Onset of winter in the mountains leads to heightened avalanche danger

 Lots of fresh snow in the mountains

A cold front moving swiftly across Tirol is bringing heavy precipitation to the southeastern regions of the land. ZAMG Weather Service anticipates 50-80 mm of precipitation between Monday (26.10.2020) at 6:00 pm and Tuesday (27.10.2020) at 9:00 am. As temperatures rapidly drop, the snowfall level on the Main Alpine Ridge will descend from 2200 m down to 1300 m in the latter part of the night. Further south the temperatures will not drop as quickly. The snowfall level there is expected to be at about 1500 m, according to ZAMG.

Overview of precipitation by 9:00 pm today, whose focus will shift southeast during the night.

Heavy precipitation in Obernberg (Brenner). Lower temperatures from the cold front are also visible.

Initial snowfall in the mountains, winds strong-but-easing, temperatures dropping (weather station Eissee on Stubai Glacier).

Between 50 and 100 cm of fresh snow is expected in the mountains along the Main Alpine Ridge from Ötztal Alps eastwards and in East Tirol.

Fresh snow forecast over 24 hrs: purple zone indicates 75-100 cm.

Avalanche danger consequences

Naturally triggered slab avalanches briefly possible

Heavy snowfall leads to a correspondingly larger load on the snowpack. In high alpine regions (above 3000m) we expect that naturally triggered avalanches will unleash as a result of this additional load, particularly on north-facing slopes, some of which will be large-sized slab avalanches. This is a direct consequence of the weak layer inside the old snowpack (discussed at length in the last Blog). As precipitation slackens off and winds ease, the likelihood of naturally triggered slab avalanches will then rapidly decrease.

By the end of this bout of snowfall, winter sports enthusiasts will probably be able to trigger this weak layer only by large additional loading and then, particularly in very steep transitions from shallow to deep snow. Nevertheless, caution is urged towards fresh snowdrift accumulations which have frequently been deposited on top of a weak layer (consisting of loosely-packed powder). Such danger spots are found more often near ridgelines at high altitudes and in lee areas behind protruberances in the terrain.

Over grass-covered slopes: snow glides

Wherever there are large amounts of snow on steep, grassy slopes, an additional danger needs to be taken into consideration: glide-snow slides and glide-snow avalanches. Caution is urged especially on hikes at the foot of steep hillsides. In our experience, due to the warm ground many such avalanches are unleashed at this juncture of the season. For examples, see the blog from 13.10.2020:  photos of Gleitschneelawinen. 

Loose-snow avalanches from rocky terrain

Loose-snow slides or avalanches are another consequence of fresh snowfall at this juncture of the season. Since solar radiation is still quite strong, the snowpack tends to become moist after the snowfall, and thereby forfeits its firmness. In sunny, rocky terrain, avalanches subsequently release more frequently from point to point wherever snowfall has been heaviest.

The upshot

Over the next few days winter sports enthusiasts who venture into outlying terrain need to have enough grasp of avalanche science to assess the risks on-site. Moreover, greater caution is called for, i.e. defensive conduct for a short time.

Our next blog will be published whenever there is a significant change in snow and avalanche conditions.

First snowpack analysis results – Ongoing old snow problem in high alpine regions – Heed temporary snowdrift problem

 Snowpack analysis in high alpine terrain

Since our last blog we have swooped far and wide in the state helicopter, dug deep in the high altitude snows in order to construct snow profiles of Tirol’s high alpine terrain. Part of this involves snowpack stability tests in order to find out how well the disparate layers of snow are bonded to each other.

Stability test on the Grossvenediger (photo: 19.10.2020) 

With avid support from the state helicopter we were able to analyze both the Grossvenediger avalanche of 10.10.2020 and the Zuckerhütl avalanche on 18.10.2020. (photo: 19.10.2020)

In a nutshell:  persistent old snow problem in high alpine regions necessitates caution on north-facing slopes in particular

Our tests demonstrated that in high alpine regions, i.e. above 3000 m, there are thin, weak layers inside the snowpack which have persisted for a long period. The tests showed good fracture propagation upon heavy loading. Caution is recommended especially on NW-N-NE facing slopes. Above 3500m the endangered aspects extend to SW and SE-facing slopes.

Snow profile Grossvenediger. The brunt of the 3 October avalanche,
the weak layer, is indicated by the arrow.

Similar situation on the Zuckerhütl: thin layer of faceted crystals wedged between two melt-freeze crusts. On top of that, a solid ‘slab’ sculpted by wind.

