Today, 25.03, we ventured into outlying terrain in Tirol’s highlands with the Alpine Police. We were able to closely analyze the 24.03 avalanche on Giggler Spitze; observe close-up an avalanche on Hochwanner in Kühtai; and obtain a good grasp of the overall situation from the air .
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| In backcountry with the Alpine Police (photo: 25.03.2021) |
In a nutshell:
In the interim a huge number of backcountry tours have been undertaken, many of them in extremely steep terrain. In view of this, there do not seem to be very many danger zones left where avalanches can trigger. The highest degree of caution is necessary where there is a near-to-surface old-snow problem. The range of aspects/altitudes listed in yesterday’s blog (24.03.) has been confirmed rather well by avalanches observed. In addition, very steep ridgeline terrain where the snow is shallow also demands more caution since near nests of depth hoar slab avalanches there can still be triggered.
Last week’s weather
Last week was heavily impacted by cold air masses from the Arctic, bringing highly variable conditions. Repeated bouts of snowfall, often strong winds. It was much too cold for this juncture of the season.
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| The upper graph shows divergences from long-term averages at the Patcherkofel station. Significantly lower temperatures in the last 10 days are visible. A new minimum was even reached. The lower graph explains the colors used. (c) ZAMG |
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| Variably cloudy weather persisted on Sunday, 21.03.2021 - northern Stubai Alps (photo: 21.03.2021) |
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| Strong winds transported lots of snow, even fashioned water sculptures. This comes from the warm phase just before the beginning of March - Lasörling Massif (photo: 21.03.2021) |
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| At the start of the week, weather improved palpably - Paznauntal (Foto: 23.03.2021) |
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| Initially variable with some precipitation and low temperatures, starting on 23.03 the weather improved. Temperatures climbed continually. |
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| In East Tirol, strong winds were frequently blowing. |
Snow cover
Skiers and freeriders were able to celebrate last week: apart from wind-impacted slopes, there was lots of gorgeous powder.
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| Powder snow dreams in the Stubai Alps (photo: 25.03.2021) |
In the meantime the sun has been doing its job with bravour. Powder exists only on purely shady slopes, melt-freeze crusts tend to dominate. On steep south-facing slopes and at intermediate altitudes the snowpack, following nights of clear skies, is becoming more and more capable of bearing loads.
Reminiscent: where yellow-orange hues in the snowpack often become visible, recalling the dust blown to the Alps from the Sahara at the beginning of February.
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| The yellow layer obtained its color from Sahara desert dust. Silvretta (photo: 23.03.2021) |
Another interesting phenomenon was also observed, fortunately irrelevant to the development of coming conditions. When on 23.03 very dry air massess penetrated at least the western regions, the fluffy, extremely light powder on the snowpack surface became moist down to a few centimetres below the surface due to intensive, short-lived solar radiation, but the snow above it remained dry. The result: skiers were surprised by abruptly sticking snow. In addition, as of late afternoon a thin melt-freeze crust formed.
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| “Radiation recrystallisation” is what this phenomenon is called. The snowpack surface is still loose and dry, beneath it a thin melt-freeze crust formed in late afternoon (which remained moist during the daytime). Silvretta (photo: 23.03.2021) |
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| Visible in this snow profile is the temperature progression responsible for radiation recrystallisation: surface temperature is negative, it is 0 degrees a few centimetres below, deeper down it is lower. (Potential for short-term formation of a weak layer due to high temperature disparity.) |
Avalanche accident Giggler Spitze on 24.03.2021 - a short analysis
As mentioned at the outset of this blog we examined the site of the recent avalanche together with the Alpine Police. The slab avalanche was impressively large: about 1600m long and 100m wide, fractured at about 2480m altitude on an extremely steep, NW-facing slope.
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A section of the accident avalanche below Giggler Spitze in the Samnaun Massif on 24.03.2021
(photo: 25.03.2021) |
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Entry tracks of the two injured persons. We carried out our stability tests just above the traverse track, where there were fissures in the snowpack. The slab unleashed where the slope got steeper.
(photo: 24.03.2021) |
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At left, one of the entry tracks. In foreground, the traverse track. Above that, a fracture.
