Outlook
Stormy southerly foehn winds which have been raging over North Tirol since yesterday, Wednesday 06.03, were the harbinger of a cold front which will move across all of Tirol tonight and bring some precipitation.
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| On the way in the field, while the cold front is pulled through in the area of the Aifner (photo: 07.03.2019) |
The focus of the precipitation will be along the eastern sector of the Main Alpine Ridge. Tomorrow, Friday 08.03, a westerly airstream will follow on its wings bringing moist air masses, but only minor amounts of precipitation to start with.
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| Rising southwesterly winds announced the perturbance soon to follow as early as Wednesday, 06.03. Eastern Simonyspitze, Venediger Massif (photo: 06.03.2019). |
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| Southerly foehn winds reached gale strength: windspeeds over 130 km/hr at the Patscherkofel wind-measuring station. |
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| The foehn storm left its marks on the Stubai Glacier as well. (photo: 07.03.2019). |
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| 5-15 cm of fresh snow is anticipated over widespread areas by Saturday morning, 09.03. In the Zillertal Alps and Hohe Tauern, 20-40 cm of fresh snow is expected. |
Winds are expected to slacken off during the course of this round of precipitation, yet still be blowing above transport velocity. Thus, easily triggered snowdrift accumulations will be generated, and they will be deep in the regions where snowfall is heaviest. The freshly drifted masses are most easily triggered on very steep, shady slopes where the snowpack surface was still powdery before the precipitation set in. Above approximately 2800 m, such avalanche prone locations are found increasingly often in other aspects as well, especially in gullies, bowls and behind protruberances adjacent to ridgelines. As the snowfall draws to a close, the snowdrift accumulations will be covered by fresh snow, making them difficult to recognize.
Ranked right behind the snowdrift masses, it is glide-snow avalanches which currently present the greatest danger in Tirol’s mountains. Since this problem has already been aired to an exhausting degree in every blog this winter, we will forego the details in this edition. Glide-snow avalanches can release at any time of day or night. All zones below glide cracks should be circumvented whenever possible, or traversed swiftly. An
accident in Vorarlberg bears poignant witness to just how unpredictable they are.
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| Glide-snow avalanches - an unpredictable threat. Landeggtal, Glockner Massif (photo: 05.03.2019). |
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| Huge clumps in the deposit area of a glide-snow avalanche on the Jöchelspitze, Allgäu Alps (photo: 06.03.2019). |
The lowermost section of the snowpack is currently well consolidated over widespread areas and stable. This also applies to central East Tirol, where wide-ranging snowpack analysis has been conducted during the last few days. The weakened old-snowpack fundament which built up due to the hesitant early phase of the winter, particularly on steep, shady slopes between 2000-2600 m, was able to consolidate quite well during the last few weeks due to the immense burden of snow and warm temperatures.
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| Snowpack analysis in East Tirol. Landeggtal, Glockner Massif (photo: 05.03.2019). |
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| The snow profile from the Lasörling Massif in East Tirol shows a well consolidated snow structure. No fractures could be generated in the more deeply embedded layers. NW; 2570; 39 degrees (©LWD Tirol). |
Further to the south in the Lienz Dolomites, an old-snow problem along the borders of the timberline is persisting on very steep slopes between about 1800 and 2000 m. Near ground level in this zone, a weak layer consisting of faceted snow crystals is evident which could still be triggered in isolated cases, primarily by large additional loading. Transitions from shallow to deep snow are particularly unfavourable.
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| A distinct layer of faceted crystals near ground level is potentially still capable of triggering in the Lienz Dolomites. Hoher Bösring, Carnic Alps (photo: 05.03.2019). |
According to ZAMG weather forecasts, little change is expected in the currently instable weather, including next week. It will turn cold, typical of high winter, and more precipitation is anticipated. Only on Tuesday can we expect an interim of foehn-influenced mild weather.
Review
We gaze back on a week of highly variable weather conditions. Two cold fronts (on Friday 01.03 and on Monday 04.03) brought fresh snow throughout Tirol. Between these bouts of precipitation and during the days which followed, sunshine alternated with cloud cover, it was frequently quite mild.
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| The weather-station graph gives a good picture of the variable weather over the last few days. On Friday, 01.03, as well as on Monday, 04.03, two cold fronts forced temperatures to plummet and brought a bit of fresh snow (shaded in red). Between those fronts and afterwards, air temperature (red line) rose markedly at times. |
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| On Friday, 01.03 (left map) the focus of the precipitation lay on the border to Vorarlberg. On Monday, 04.03 (right map), the heaviest snowfall was in the southern regions. |
Winds accompanying the snowfall transported the fresh snow and generated small snowdrift accumulations. These drifts were triggerable, at most, only on steep, shady slopes at high altitudes and in high alpine regions. This was because a weak layer is always necessary for a slab avalanche to trigger:
Prior to the snowfall on Friday, 01.03, the snowpack surface was highly irregular (see blog of 28.02). Only on very steep, shady slopes were there any loosely-bonded, faceted snow crystals to constitute a weak layer for a slab on top of it. Prior to the snowfall of Monday, 04.03, it was solar radiation and warmth which had the biggest effect on the snowpack surface. This time, too, a weak layer in the form of loosely-bonded powder was evident only on north-facing slopes.
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| The southerly wind which arose on Monday, 04.03, brought snow transport anew and generated snowdrift accumulations. Venediger Massif (photo: 04.03.2019). |
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| The snowdrift accumulations, deposited especially in gullies, bowls and behind terrain edges on shady slopes at high altitudes and in high alpine regions, were generally small and trigger-sensitive only for a short time. Rieder Wetterkreuzspitze, northern Stubai Alps (photo: 04.03.2019). |
During the phases of beautiful weather between the two bouts of precipitation, as well as on Wednesday, 06.03, temperatures again rose measurably. As a result of solar radiation, numerous moist loose-snow slides released on steep, south-facing slopes.
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| As a consequence of solar radiation, the likelihood of moist loose-snow avalanches triggering on south-facing slopes increased. The melt-freeze encrusted, smooth ground surface provided a perfect slide for the gliding snow masses. Friedrichskopf, Schober Massif (photo: 06.03.2019). |
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| The snowpack on south-facing slopes suffered visibly from the high temperatures of the last few weeks. For that reason, the danger of glide-snow avalanches has diminished somewhat in East Tirol. (photo: 06.03.2019). |
On shady slopes, the snowpack surface above approximately 2200 m remained dry for the most part. Wherever wind had no impact on the snow, great backcountry touring conditions could be enjoyed.
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| The joys of the descent: beneath fresh snow the old-snowpack surface could still be felt in the Lasörling Massif. (photo: 06.03.2019). |
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Whoever is currently en route in outlying terrain needs, above all else, to be able to assess the signs of wind impact. Fresh snowdrift accumulations in very steep, particularly in shady ridgeline terrain, need to be circumvented whenever possible. (photo: 06.03.2019).
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