Short analysis of reported avalanches

Avalanche accident Grossvenediger

As reported in the last blog, there was a fatal avalanche accident on the Grossvenediger on 10.10.2020. A winter sports enthusiast was caught in the deadly swathe of a slab avalanche just below the summit and plummeted over rocky terrain. The avalanche was triggered as he traversed a short gradient of about 10 to 20 degrees near the foot of the main slope where the snowpack was shallow.

Avalanche accident Grossvenediger: the primary slab avalanche was triggered near the zone marked. A secondary fracture can be observed below the rocky area resulting from the additional loading of the plummeting snow masses. The person was buried just before the end limits of the avalanche deposit.
(photo: 13.10.2020)

View from the summit of the Grossvenediger to the avalanche zone (now drifted again). The flat bottom of the slope and the plummet zone which follows it are visible.  (photo: 19.10.2020)

Avalanche Zuckerhütl

In early afternoon on 18.10.2020, a report came in of an avalanche on the Zuckerhütl in the Stubai Alps. A large-sized slab was triggered on the northern flank when a person was descending a slope (gradient about 50°). It was at first unclear if persons were buried, since the avalanche swept over the ascending tracks to the Zuckerhütl, so an extensive search was launched. The alarm was halted about two hours later after the Alpine Police exploration and Rescue Squad searches. One person was swept along but not buried, and escaped without injury.

Slab avalanche on the Zuckerhütl. The photo was taken just after the avalanche. The arrow shows the spot of the descent where the skier was when the avalanche was triggered. The circle shows a person who then entered the slope. The avalanche fractured right below that point. (photo: 18.10.2020)

It is evident that the weak layer was widespread and cohesive. The lateral, lower borderline of the avalanche fracture (and thus, the lower limit of the weak layer that unleashed the avalanche) lies at about 3350 m. Glacier ice became visible when the snow masses were swept away. The weak layer was thus also in the zone of glacier ice (visible in the photo above a hard snow fundament, see Zuckerhütl snow profile above). 

Avalanche Tiefenbachferner

Schneebrettabgang Tiefenbachferner, 3100m, NO-seitig

No one was buried in this avalanche. Just like in the other avalanches, the weak layer consisted of faceted crystals and lay atop the rain crust which formed on 03.10.2020, as observed by Peter Raich.

 

Naturally triggered slab avalanches? Almost none.

Hovering above the Main Alpine Ridge, we espied but one single naturally triggered slab avalanche in high alpine terrain, in the Zillertal Alps. These observations coincide with our snowpack analysis: for a weak layer to trigger usually requires heavy additional loading in extremely steep terrain.

Naturally triggered slab near a ridgeline in the Zillertal Alps (photo: 19.10.2020)

On north-facing slopes at high altitude, isolated snowdrift problem in shady terrain

Apart from the old snow problem mentioned above we also discovered a superficial snowdrift problem. This occurs on extremely steep, shady slopes relatively near ridgelines above 2800 m, where older, hardened wind crusts (atop decomposed snow) can be triggered by winter sports enthusiasts. The higher recent temperatures will gradually shift this problem to higher altitudes.

A hard slab fracturing at 2870m near the Kaunertal Glacier (photo: 22.10.2020)

Snow profile from above picture. Very hard surface, then a thin layer of decomposed snow crystals.

On the other hand, freshly generated snowdrift accumulations currently do not trigger easily, since the surface is moist up to high altitudes or else sculpted by wind, making the fresh drifts well bonded.

Fresh snowdrifts in high alpine regions (photo: 18.10.2020)

Otherwise noteworthy:

The snow is melting.

The last few days have been marked by foehn influence and rising temperatures, with corresponding effect on the snowpack: it has started to melt,  become moist up to high altitudes. On south-facing slopes the snowpack is currently isotherm up to over 3000 m, in other words 0 degrees Centigrade.

Weather station data on Pitztal Glacier. Since the last blog, weather has been highly variable with increasing foehn impact.

The red line shows the snow temperature: everywhere zero degrees at 3000 m on south-facing slopes

Solar radiation and mild temperatures: view from Wurmkogel in the Ötztal Alps toward Timmelsjochstrasse (photo: 20.10.2020)

Danger: rocks

Whoever is currently in outlying terrain and not on glaciers needs to master ‘rock skiing’ and beware injuries from the stones jutting out of the snow or lurking just beneath the surface.

Danger of injuries from rocks. Kaunertal (photo: 22.10.2020)

Sahara sand is in the air.

View towards the south from Kaunertal Glacier. The yellowish hue comes from sand blown north from the Sahara. (photo: 22.10.2020)

Overview of Sahara sand plumes  (c) ZAMG

Our next blog will be published whenever there is a significant change in snow and avalanche conditions.

