Immense amounts of fresh snow, storm winds, variable snowfall altitudes
The ZAMG forecasts have been confirmed: in southern regions it will snow intensively. In East Tirol the expectations are currently for 50 cm of fresh snow on the valley floors (with significant proportion of rainfall), elsewhere generally 100 cm. In the upper Gailtal/Lesachtal, according to current models, 100-150 cm is anticipated, and in the mountains 150-200 cm; in the Carnic Alps up to 250 cm. In regions along the Main Alpine Ridge, about 60 cm of fresh snow is expected in the valleys, in the mountains 100-150 cm (up to 200 cm in Timmelsjoch region).
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| 72-hr fresh snow forecast, 03.12. 07:00 – Sunday, 6.12. 7:00 am |
Furthermore, winds in the mountains will be blowing at storm strength.
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| Wind forecast for Friday, 4.12.2020 10:00 pm. Winds will be southerly. |
Making the situation worse is the fact that the snowfall level will vary intensely within a short time frame. This is due to layers of air in the atmosphere which are near 0°C over wide swaths of altitude.
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| Precipitation and wind forecast for southern East Tirol. The very high intensity over a long period of time is quite unusual. Wherever precipitation is both intense and persistent, likelihood of snowfall down to low lying areas is far higher than where intensity is low. (in that case, rainfall level according to ZAMG Weather Service forecasts would then be 1500 m, possibly higher.) |
Current snowpack analysis
The most important thing prior to such events is to have as precise an analysis of snowpack layering as possible, along with a review of recent weather:
After the last major snowfall with cold front on Thursday/Friday, 19-20 November, there followed predominantly mild, dry, sunny weather in the mountains. Last Sunday, 29.11, cold, moist air masses penetrated down to low lying areas, thereby generating a brief period of high fog over widespread areas in Tirol, above which the skies were cloudless. Between Tuesday, 1.12, and today, Thursday, 3.12, there was 5-20 cm of fresh snow registered widespread in Tirol, focal point was in the southern regions. On shady slopes above the timberline this fresh snow was deposited generally on an already cohesive, area-wide snowpack. On sunny slopes, it fell mostly on bare ground up to high altitude (generally 2500 m).
Impressions of snow distribution according to altitude and aspect
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| View from Adlersruhe towards the south. The photo was taken last Sunday, 29.11. Visible in the valley is the high fog referred to above. |
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| Typical: southern aspect bare, shady aspect old snow, Hochtennscharte (photo: 30.11.2020) |
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| A blanket of snow on shady slopes in Defereggental (photo: 01.12.2020) |
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| In southern East Tirol: above the timberline, a cohesive, area-wide snowpack (photo: 01.12.2020) |
At least on shady slopes, the fresh snow will be deposited on an unfavourably layered old snowpack.
Our snowpack analysis, together with the pictures provided by our models, draw a clear picture: due to variable conditions in October and dry, fine weather in November, numerous crusts formed inside the snowpack, alternating with numerous soft layers. This is especially true in shady terrain above the timberline, in high alpine zones (above 3000 m), and to some extent in sunny terrain. Have a look at the details of some snow profiles:
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| Succession of crusts and soft layers ("crust-sandwich"), Gurgler Massif (southern Ötztal Alps), 2670m, NW, 30°, date of profile: 27.11.2020 (c) LWD Tirol |
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| Recent snowfall, then a succession of crusts and expansively metamorphosed (faceted) crystals, Pitztal Glacier (Weisskugel Massif), 2840m, N, 32°, date of profile: 03.02.2020 (c) Markus Kogler |
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| A fracture was marked here beneath a thin crust on a south-facing high alpine slope, Tiefenbach Glacier (Weisskugel Massif), 3270m, S, 30°, date of profile: 24.11.2020. (c) LWD Tirol |
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| Fascinating in this flat ridgeline terrain: thin layers of surface hoar covered by snow, generated by Nigg Effect, Wurmkogel (Gurgler Massif), 2920m, date of profile: 27.11.2020 (c) LWD Tirol |
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| Stable lower fundament; upper part: fresh snow and snowdrifts prone to triggering. Profile near Tirolean border in the Allgäu Alps (Ifen, Hahnenköpfle), 2100m, NO, 24°, date of profile: 03.12.2020 (c) LK Kleinwalsertal |
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| Snowpack simulation using data from Eselrücken weather station, Virgental. Left: snow depth development including layering. The increasing snow depths starting on 3.12 at 7:00 pm form the prediction of snowpack layering in coming days. Right: current simulation for 3 December 2020, 7:00 pm. This shows snowpack layering for a north-facing slope 38° gradient. Easily recognized again: succession of pronounced crusts and loosely packed layers. |
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| Typical of current situation: loosely packed surface, then a crust, loosely packed snow beneath that, then another crust... The more recent powder snow is lacking in this photo from 27.11 (photo: 27.11.2020). |
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| Photo in southern East Tirol at 1500m from 02.12.2020. Expansively metamorphosed snowpack. |
Effects on avalanche danger levels
One thing is certain: avalanche danger levels will increase rapidly and markedly as precipitation intensifies, together with wind and slowly rising temperatures. The wide-ranging snowdrifts will bond poorly with the loosely packed surface snow, at least on shady slopes above the timberline. In addition, the load of the snowpack will continually increase over the course of the snowfall so that fractures inside the old snowpack will become ever more likely.
If we assume that predictions of fresh snow are correct, the size of avalanches will ongoingly increase during the course of the snowfall. We expect large-sized naturally triggered slab avalanches to trigger starting on Saturday. This will occur increasingly in terrain which is quite shady. In the regions of major precipitation, it is also possible that during the snowfall, particularly during varying wind impact or graupel deposits, weak layers form which in turn make slab avalanches possible in other aspects, too.
On top of that, the danger of glide-snow avalanches will increase on grass-covered slopes. Their likelihood rises through the impact of rainfall.
Not to be underestimated: the danger of trees breaking/toppling beneath the snow load.
Further to the north, where less snowfall is anticipated, winter sports enthusiasts should keep in mind that snowdrift accumulations in shady terrain above the timberline can easily be triggered. Furthermore, slab avalanches (expected to be only of medium size) can trigger naturally in those regions.
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| The quiet before the storm: start of foehn impact on Main Alpine Ridge, southern Ötztal Alps (photo: 03.12.2020) |
