Attention: daytime avalanche danger cycle
At the moment (4 February) we are experiencing an unseasonably warm weather phase. The snowpack at low and intermediate altitudes, which was already moist, is slowly becoming thoroughly wet. On sunny slopes the moisture content of the uppermost layers is marching forward, up to higher altitudes. More moisture in the snowpack means less firmness and, thereby, heightened likelihood of avalanches. The situation becomes even more interesting when water penetrates as far as the weak layers, generally at mid-level in the snowpack. This scenario depends on a great many factors, primarily the weather, of course, but also (diffuse) solar radiation, humidity of the atmosphere, air temperature, wind impact, residual clouds, plus the respective altitudes and aspects. In other words, a variety and complexity which can only be estimated on-site. What is unvaried and unchangeable: there is increasing potential for more naturally triggered wet-snow and glide-snow avalanches especially below about 2300 m.
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| Heightened glide-snow avalanche activity, as here in Ausserfern. Decisive: rain impact on 02.02.2021 (photo: 02.02.2021) |
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| Glide-snow avalanches in southern East Tirol (photo: 28.01.2021) |
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| Naturally triggered avalanche in Pitztal, Neurer district, on 03.02.2021 at 1:00 pm. Fracture occurred at 2350 m, north. The impulse (probable cause) came from a loose-snow avalanche in rocky terrain unleashed by diffuse solar radiation plus warmth. (photo: 03.02.2021) |
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| Fresh, wet slab, 1850 m, south. Sellraintal (photo: 03.02.2021) |
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| Snow profile for above photo. The snowpack at this altitude and in this aspect is isotherm – 0° (red line). Water penetrated down to the weak layer, raised proneness to triggering. |
On the other hand, wet loose-snow avalanches on extremely steep, sunny slopes can be expected even at very high altitudes. More loose-snow avalanches were observed as early as 3 February. According to current weather forecasts, these avalanches could become even more frequent on Saturday (6 February).
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| Wet loose-snow avalanches in rocky terrain. Sellraintal (photo: 03.02.2021) |
Due to the prevalent old-snow problem it can be expected that the impulse of a loose-snow avalanche will cause fractures deeper down inside the snowpack. Thus, avalanches can grow to large size, in isolated cases even to very large size.
Let’s have a look at two such triggering-scenarios for activating a weak layer...
- Water seeps down to the weak layer
- Impulse from additional loading of a wet loose-snow avalanche (in extremely steep terrain)
...Based on current weather data, a triggering from the impulse of a wet loose-snow avalanche is more likely.
Attention: continuing old-snow problem. Caution esp. at altitudes between 1900m and 2300m.
The old-snow problem was discussed thoroughly in our last blogs. It continues, i.e. it still matters. This problem is most striking between 1900 and 2300 m in all aspects. ALL the recent avalanche accidents occurred in this altitude span. The situation must be viewed as treacherous, especially because trigger-sensitivity of the snowpack is difficult to assess even for a trained eye. These avalanches release most easily in spots where snow is shallow and in transitions from shallow to deep snow. In isolated cases, remote triggerings from flat terrain are still possible.
Slab avalanches can grow to dangerously large size for backcountry skiers due to the far-ranging spread of weak layers and thus, the huge potential spread of fracture propagation.
If we compare the situation with the one over last weekend, we can assume that the likelihood of an avalanche triggering has actually decreased. Yet the consequences of an avalanche continue to be quite high, due to the potential size of a release. Low probability - high consequence.
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| Visible in this snow profile from Pitztal, about 2150 m, NE, is the thin, loosely-packed weak layer. Stability tests showed good fracture propagation from moderate loading. At this altitude/aspect the snowpack still has temperature reserves. A thoroughly wet weak layer is unlikely in the next few days. |
Analysis of avalanche accidents of this last week
In the mountains of Tirol, 5 persons died in avalanche accidents in the last week. Here a brief analysis.
