Snowpack becoming increasingly wet - Likelihood of naturally triggered avalanches rising
Over the last few days it was quite warm, amid variable weather conditions. The snowpack became ever moister, also at high altitudes, thoroughly wet even down to lower layers. In the southern regions this was not the case, due to temporarily poor weather. It was similar in parts of North Tirol last night (28.04-29.04) where outgoing longwave radiation of the snowpack was severely reduced regionally.
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| Web camera photos aid evaluation of nighttime cloud cover |
Avalanche releases have been limited. Mostly they were (isolated) wet loose-snow avalanches.
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| Loose-snow avalanxches near Zuckerhütl in the Stubai Alps (photo: 23.04.2021) |
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| On Sunday, 25.04, warm temperatures made the snowpack on shady slopes thorougly wet. Thereby, some loose-snow avalanches were activated in extremely-steep terrain. (Griesskogel Massif) |
Since yesterday (28.04) there have again been naturally triggered avalanches observed. This seems to be a sign that naturally triggered avalanche activity will increase in the next few days. This will strike the shady slopes most of all, particularly where the snowpack is not yet thoroughly wet. Currently we foresee an altitude band at 2400 m with certain disparities upwards and downwards. But there is potential for slab avalanches also on sunny slopes. Fractures could occur increasingly in the near-surface layers. There, a melt-freeze crust, which is deposited atop loose, wet, melt-freeze forms, could provide the requisite slab.
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| Naturally triggered slab avalanche at 2300m, in rear Pitztal near a moraine (photo: 28.04.2021) |
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| Visible in the photo: melt-freeze crust still capable of bearing loads, beneath that a thoroughly wet, loose layer: a potential weak layer for slab avalanches. Kaunertal. (photo: 28.04.2021) |
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| Glide-snow avalanche on shady slope at 1600m, Tannheimertal (photo: 28.04.2021) |
Snow quality is currently poor, by and large. Only above about 2700/2800m does it improve.
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| A thin line between “fracture” / breakable melt-freeze crust / capable of bearing loads. Spots where you break through are becoming more frequent. (photo: 28.04.2021) |
Short review
Last week was truly April weather, fully variable. In North Tirol the conditions were more pleasant than in East Tirol - due to foehn influence. It was really good on the weekend, in particular on Saturday, 24.04, when a short thrust of dry air penetrated the land. In spite of warmer temperatures the snowpack was hardly weakened. Favorable conditions dominated, complete with daily daytime avalanche danger cycle.
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| The best day last week was surely Saturday, 24.04. The dry air masses are visible. The snowpack had adequate outgoing longwave radiation during the nocturnal hours. In the interim the snowpack has become wet, melting point 0°. Water can now seep unhindered into the lower layers. |
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| A similar picture from Eselrücken in northern East Tirol. |
According to ZAMG Weather Service forecasts, the month of April numbers among the coolest Aprils since 1997 and is one of the eight driest in the history of weather measurement (since 1858!).
Outlook for the next few days
Between today, 29.04, and tomorrow, 30.04, a weak cold front will traverse the land, according to ZAMG Weather Service. Temperatures will drop about 1-2°. In the mountains a strong-velocity foehn wind will prevail in the foehn lanes. Variable conditions will persist on Friday, 30.04. It could prove to be a ‘steamy’ day with lots of diffuse radiation which will reinforce the process of snowpack moistening. On 01.05 conditions will part ways: pleasant in the north, less so in the south. Still mild. On Sunday, 02.05, a cold front will send temperatures significantly lower. Snowfall level in the southern regions will be higher than in the north, between 1200 and 2000 m. Fresh snow: about 20-30 cm in the mountains. The wet-snow threat will briefly recede. But at high altitudes, mostly near ridgelines, a snowdrift problem will prevail, even if limited.
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| 12-hr fresh snow forecast 29.04. - 30.04.2021 |
