Following a week of high-pressure weather conditions and generally quite favourable conditions, winter sports enthusiasts have to realign their sights: starting on 4 January wind and snowfall will swiftly lead to heightened avalanche danger. Freshly generated snowdrifts on W/N/E facing slopes, particularly in wind-protected zones, will be easily triggered by backcountry skiers and freeriders.
The weather this last week
Mostly high-pressure conditions
During the last week, i.e. as we crossed over into the new year, a high-pressure front dominated. From the northwest, very mild and dry air masses flowed over the Alps. At ground level, cold air formed layers leading to huge temperature disparities between mountain peaks and valley floors (temperature inversion).
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| Generally fabulous weather for winter sports. En route in Virgental (photo: 31.12.2019) |
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| On 27.12, moist air masses lodged against the Alpine flanks. Thereafter, very dry and mild air masses flowed in. At high altitudes it was often quite windy. Station Gallreideschrofen in Gschnitztal. |
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| A similar picture in East Tirol Particularly on 28.12, strong winds were blowing in the heights. |
Quite windy at high altitudes
This last week was subjected to repeated bouts of heavy wind impact, particularly in the southern regions at high altitudes. On 02.01, winds at high altitudes will again intensify, especially in the classic foehn-influenced zones and on the Main Alpine Ridge. On 03.01 the high-pressure conditions will diminish in force, the airstream will shift from southerly to westerly and the air will slowly gain in moisture.
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| Heavy wind impact (photo: 28.12.2019) |
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| A typical picture on 28.12 in East Tirol: huge snow plumes on the mountaintops. |
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| View from Tux Alps towards Main Alpine Ridge, where snow plumes are visible. (photo: 28.12.2019) |
Snowpack development
Wind-impacted snowpack surfaces, esp. at high altitudes
Due to wind influence the snowpack surface is highly irregular, at least at high altitudes. In addition, shallow-snow spots alternate with deep-snow spots. Ridgeline zones were often utterly windblown.
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| Erratic snowpack surfaces in northern East Tirol (photo: 29.12.2019) |
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| Huge wind swirls above Nauders (photo: 29-12-2019) |
Thoroughly wet snowpack, esp. on sunny slopes
The warm temperatures and solar radiation made the uppermost layers of the snowpack (at very least) moist on E-S-W facing slopes in particular. This, in turn, weakened the snow. In extremely steep terrain, increasingly frequent moist loose-snow avalanches were observed for that reason.
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| Conditions like in springtime on sunny slopes. Central Lechtal Alps (photo: 01.01.2020) |
Expansively metamorphosed snow in cold-air zones and in shady terrain
During star-studded nights, the snowpack cooled down massively. This reinforced the formation of loose crystals near the snowpack surface. It often resulted in decomposed snow or faceted crystals, in some places also surface hoar.
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| Surface hoar at 1200 m in the Mieming Massif (photo: 29.12.2019) |
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| On New Year’s Day, many valleys east of Innsbruck were swathed in fog. At the upper borderline of the fog, surface hoar often formed, but persisted only on shady slopes. |
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| So-called burled powder is pictured above. Terrific for skiing, but already somewhat metamorphosed. This requires caution in case of approaching snowfall. En route in Sellrain, Fotscher Windegg. (photo: 29.12.2019) |
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| A loosely-packed snow cover in which skiers break through the snow down to the ground. Poor snowpack structuring for approaching snowfall. This is a wind-protected shady slope at 2090 m. |
Frequently good snow quality
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| Great powder in the northern Stubai Alps (photo: 01.01.2020) |
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| Dream-come-true descent in Lechtal Alps (photo: 29.12.2020) |
Snowpack analysis
During our snowpack analysis we concentrated not only on the snowpack structuring of the surface layers, but also on the processes unfolding inside the snowpack near to the upper surface, specifically, on the generation of potential weak layers corresponding to Danger Pattern 4 (cold on warm), both from 21.12 and from 24.12. On those dates, moister and relatively warmer layers were covered by new fallen snow. Our conclusion to date: from region to region, faceted crystals have formed at the borderline to the layers which might constitute weak layers for the snowdrift accumulations deposited on top of them, but only in rare cases. Most suspicious in this respect is a narrow altitude band between 2100 and 2300 m on shady, very steep slopes.
