 20 cm from place to place, decreasing to 5-10 cm further to the east. On 14 December a warm front will move over Tirol. In the mountains it will remain stormy. Major wind direction: westerly. Above about 1500 m, about 20 cm of fresh snow is expected.
The stormy conditions will ratchet up avalanche danger somewhat. Regions with considerable avalanche danger will be more widespread. Rainfall at low altitudes will have a detrimental effect on the snowpack in those regions where precipitation is heaviest. If precipitation is heavy, small, moist loose-snow slides are possible. In wind-protected and non-rain impacted regions, the conditions are generally more favourable.
The main danger still stems from freshly generated and, to some extent, older snowdrift accumulations. Caution is urged, most of all, in steep terrain near ridgelines, in gullies and in bowls, as well as behind protruberances in the landscape in general. Winter sports enthusiasts require experience in on-site assessment of avalanche dangers.
