Difficult avalanche prognosis
We are at a point in the season where forecasting avalanches is particularly challenging, largely because even miniscule divergencies in meteorological / nivological parameters can have astoundingly big effects on avalanche danger. Specifically, this is the current rostrum of factors which interplay:
- Nocturnal skies overcast
- Clouds forming at various altitudes during the day
- Local rain showers
- High air moisture
- (diffuse) Solar radiation
- Changing air temperatures
- Changing temperature of snowpack surfaces / thawing point
- Temperature reserves inside snowpack plus water seepage into snowpack
- Snowpack layering highly variable in different aspects and altitudes
Currently, we anticipate for Good Friday (02.04.2021) that nighttime skies will be largely overcast, local showers likely. The snowpack will not be able to cool sufficiently. Conditions will already be unfavorable in early morning. On shady slopes at high altitude it will be better. The trouble is, you can’t get there without passing through an unfavourable zone....
Further to the south, on the other hand, nighttime skies are clearer, lesser tendency to showers. But there, too, conditions are truly favorable only at high altitudes, since the melt-freeze crusts which form during the nights are breakable due to the already massive water seepage.
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| View from Meiler Refuge in Karwendel, looking north. Cloud cover on the increase. (photo: 01.04.2021) |
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| View from Zugspitze, looking south. Less cloud cover moving in. (photo: 01.04.2021) |
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| On 1 April, rise of thawing point towards 0°C, but with flattening curve of snowpack surface temperature. Both together are a sign the snowpack is becoming thoroughly wet. |
Wet-snow avalanche activity. Observations in backcountry.
Recent days lay under the spell of wet-snow avalanches and on-site snowpack analysis. We were spared a massive cycle of wet-snow avalanches, but every single day naturally triggered avalanches were reported or observed by us. Slab avalanches, for example, triggered nearly exclusively in near-surface layers (snowfall from mid-March). What stood out: the high proneness to triggering of the snowpack in steep terrain wherever it was thoroughly wet. Also conspicuous: such danger zones were very widespread in outlying terrain. These striking criteria were mirrored in the ongoing recent danger forecasts.
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| Naturally triggered slabs of recent days: in Weisskugel Massif (photo: 01.04.2021) |
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| Avalanche path in Pollestal. Also visible: a glide-crack (photo: 01.04.2021) |
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| Kaserer Winkel in western Tux Alps. Photo: 7:06 am, 1 April 2021 |
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| Kaserer Winkel in western Tux Alps. Photo: 12:14 pm, 1 April 2021. The naturally triggered wet-snow avalanche triggered at 2200m in E-NE aspect. |
On-site snowpack analysis
Sunny slopes
On sunny slopes the snowpack’s heightened proneness to triggering in near-surface layers was confirmed.
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| Arrow points to near-surface weak layer, thoroughly wet, at 2215m, S, in Silvretta |
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| This slab in the Weisskugel Massif was triggered by a skier in a near-surface weak layer of faceted crystals beneath a crust (photo: 01.04.2021) - very similar to the profile above. |
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| A “crust sandwich” on a south-facing extremely steep slope at 2850m, northern Stubai Alps. The lower very thick crust initially has a stabilizing effect, but loses firmness due to water seepage. |
Shady slopes
Since shady slopes were not yet thoroughly wet (and thereby weakened) during the massive avalanche cycle around 23 February, we concentrated on the approaching water seepage on shady slopes. At high altitudes there are still sufficient reserves of cold. Below about 2200m, heightened proneness to triggering is very possible in northern regions locally on shady slopes.
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| Important: temperature reserves. Apart from near-surface layers, the snowpack is isotherm nearly everywhere. North, 2150m. Arrow points to possible weak layers, triggerable by water seepage. |
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| Photo for the profile above. Lukas, one of our civil service helpers, points to two potential weak layers with water seepage (photo: 01.04.2021) |
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| Shady slope, 2740m, snowpack still cold, stable at this spot |
Snow quality
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| Typical of the season: swiftly alternating snow quality with altitude and aspect. Everything is on offer: superb firn, still great powder, very hard surfaces, more melt-freeze crusts and, of course, a thoroughly wet snowpack in the afternoon. (photo: 30.03.2021) |
Outlook
According to ZAMG Weather Service, the influence of the high-pressure weather front is diminishing. While in northern regions some rainfall is scheduled, southern regions will be more pleasant. Temperatures are gradually dropping. The wet-snow threat will recede significantly by Holy Saturday at latest. Following a very sunny Easter Sunday, and partly sunny Easter Monday, a cold front from the northwest will reach us and bring fresh snow....
The team at Avalanche Warning Service Tirol wishes everyone a Happy Easter!