Avalanche accident Juifenalm (northern Stubai Alps) on 30.01.2021
Two backcountry tourers were in their descent from “Auf dem Sömen” in Sellraintal when a slab triggered while they traversed a steep (35°gradient) slope. Both were swept along with the avalanche and completely buried. One of them managed to release the airbag, but was so thoroughly overpowered by snow masses while lying prone, belly down, that it was not possible to get free of the load. The accident was noticed on the following day. In the course of a nocturnal search, the persons were located and dug out of the snow masses.
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| Avalanche near Juifenalm. Arrow points to entry track. Spot of burial is encircled. (photo: 01.02.2021) |
The fracture zone was at about 2140 m altitude. The avalanche was about 150 m long, 30 m wide. The average fracture depth was 40 cm. The slope faced west.
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| In foreground, snow-burial spots are visible. (photo: 01.02.2021) |
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Snow profile at the foot of the slope orographically left of the avalanche. The arrows mark two loosely-packed layers, between them a thin melt-freeze crust which formed just before Christmas.
(photo: 01.02.2021) |
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| Snow profile corresponding to the above photo. Relevant for the accident was the lower fracture point (yellow line at margin) |
Avalanche accident Widdersbergsattel (northern Stubai Alps) on 30.01.2021
One person was in the ascent towards Widdersbergsattel (point of departure: Axamer Lizum). A slab triggered in which the person was swept along and completely buried. Two persons on nearby Widdersberg saw the avalanche release after some delay, skied to the avalanche path, were able to receive a beacon signal and dig out the person. All attempts at resuscitation failed. In this avalanche release, it is no longer possible to determine whether the person was near the foot of the slope during the avalanche or was on the steep slope (35° gradient). The slab was 160 m long, 125 m wide, average fracture depth was about 50 cm (maxima 200 cm). The avalanche slope faced east. The fracture zone lay at about 2250 m.
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| Avalanche beneath Widdersbergsattel, Kalkkögeln. Photo taken by the Alpine Police during their search. (photo: 30.01.2021) |
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| Orographically right avalanche fracture (photo: 31.01.2021) |
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| Snow profile orographically right of avalanche path, near spot of burial. The fracture occurred in a loosely-packed, faceted layer beneath a thin melt-freeze crust. |
Avalanche accident Neunerkogel (northern Stubai Alps) on 30.01.2021
A young freerider skied from the Drei-Seen Lift towards Kühtai below the Finstertal Reservoir wall in the direction of the valley. Below a small, extremely steep slope the freerider lost a ski. When attempting to retrieve it, a slab triggered which buried the person. Two accompanying skiers were able to escape the avalanche path. The depth of the spot of burial was about 40 cm. The freerider did not have a transceiver, making the search more difficult. An avalanche dog was finally able to locate the person. Attempts at resuscitation failed.
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| Slab avalanche below Neunerkogel. The accident slope was full of skiing tracks, at least in the lower part. The fracture propagated over rocky terrain. (photo: 30.01.2021) |
The slab avalanche was about 150 m long, 80 m wide. The fracture was average 70 cm deep.
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| Snow profile Neunerkogel – a situation quite similar to the other avalanche accidents. |
Avalanche accident Arbeserkogel (eastern Tux Alps) on 01.02.2021
Two persons walked across the ski piste on the Arbeserkogel south of Schwaz. From there their route led them along a mountain shoulder towards Kellerjoch. An extremely steep north-facing slopes was selected as their descent. When one person skied into the slope, a very large slab triggered almost immediately. The person was fatally injured. A ground-search of the terrain was considered too dangerous, so a helicopter was engaged. It became evident later that the person’s transceiver was turned on, but the battery was too weak to send/receive a signal. After an extensive search the person was finally located by the helicopter with a Recco device.
