Current problems: small-spread snowdrifts, old snow
Quite favourable conditions currently reign in Tirol. Avalanche danger is moderate to low today (Friday 21.02.2020).
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| En route in Defereggental. On sunny slopes, springlike conditions often prevail, including corn snow during the descent. (photo: 18.02.2020) |
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| Avalanche danger map for Friday, 21.02.2020 |
We currently have 2 avalanche problems to deal with: a snowdrift problem & an old-snow problem. The snowdrift problem is far easier to evaluate than the old-snow problem.
Snowdrift problem
What matters for the snowdrift problem is to pay heed to fresh snow transported by winds, most triggerable at high altitudes, particularly on shady slopes. The big advantage: if you have experience assessing avalanche dangers on-site, these drifted masses are easily recognized, thus, circumvented.
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| Fresh snow being transported by wind. If solar radiation is diffuse, the drifts tend to bond swiftly with the loose powder snow beneath them in this season. Silvretta (photo: 17.02.2020) |
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| The likelihood of fresh snowdrifts triggering tends to increase with ascending altitude. Snow plumes in the Venediger Group (photo: 18.02.2020) |
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| The most recent past: sometimes variable, often sunny, inordinately warm, frequently windy. |
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| Similar situation on Patscherkofel. Visible in the graph: the generally low level of precipitation. |
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| Cold fronts brought only small amounts of fresh snow. |
Old-snow problem
The old-snow problem is more complex, and needs to be examined in more differentiated ways. These are faceted, loosely-packed snow crystals which lie deep inside the old snowpack and potentially constitute a weak layer for slab avalanches. However, the old-snow problem is not widespread; it is limited to little-tracked, very steep to extremely steep backcountry touring terrain.
On the one hand, we are dealing with expansive metamorphosis of snow launched by the danger pattern “cold on warm.” This process began with the rainfall on 3-4 February. The weak layer is not the same everywhere. Most often, our snowpack analysis uncovered significant layers at altitudes around 2300 m on north-facing slopes. Since this zone currently has little snow, the necessary ‘slab’ for a slab avalanche is lacking. Thus, there are relatively few avalanche prone locations.
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| Layer of faceted crystals between two rain crusts. The rain crusts formed on 3 February due to a briefly fluctuating snowfall level. The layer of faceted crystals subsequently formed between the crusts. |
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| Visible in this snow profile from the Lower Inn Valley: only a partial fracture could be generated in the relevant layer. We received repeated reports particularly from the lowlands about settling noises, especially in flat terrain above the treeline, which emanated from this layer. |
We also found faceted crystals as a result of the extended period of fine weather prior to 28 January, particularly in W/N/E aspects. Due to the warmth which has prevailed since then, we assume that these layers can only be triggered in W/NW to N to E/NE facing terrain between 2600 and 3000 m. The prerequisite: large additional loading in zones where the snow is shallow or in transitions from shallow to deeper snow.
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| Thin faceted layer between two melt-freeze crusts at 2875 m on extremely steep NW slope in Silvretta. The lower melt-freeze crust formed until 18.01, the upper until 28.01. Two persons were in the immediate vicinity of this snow profile when a slab avalanche triggered. |
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| Avalanche for the above profile. Snow profile location and location of persons when the avalanche triggered are circled. Haagspitze - Silvretta - Bieltal side (photo: 17.02.2020) |
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| Fracture point of the avalanche on 17.02. Haagspitze including descent track. The avalanche released when one person was in the upper part of the fracture zone. (photo: 17.02.2020) |
Apart from this avalanche, we are aware of only one other avalanche due to the old-snow problem (despite the marvellous weather last weekend, 15-16 February). This underscores our assumption that there are relatively few danger zones where avalanches can be triggered in the old snow. This one was in freeriding terrain of the Sölden ski area on a very steep east-facing slope at 2500 m. This avalanche also caused no injuries or damages.
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| Avalanche on 16.02 in freeriding terrain of Sölden Ski Area (photo: 17.02.2020) |
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| Entry track for avalanche on 16.02 in freeriding terrain of Sölden Ski Area (photo: 17.02.2020) |
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| Quite typical of the current situation in steep sunny terrain: sequence of hard crusts and soft layers near the surface, also in varied form in middle part of the snowpack. The soft layers are frequently riddled with frozen melt-freeze canals and blanketed by several hard layers. Thus, little likelihood of triggering. |
Short review of last week’s weather
As an addendum to the above weather graphs, here a verbal description of the weather by our apprentice Marco Knoflach:
“At the beginning of last weekend (15-16 February) an intermediate high took shape which brought us springlike conditions in the middle of February. As a result of the westerly-to-southwesterly air current, very dry and mild air masses were fed our way. The zero-degree level ascended to 3500 m on Sunday, 16 February. In addition to the very mild air, strong foehn winds in the classic foehn-exposed wind lanes made the mercury climb a further notch. Highest temperatures released by the ZAMG Weather Service for the period 15-17 February were approximately 10 degrees above average. On Monday night (18.02.2020) a cold front swiftly moved in, and in its wake came drier and colder air masses. On Wednesday (19.02.2020) the next cold front passed through, before a brief intermediate high calmed the conditions today (20.02.2020).”
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| A swiftly moving cold front passed through this week. View from Senderstal towards the northwest. (photo: 19.02.2020) |
Weather and Avalanche Outlook
“According to ZAMG Weather Service, on Thursday night (21.02.2020) a weak cold front will brush past us, lowering temperatures, bringing brisk-to-strong winds and light showers. On the coming weekend, an outlier of the high over the Azores will generate the weather, again creating springlike conditions.”
