Unusually warm: time window shrinking for backcountry tours. Naturally triggered avalanches increasing.
We are currently experiencing a phase of unseasonably warm weather. The zero-degree level today lies at 3300m, according to ZAMG Weather Service. Despite nights of clear skies and outgoing longwave radiation with resultant cooling, the snowpack rapidly softens up during the daytime. Avalanche danger thus increases correspondingly fast. Currently we are at the critical level of a “3” as of afternoon, in other words, very near to Danger Level “high.”
Yesterday (30.03) increasingly frequent naturally triggered wet-snow avalanches (mostly loose-snow and slab avalanches) were observed. This trend will intensify. That means avalanche prone locations for avalanches person-triggered and naturally-triggered will further increase during the course of the day.
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| Typical of current situation: on sunny slopes slab avalanches tend to fracture in near-to-surface layers, after snowfall starting on 05.03. (photo: 30.01.2021) |
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Temperature rises, snowpack surface becomes wet during the day, snow melts.
Muttekopfhütte station (eastern Lechtal Alps) |
A trap for experts: spots with shallow snow on shady slopes (very steep, esp. near ridgelines)
The recent avalanches involving persons which became known were by and large results of a local old-snow problem where daytime warming did not play a part. Such avalanche prone locations continue to exist at high altitudes in shady terrain. Slab avalanches can trigger there particularly where the snow is shallow in steep-to-extremely-steep ridgeline terrain. Apart from the risks of being swept along, the dangers of falling also demand attention. The avalanche accident below Brennerspitze on 28.03.2021 fell into this category. Therefore, an analysis of that incident follows here in more detail:
Brennerspitze avalanche accident in the Stubai Alps on 28.03.2021
Yesterday (30.03) we analyzed the avalanche accident together with the Alpine Police. This was a hard slab which was about 25m wide at the point of fracture and all-in-all was 1200m long. At its widest the avalanche path was 90m. It triggered immediately when a person skied into the 45° steep north-facing slope at 2870m altitude. The person was caught by the avalanche, swept over rocky terrain and ultimately totally buried in snow. Resuscitation attempts were made, the person was flown to the hospital.
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| Slab avalanche Brennerspitze, one person buried in snow (photo: 30.03.2021) |
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| Upper arrow points to the zone where the person skied into the slope. Lower arrow is the track at fracture point. The fracture depth there was about 100cm, 200cm slightly displaced, up above 20-30cm (photo: 30.03.2021) |
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View from ridge: here the skier jumped into the slope. Due to heavy additional loading, the initially penetration of the hard snowpack was slightly deeper. Right of the track, a fracture is visible. However, the slab subsequently triggered laterally, and somewhat lower. Burial point in the path is indicated.
(photo: 30.03.2021) |
We carried out our snowpack examination in the immediate vicinity of the entry track. A major factor of this avalanche was the sequence of hard-and-very-soft layers. The uppermost part of the snowpack was massively wind-impacted and very hard. Beneath that we found a very soft layer of depth hoar and faceted crystals. Further down, other hard and soft layers alternated.
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| Snowpack examination at entry track (arrow). Shallow snow, hard upper layer, snow was very loose near the upright saw in photo - depth hoar and faceted crystals. (photo: 30.03.2021) |
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| Snow profile orographically right of entry track. The block broke after being sawed off at the side and rear. |
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| Profile orographically left of entry track. Two fractures were initiated here. Major avalanche factor: the lower fracture. |
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Alpine Police at the point of burial (photo: 30.03.2021)
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