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| Avalanche accident Arbeserkogel. The arrow points to the entry track, the spot of burial is encircled. The avalanche was about 890 m long, 220 m wide, average fracture depth was about 70 cm. |
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| Avalanche fracture. Arrow points to the entry track. (photo: 01.02.2021) |
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| Entry track. Deep snow at the beginning is clearly visible. The track leads towards a shallow-snow zone just below the tree. A few metres below that, the avalanche triggered when the person was in the descent. (photo: 01.02.2021) |
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| Snow profile near spot where avalanche triggered. This was a shallow-snow spot. The two arrows point to two loosely-packed, faceted layers. Between them, a crust. (photo: 02.02.2021) |
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| Snow profile for the above photo. |
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| For comparison, a second snow profile made at a spot above the fracture point where the snow was deeper. Here, far greater loading was necessary to trigger the weak layer. |
Review of the last week, particularly last weekend
Last week was full of highly variable conditions. The sequence of cold fronts and warm fronts brought about 100 cm of fresh snow, accompanied by storm-winds. Further east, snow depths decreased successively, generally amounted to 20-50 cm.
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| highly varied... |
When the warm front arrived on 28 January, avalanche danger levels increased significantly. Numerous avalanches triggered naturally, including very large releases in western regions. On Saturday, 30 January, very large-sized avalanches were artificially triggered in western regions where snowfall had been heaviest. During the following days, numbers of large-sized, even very-large sized avalanches triggered naturally over the course of the day. Most of them were caused by additional loading due to wide-ranging snow transport; some by the weakening of the snowpack from diffuse solar radiation.
Estimated danger for Saturday, 30 January 2021
Due to a number of inquiries about our ongoing assessments of avalanche danger which were re-assessed at “high” on Thursday and Friday, subsequently re-assessed at a critical level 3 on Saturday after the warm front passed through, we publish here a retrospective view of the estimated danger of the situation: how it was seen and how it turned out.
In retrospect, ranking the western regions (Paznaun, Arlberg) at “high” on Saturday would have been more accurate. The most important factor for lowering it to a critical level 3 was that naturally triggered avalanche activity had been so massive as the warm front passed through (Thursday 28.01 – Friday 29.01) and that so many avalanches had already released naturally. We attempted to point out major factors in the Avalanche Bulletin which, all in all, corresponded pretty accurately to the actual situation as it developed.
What was given too little consideration – and this proved decisive – was the vast wide-ranging slab which as a result of the warm front formed above the already existing snow that had fallen since mid-January, along with the weak layer which lurked below this deep layer. This new-formed slab reinforced fracture propagation, particularly in the western regions where snowfall had been so heavy, to an extent which is seldom seen.
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| When an avalanche triggered, presumably naturally, several avalanches followed in sequence in the entire mountain basin, generating colossal fracture propagation on Fliesser Berg. Fortunately, the photographer and person accompanying him, who were in moderately steep terrain at that moment, were not caught in the avalanche visible in foreground. (photo: (c) Noah Ladner, 30.01.2021) |
Particularly in accident zones, this newly formed slab was not all that deep (overall snowfall there was 20-30 cm, although amid strong to storm-velocity wind impact). Stability tests carried out before the weather fronts arrived demonstrated that the weak layer referred to above was no longer very prone to triggering. Evidently, even this minor amount of fresh snow combined with wind was sufficient, through the generation of a marked slab, to create high proneness to triggering and large-sized avalanches. What also proved consequential was the rain crust which formed on 21-22 December up to about 2400 m which then, during the cold phase, was destroyed down to at least 100 metres below. Above and beneath this rain crust we find, evenly distributed over wide-ranging areas, faceted, loosely-packed and ongoingly trigger-sensitive layers, especially at altitudes of about 1900-2300 m, as was described in the Avalanche Bulletin.
Thus, this has been an extremely challenging time for everyone, including for us avalanche forecasters. It was intensified by a high degree of uncertainty regarding weather forecasts. All in all, a profoundly complex avalanche situation.
In future when they arise, we will try to approach and analyze such complex situations even more comprehensively. In retrospect, this particular situation recalls the avalanche accident Geier in the Wattener Lizum on 06.02.2016, and that on 08.03 and 09.03.2017, which had similar avalanche potential.