(photo: 25.03.2021) |
Immediately conspicious, and the most probable cause of the accident, was a shallow-snow zone near the ridgeline. There, beneath the snowdrifts we found layers of loosely-packed, expansively metamorphosed (faceted) snow crystals. Stability tests there initiated a fracture which then also propagated further. We also discovered nests of depth hoar here and there. Thus, there was lots of depth hoar beneath the snowdrifts in this general vicinity and precisely there, the snowpack was particularly easy to trigger. From such a spot, the fracture was presumably initiated and further propagated to other quite stable zones.
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| Snow profile with stability test above the avalanche fracture, in orographically right section of the avalanche. The snow block released in a loosely-packed layer of depth hoar. To the left of the shovel, a fracture inside the snowpack is visible which was generated by the avalanche. (photo: 25.03.2021) |
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| The snow profile of the photo above it. The arrow points to the weak layer. |
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| Loose snow on the shovel was what composed the weak layer. (photo: 25.03.2021) |
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| At left, the entry tracks. (photo: 25.03.2021) |
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| The deepest fracture was about 200 cm. Fracture down to very stable zones. (photo: 25.03.2021) |
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| View of plummet path as far as deposits in forest lanes (photo: 25.03.2021) |
Other recently observed avalanches
Right off the bat, recent avalanches have one thing in common: they all released in extremely steep terrain.
Below the Hochwanner in Kühtai, one person was caught by a slab avalanche during a descent on 25.03 and swept along. The person sustained no injuries. This was an extremely steep NE-facing slope. The fracture occurred at 2350m. The avalanche was about 300m long and 50m wide.
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| Avalanche Hochwanner. The person skied from above left into partially rocky terrain. At bottom of photo, the rescue helicopter arriving at the site is visible. (photo: 25.03.2021) |
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| Snow profile of orographically left fracture zone near the entry track: beneath a thin melt-freeze crust is a faceted layer - the weak layer. On top of that, recent snowdrifts. |
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| Hoher Burgstall avalanche on 23.03.2021, NE, 2500m. We assume snowpack layering is very similar to the Hochwanner avalanche. (photo: 25.03.2021) |
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| Grünbergspitze avalanche. Presumably naturally triggered. Weakened snowpack due to solar radiation. appx. 2700m, south. (photo: 24.03.2021) |
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| Naturally triggered avalanche near Kögele - northern Stubai Alps. This entire slope also unleashed a slab avalanche in mid-January 2021. NE, 2200m (photo: 25.03.2021) |
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Thialkopf avalanche - north, near timberline - similar to the one reported in Wattental, Tux Alps.
(photo: 21.03.2021) |
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| The arrows point to small slab avalanches which triggered in the Tux Alps at the start of the week. Weak layer was presumably a wind-loaded loosely-packed powder layer, but not 100% certain, since we did not perform on-site analysis. (photo: 24.03.2021) |
With intensifying solar radiation and rising temperatures over the last few days, loose-snow avalanches became more frequent. Most were relatively small, somewhat larger east of Wipptal. There on 23.03, there was more intensive diffuse radiation. Some glide-snow and isolated slab avalanches were also observed there. The slab releases were mostly on sunny slopes, near ridges in extremely steep terrain.
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| Deposit of a loose-snow avalanche in rear Zillertal (photo: 23.03.2021) |
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| Recently triggered loose-snow avalanches (photo: 25.03.2021) |
Bald zieht (wieder) das Frühjahr ein
Tomorrow on 26.03 it will again be sunny. On foehn-exposed summits, brisk SW winds are expected. On Saturday, 27.03, a cold front from the northwest will swiftly move across the land. Snowfall level is expected to be between 1000 and 1500m. Generally, 5 cm of fresh snow is anticipated, 10 cm in the northern barrier cloud regions. After the front has passed, skies will clear up more rapidly in the western regions than in the east.
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| Clear skies following the cold front on Saturday, 27.03.2021 |
In wind-protected shady terrain at high altitude, small sometimes trigger-sensitive snowdrift accumulations have formed. In high-alpine regions this is possibly due to Nigg-Effect, possibly near ridgelines atop surface hoar.
Day by day starting on Sunday, 28.03, it will become increasingly sunny and milder. What matters most as of that time is the degree of moisture inside the snowpack. We expect a marked daily cycle of avalanche danger to start on Wednesday at latest.
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Due to the thoroughly wet snowpack, as expected, also more frequent glide-snow avalanches are coming. (photo: 20.03.2021)
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