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| The arrow points to a moist layer from 24.12. |
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| Immediate vicinity of prior snow profile site: a melt-freeze crust developed from a moist layer. Bordering on that, no formation of faceted crystals. |
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| South-facing slope at 2000 m, a thin weak layer expanded inside a warm layer on 24.12. Warmth had a favourable effect. |
As the avalanche accident on the Gamskarspitze on 31.12.2019 demonstrated, there are loosely-packed layers beneath the wind crusts in zones where snow is shallow. These are isolated avalanche prone locations which occur particularly in high altitude zones.
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| The arrow points to a potential weak layer for slab avalanches below a hardened, wind-impacted layer. Large additional loading would be necessary to trigger this layer. |
In isolated cases, there are faceted crystals beneath old melt-freeze crusts which can be triggered. W/E facing slopes above about 2500 m, S-facing slopes above 2800 m. This is a rain crust on shady slopes, formed in mid-November, in an altitude band between 2100-2400 m. Triggering seems likely only by large additional loading.
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| Fracture of faceted crystals beneath a melt-freeze crust, on shady slope at 2300 m in the southern Ötztal Alps (photo: 01.01.2020) |
Avalanche events
Over the last week, there were many winter sports enthusiasts in the mountains. In proportion to these numbers there were relatively few who triggered avalanches, a sign that the snowpack is well structured. An overview can be found here: one avalanche ended with a fatality on the Gamskarspitze in the western Verwall Massif. See previous blog.
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| Avalanche Scheibenspitze in Navistal (photo: 31.12.2019). The prerequisites seem to have been the same as for the avalanche accident on Gamskarspitze: shallow snow, wind-impacted, extremely steep, shady, 2400 m. |
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| Avalanche Kesselspitze. Snowdrift accumulation triggered when a skier sprang into it on the slope. Arrow: ski tracks of two skiers who skied out of the avalanche path. Circle: two other persons on their descent. (photo: 29.12.2019) |
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| Avalanche in Huilahner Kogel on 29.12, southern East Tirol (photo: 31.12.2019) |
Increased glide-snow avalanche activity
The snowpack was already quite moist before 21.12, then was made thoroughly wet by the warm temperatures on sunny slopes. This reinforced gliding snow masses over steep, grass-covered slopes. Glide-snow avalanches were observed in North Tirol and in East Tirol.
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| Hot spots of glide-snow avalanches were in the Arlberg and Ausserfern regions, as well as the Tux Alps. (photo: Arlberg region on 01.01.2020 |
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| Glide-snow avalanche near Berwang in the Ausserfern region (photo: 29.12.2019) |
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| A serious danger to be considered on all backcountry tours, Ausserfern. (photo: 28.12.2019) |
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| Glide-snow avalanches sometimes grew to large size. Tux Alps (photo: 31.12.2019) |
What’s next?
As referred to above, avalanche danger is expected to swiftly increase, and steadily in wind-influenced regions. This applies particularly to terrain where the snowpack is currently composed of loosely-packed crystals, and it especially includes shady terrain.
When on 4 January two front systems collide on the northern flank of the Alps in swift succession and new fallen snow is the result, also lower altitudes will be influenced by this event wherever precipitation is heavy. And in particular, wherever fresh snow is deposited on top of surface hoar.
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The danger will rise to considerable. Winter sports enthusiasts need to adjust to the new situation and the changed conditions - and consequently circumvent the freshly-generated snowdrift accumulations in steep terrain. For all untertakings in outlying terrain, great experience in assessing avalanche dangers on-site is essential.
Appendix: on 02.01 at 5:17 pm, Tirol headquarters reported an avalanche below the Nuremberger Refuge in the Stubai Alps. It is assumed that some persons were buried in snow. Strong-velocity winds are currently blowing on-site.
