New weak layers possibly forming
The recent instable weather with its immense temperature fluctuations is giving us a headache. The reason: it provides perfect prerequisites for Danger Pattern "cold on warm" (dp.4). This syndrome is known for forming weak layers inside the snowpack after a certain post-snowfall period. As of today, 15.04.2021, we have evidence which points to this threat: in the Glockner Massif, settling noises were reported on sunny slopes above 2700m. On Tschadinhorn in the Schober Massif a southwest-facing gully triggered a 100m long and equally wide slab avalanche at 2700m due to additional loading. Not far from Tirol, in the Sesvenna Massif, signs point to a near-surface weak layer being particularly prone to triggering, particularly at high altitudes on sunny slopes. Recollections of a similar situation come to mind.
Evidence is still lacking to what extent shady terrain is being visited by this development. The fact is, at least the snowpack on north-facing slopes was moist up to high altitudes last weekend and was blanketed over by cold fresh snow.
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A slab triggering due to loose-snow avalanche. Type of weak layer unknown.
2700m north, Lüsener Villerspitze |
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| Since the beginning of this month, alternating warm and cold phases have dominated. |
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| Marked temperature disparities this week. To start with, cold; then warm as snowpack surface moistened; then cold again with the snowfall. Above, one of the stations with the greatest amounts of fresh snow measured. Franz-Senn-Refuge in the Stubai Alps. |
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| Similar in southern regions. There, winds were stronger. Zischke station im Defereggental |
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| 24h-differences in snow depths on 12–13 April 2021 |
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| One of the spots where snowfall was heavy last week: Halltal in the Karwendel. (photo: 14.04.2021) |
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A frequent companion during snowfall: graupel. Gleirschtal in northern Stubai Alpls.
(photo: 12.04.2021) |
Problem zones in near-surface layers
In order for a slab to form you need not only a weak layer, but also a bonded snow mass on top of it. The latter is found frequently at high altitudes due to short periods of wind impact, but also on sunny slopes where the snowpack slowly settles and condenses due to solar radiation in spite of low temperatures. What is currently important: we are faced with near-surface problem zones. Potential weak layers: either faceted crystals (due to dp.4, briefly loose, blanketed powder) or massive layers of embedded graupel locally.
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In this profile the slab is lacking. The fresh snow is loosely-packed. Potential weak layers:
graupel and the faceted crystals near the surface (above arrow). |
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| A quite thin slab above a thin, loose weak layer (arrow symbolizes the potential near-surface problem zone). Also interesting, the temperature leap near the surface: short solar radiation led to snowpack moistening a few cm beneath the very loose surface, so-called “radiation recrystallisation.” |
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| Main concentration in marked zone. Easily visible: the layer of Sahara dust from the beginning of February. Jamtal, Silvretta. (photo: 14.04.2021) |
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Indicative of ridgeline high-altitude terrain: wind impact with snowdrift accumulations
on both shady and sunny slopes. |
Still lots of good powder
Despite the above considerations and developments which are devilishly hard to evaluate, we also have to focus on the fantastic powder snow of last few days.
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| A powder dream-come-true in the Gurgler Massif. Hinterer Seelenkogel (photo: 14.04.2021) |
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| Similar situation in the Jamtal (photo: 14.04.2021) |
Numerous loose-snow avalanches
Loosely-packed powder snow reinforces a heightened likelihood of loose-snow avalanches triggering at this juncture of the season. The below-average temperatures slow down this activity somewhat, but numerous loose-snow avalanches were observed particularly on 13.04 and 14.04.
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| Deposits of freshly triggered loose-snow avalanches. Northern Stubai Alps (photo: 14.04.2021) |
Caution: cornices
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Another danger lurks in ridgeline terrain: cornices. Do not underestimate them! Grossglockner
(photo: 15.04.2021) |
Outlook
It will remain cool. An intermediate high will bring sunshine on Friday, 16.04, and Saturday, 17.04, at least. Thereafter a northern air current will keep things unstable. Initially, not much will change in the avalanche situation. We assume quite favorable conditions at least at low and intermediate altitudes; and a treacherous situation at high altitudes from place to place. Solar radiation will reinforce bonding of the near-surface layers.
Incidentally, stability tests of near-surface layers conducted by persons who know how to conduct them can help us track down weak layers. We are interested in such observations. Please send the results (including altitude) directly to our headquarters: lawine@tirol.gv.at - Thank you for your support.
Only a few lifts are still open.
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Dream-come-true conditions, such as here in Kühtai ski area, open until Sunday, 18.04
(photo: 14.04.2021) |
