This last week has again been highly variable: sunshine, clouds, some precipitation. The extremely cold phase came to an end on 16.12, subsequent temperatures rose continually. Three phases of wind were observed: on 13.12 a strong southerly wind prevailed, on 16.12 winds shifted to westerly, on 19.12 winds again were southerly from region to region.
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| Weather station data from Plojen station / Planskopf in the Serfaus ski area |
Danger situation
In the EUREGIO Tirol-South Tirol-Trentino, low and moderate avalanche danger levels prevailed over widespread areas. The main problem stemmed from freshly generated snowdrift accumulations (frequently adjacent to ridgelines) as well as gliding snow, also old snow from region to region.
Snow situation & snowpack
Tirol was draped in winter garb in many places. Nonetheless, the possibilities for backcountry tours continue to be very limited, at least at low and intermediate altitudes.
Tirol was draped in winter garb in many places. Nonetheless, the possibilities for backcountry tours continue to be very limited, at least at low and intermediate altitudes.
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| En route in the Stubai Alps. View into Senders Valley. Contact with stones cannot be ruled out... (photo: 18.12.2018) |
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| Ski tracks near Jochberg in the Kitzbühel Alps. Grassy ground rises to meet the skiers. (photo: 15.12.2018) |
We utilised the week sharpening our picture of snowpack layering in Tirol a further notch. All in all, it looks good. Least favourable by comparison is the layering on shady slopes starting at about 2300 m up to maximum 2800 m, where there are weak layers of faceted crystals inside the snowpack increasingly frequently, surrounded by melt-freeze crusts. (The crusts became thinner and softer during the most recent cold phase.) Weak layers are generally not cohesive over wide areas, for that reason tend to occur mostly near bowls or in the vicinity of small concavities in the terrain. Glide cracks and settling noises (whumpf!) were not observed during this last week.
The structure of the snowpack surface is striking: during the most recent cold period, it became quite loose in wind-protected terrain. At least above the timberline there was frequent surface hoar evident, which then got covered in western regions of North Tirol. In isolated cases we received reports of Nigg Effect (surface hoar forming near ridgelines).
The wind impact referred to at the outset of the blog was also noticeable, particularly above the treeline. Increasingly frequently there are wind swirls evident, and thin melt-freeze crusts on sunny slopes in particular.
Avalanche activity
Last week, the report of a slab avalanche triggered by a skier in Fimbertal in the Silvretta arrived at headquarters in Tirol. Elsewhere it was rather quiet. However, activity involving glide-snow sluffs and glide-snow avalanches increased significantly on grassy slopes. The rise in temperatures reinforced this activity.
Outlook
Right on time for Christmas, warm-thaw weather announces its imminent arrival. It will soon become measurably milder. The precipitation will be rain at least up to intermediate altitudes. Winds will intensify and reach storm strength in some places.
The effect on the snowpack and avalanche danger levels is complex:
In regions exposed to rainfall, the snowpack surface will become moist, the loose layers of snow nearest to the surface will be destroyed. This is actually a positive development.
At higher altitudes, on the other hand, snowdrift accumulations will be generated afresh. Particularly where the drifts are deposited in very steep terrain which has been wind-protected to date, the proneness to triggering can be expected to increase. This trigger sensitivity will increase further as precipitation sets in and higher temperatures arrive.
In any case, the likelihood of glide-snow avalanches will increase significantly. Due to the higher temperatures and the rain, friction near ground level diminish, thereby raising the likelihood of releases.
Upshot: gliding snow on steep, grass-covered slopes and freshly generated snowdrift accumulations at high altitude will be the main danger.
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| Sequence of crusts and faceted crystals. Stubai Alps. NW, 2300 m (photo: 18.12.2018) |
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| Weak layer of faceted crystals near ground-level on Predigberg in the Silvretta, N, 2100 m. (The old snowpack at this altitude is spatially limited, rarely cohesive or area-wide.) (photo: 18.12.2018) |
The structure of the snowpack surface is striking: during the most recent cold period, it became quite loose in wind-protected terrain. At least above the timberline there was frequent surface hoar evident, which then got covered in western regions of North Tirol. In isolated cases we received reports of Nigg Effect (surface hoar forming near ridgelines).
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| Surface hoar at the edge of the forested zone in the Kitzbüheler Alps (photo: 15.12.2018) |
The wind impact referred to at the outset of the blog was also noticeable, particularly above the treeline. Increasingly frequently there are wind swirls evident, and thin melt-freeze crusts on sunny slopes in particular.
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| Wind ripples on the snowpack surface in the Silvretta (photo: 18.12.2018) |
Avalanche activity
Last week, the report of a slab avalanche triggered by a skier in Fimbertal in the Silvretta arrived at headquarters in Tirol. Elsewhere it was rather quiet. However, activity involving glide-snow sluffs and glide-snow avalanches increased significantly on grassy slopes. The rise in temperatures reinforced this activity.
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| Photo left center, a freshly triggered glide-snow avalanche. Additional glide cracks and ‘wishbones’ bear witness to increased activity. Silvretta. (photo: 18.12.2018) |
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| Glide-snow avalanches are a permanent threat in regions where there has been heavy snowfall. Hochfügen (photo: 13.12.2108) |
Outlook
Right on time for Christmas, warm-thaw weather announces its imminent arrival. It will soon become measurably milder. The precipitation will be rain at least up to intermediate altitudes. Winds will intensify and reach storm strength in some places.
The effect on the snowpack and avalanche danger levels is complex:
In regions exposed to rainfall, the snowpack surface will become moist, the loose layers of snow nearest to the surface will be destroyed. This is actually a positive development.
At higher altitudes, on the other hand, snowdrift accumulations will be generated afresh. Particularly where the drifts are deposited in very steep terrain which has been wind-protected to date, the proneness to triggering can be expected to increase. This trigger sensitivity will increase further as precipitation sets in and higher temperatures arrive.
In any case, the likelihood of glide-snow avalanches will increase significantly. Due to the higher temperatures and the rain, friction near ground level diminish, thereby raising the likelihood of releases.
Upshot: gliding snow on steep, grass-covered slopes and freshly generated snowdrift accumulations at high altitude will be the main danger.