Wintery conditions in some high alpine regions. Latest news: first avalanche victim of the season.

 Temperatures low, precipitation, snow in the mountains

After a particularly rainy August (including first snowfall down to nearly 2000m on 4 August) a warm September followed.

Very early and short-lived winter joys on Stubai Glacier, 5 August (photo: 05.08.2020)

Starting on 22 September, it turned wet again. On 26.09 a massive onset of winter occurred, with snowfall down to low altitudes.

Lots of snow in many parts of Tirol for this early in the season

70 cm of fresh snow at Innsbruck Refuge in the Stubai Alps (photo: 26.09.2020)

Simply too cold for this juncture of the season. Rettenbachtal (photo 27.09.2020)

Due to a stormy foehn phase bringing rainfall up to high altitudes on 2-3 October, most of the snow on the ground melted quickly.

The ellipses mark the precipitation and stormy southerly air current. In the rectangle is the heavy snowfall with dropping temperatures on 25-26.09.2020

It filled the brooks to the tipping point in some valleys, e.g. Gschnitzbach in Wipp Valley, where the flooding record of the century was nearly reached.

Water level of Gschnitzbach © hydro online

Gschnitzbach (photo: 03.10.2020)

The first two weeks of October were cool and variable with several rounds of fresh snow, some of which fell down to low altitudes. There were also strong winds blowing in the mountains.

Wintery in high alpine regions

The instable weather often led to deep winter conditions in high alpine regions, which of course pleased the glacier ski areas enormously. There are already good skiing conditions there.

Skiing operations of Stubai Glacier (photo: 08.10.2020)

Backcountry ski tourers also got a first run for their money after heavy snowfall on 26.09. Our observer Lukas Ruetz researched this bout of unusual snowfall and backcountry skiing in autumn: Worth reading!

On 12.10 we received a report from our observer Thomas Mariacher about a possible backcountry tour (for a short time) up the Figerhorn in East Tirol.

Ski tour with only 20 cm of snow, thanks to the grassy ground beneath it: Figerhorn (photo: 12.10.2020)

Regional avalanche danger in autumn. First avalanche victim of the season.

While we were writing this blog we received the news of the first avalanche victim of the season. A backcountry skier on the Grossvenediger who had been missing since Saturday 10.10 was found dead on 13.10. Just below the summit he was swept off by a breaking slab and plummeted over rocky terrain.

Grosser Geiger towards Grossvenediger. The arrow shows the approximate area where someone was swept away by a slab avalanche on 10.10. (photo: 09.10.2020) 

A photo on tirol.orf.at shows a fracture below the rocky flank. This must have been a secondary fracture due to the additional weight of the snow masses unleashed. In the secondary fracture you can see the glacier ice peeking through, an indication of a weak layer deeper down inside the snowpack. We don’t yet have reliable information about the background conditions of the avalanche. Signs point to a primary fracture below the summit stemming from a weak layer as the cause. (Initial information points to a 75-cm fracture.) This coincides with the avalanche below the Königspitze in South Tirol on 27.09.2020, where two mountaineers were caught. Details can be found in the blog of our South Tirolean colleagues. What is conspicuous is the moist snowpack surface prior to the release and the huge drop in temperature, with snowfall on 26.09. We assume that as a result of this, a persistent weak layer formed very swiftly in accordance with Danger Pattern no. 4 (cold on warm). Both avalanches occurred at nearly the same altitude. The accident area faces southwest, is at about 3600 m.

Glide-snow slides / Glide-snow avalanches

A recurring pattern in early winter following heavy snowfall is seen in the glide-snow slides and avalanches. Atop a relatively warm grassy slope, fresh snow can easily slide off and place persons in danger. Particular caution is called for on hikes below grass-covered slopes right after snowfall.

Numerous glide-snow slides in Ausserfern on 27.09.2020

Arrows mark fracture spots of glide-snow slides above St. Veit in Defereggen (photo: Vitus Monitzer, 12.10.2020)

Loose-snow slides / Loose-snow avalanches

Another recurring pattern: loose-snow slides and avalanches which trigger immediately after snowfall in extremely steep terrain. The intensive solar radiation in early winter can rapidly soften the snowpack surface and destabilize it. This is a type of avalanche which is relatively easy to predict, especially when the snow masses are moist. It places persons in high danger.

Arrows show the loose-snow slides below the Grossglockner due to solar radiation. © foto-webcam.eu

Afterword: As soon as we receive details about the snowpack layering in high alpine regions, as well as information about the avalanche accident, we will publish a new blog.