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Summary
The outstanding weather forecast for the weekend permits us to look forward to making good use of the stable snowpack.
Influential weather factors of recent days were the cold fresh snow plus wind; fluctuating temperatures; heightened solar radiation; and intermittently strong settling of the snowpack. Depending on altitude and aspect, these factors became more, or less, relevant.
Review
The latest snowfall of Monday (25.03) and Tuesday was accompanied by plummeting temperatures (15 degrees lower) and regionally strong-velocity winds. Trigger-sensitive snowdrifts were generated which were predominantly small, easily recognized, deposited behind crests and in gullies and bowls.
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| A few centimeters of fresh snow fell on Monday, 25.3, and Tuesday, 26.3, amidst cold temperatures and regionally strong winds, as here on the Plattkopf in the Zillertal Alps. |
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| Beneath melt-freeze crusts, weak layers often lurk which can serve as a fracture surface for slab avalanches. Here in Navistal, settling noises on south-facing slopes were heard and a slide remotely triggered. (photo: 24.03.2019). |
Loose-snow avalanches can be either dry or wet; the latter type often occurs in combination with daytime warming. In the afternoon, the danger of being swept along by the snow also increases in steep, rocky terrain.
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| Loose-snow slides are a dominant picture every spring. At the moment, they are small. In photo from Härmelekopf, the negative factor is that these slides can cover huge distances; the positive factor is slabs cannot be triggered by them. (photo: 24.03.2019). |
Gliding snow activity has receded and since the last major rainfall at mid-month only small areas have been set into motion. Like with other ongoing alpine dangers however, this does not mean they cannot become threats in each individual case.
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| Gliding snow release in Northern Massif, shot from a web cam on 24.03.2019. |
The good conditions were taken advantage of during the last week, permitting ascents and descents on steep routes. The cold fresh snow was deposited atop a stable and irregular surface.
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| Gully on north-facing slopes in the Stubai Alps used for the ascent to 3000 m and the descent. (photo: 23.03.2019). |
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| Man and dog get their reward when the fresh snow can be taken advantage of in good time, as here in Sellrain. (photo: 26.03.2019). |
Since the problems referred to are limited spread, the map of the Avalanche Bulletin is full of green and yellow spaces. In other words, the weekend promises to be splendid. For tour planning, springtime conditions are the necessary axis of orientation, wintertime problems are withdrawing to the high alpine regions.
Snow situation
Thanks to snowfall at the beginning of the week, we are in positive zones as regards the 72-hour overall sum of fresh snow in northern Tirol (up to 30 cm in Lechtal and Karwendel), while the balance around the Main Alpine Ridge is negative.
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| Snow distribution of last precipitation. In distinction to the last blog, the fresh snow outdistanced the snowmelt. |
In the valleys, spring has made its entry. It means that skis have to be carried on your back on ascents which begin at low altitudes. Don’t forget that it is still winter up at higher altitudes, complete with immense masses of snow.
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| Rapidly accumulating amounts of snow with ascending altitude, e.g. at the interface of Northern Massif and Innsbruck, bear witness to the highly distinct worlds of summer and winter. |
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| Striking for this winter: the north/south distinction. In Ausserfern the snow depths are far above average, while the reserves from February in East Tirol have already evaporated. |
Moisture
Model calculations demonstrate a very homogenous picture of snow temperature. These can only be interpreted as an indicator, since the calculation is coupled to air temperature.
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| The model of average snowpack temperature calculated by SNOWGRID reaches the freezing point over far-reaching areas. |
Thorough moistening of the snowpack proceeds apace, naturally dependent on altitude and aspect. On sunny slopes the snowpack is isotherm up to high altitudes, especially on steep south-to-west facing slopes where the snow is shallow. On shady slopes and in general at high altitudes, there is still powder and loosely-bonded snow.
Stations where the snowpack surface temperature is measured indicate when the snowpack becomes thoroughly wet. The measurement is technically delicate, however, and subject to error when the snow on the surface becomes warm.
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| The surface temperature nears the maximum of 0 degrees for ever-longer periods, as here NW at over 2100 m; in other words, snowmelt. As a result of the cold front starting on the 25th, the snowpack surface was repeatedly cooled. However, it is starting to climb again and will reach 0° on the weekend. |
Ordinarily, we refer to an isotherm snowpack (continuing constant temperature) when the temperature remains constantly at zero degrees. The snowpack then has no further buffer of cold and becomes throughly wet. Nocturnal outgoing longwave radiation can cool the snowpack, but only from the surface. This buffer (the zone of negative snow temperature between surface and ground) is currently a perfect illustration of the varied conditions.
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| Profile 1: W, 1900 m, 30°, Karwendel. The snowpack is isotherm and thoroughly weakened by moisture (©LWD Tirol). Profile 2: E, 1900 m, 25°, Verwall. The sun was sufficient to generate crusts, but insufficient to warm the whole snowpack. Nonetheless, there is no longer much leeway. (©Fun). Profile 3: N, 2700 m, 30°, Stubai Alps. Low air and snow temperatures in shady high alpine terrain. (© Lukas Ruetz). In red: the snow temperature. |
Outlook for the weekend
According to ZAMG weather forecasts, residual clouds will disperse on Thursday, and the nighttime skies will often be clear. Following a chilly Friday morning with high fog in some areas, it will turn warmer during the daytime. On Saturday it will be springlike, with the zero-degree levels at about 2700 m. On Sunday, some light clouds are expected which could become dense and generate showers on Monday.
The nights will be clear, which means the snowpack will have strong outgoing radiation and freeze on the surface. This effect is countered by high air moisture (see illustration below) and the forecast inversion on Saturday, i.e. when the atmosphere releases more warmth to the earth’s surface).
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| Following a phase of low air moisture which enhances direct condensation of snow into water, since Monday (25.3) a phase of high air moisture has followed. This enhances snowmelt without condensation to the surface, on the one hand; and on the other, releases energy to the snowpack like clouds and amplifies the classic greenhouse effect. High air moisture effectively counters nocturnal outgoing radiation. |
All in all, the daytime danger cycle of fluctuating dangers needs to be given greater attention at low altitudes and on sunny slopes. That means avoiding the breakable crusts. At higher altitudes, isolated snowdrift accumulations might be triggerable until greater settling has taken place. Caution is urged wherever these drifts are veiled by loosely-bonded slides. The snowpack layering is highly varied. Careful planning can increase the enjoyment.
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| Shortwave solar radiation is the greatest energy input into the snowpack, it can swiftly react to temperature limits. Shown here on the Schwarzhorn before the latest snowfall. (photo: 24.03.2019). |
(This blog was generated by our practicing apprentice Michael Reisecker.)
