Avalanche Bulletin Wednesday, 23.04.2014, at 07:30
Heed daytime danger cycle. End of daily avalanche bulletins
Avalanche danger levels are contingent on altitude: above approximately 1600m the danger is moderate; below that altitude, generally low due to lack of snow. There are currently only few danger zones left, slab avalanches which can release by large additional loading are most likely in extremely steep, shady terrain at 2400-2600 m where snow is shallow. Loose avalanches can be triggered by skiers only in craggy terrain where the snowpack is wet. Gliding avalanches are indicated by glide cracks in the snowpack surface, likeliest on steep, grassy slopes in southern East Tirol where snow depths are greatest. Danger zones are creeping to higher altitudes, increasingly in shady terrain where the snow base is wet for the first time this season. After nights of clear, dry skies, the situation is always more favourable than following wet, cloudy nights.
The snowpack’s outgoing nocturnal radiation was adequate after midnight especially in eastern regions, permitting a breakable crust to form, higher up a crust capable of bearing loads. The variable April weather conditions will swiftly moisten the snowpack again today. Significantly weak layers where slab avalanches threaten are rare, most likely on shady slopes at 2400-2600 m, where a layer near the ground, formerly of faceted crystals, could release with increasing wetness. As throughout the winter, in northern regions below-average snow depths prevail, particularly sunny slopes are bare to above the treeline. In southern East Tirol, snow depths are above average for this juncture of the season.
Alpine Weather Forecast (ZAMG-Weather Service Innsbruck)
Mountain weather today: Sunshine in most regions during the morning; as of midday, convective cloud build-up is anticipated, bringing rain showers and thunderstorms to the Northern Limestone Alps in particular, as well as the upland peaks. It will fall as snow or graupel as of about 2300 m. On the Main Alpine Ridge and the southern flank of the Alps conditions are somewhat more stable, the likelihood of showers is lower. Temperature at 2000 m, +4 degrees; at 3000m, -2 degrees. Light easterly winds, occasionally brisker due to local circulation and showers.
Short Term Development
The daily avalanche bulletins come to an end today, 23 April. In case of major changes, updates will be published. We extend warm thanks to all those who assisted us with their valuable observations, news and reports over the course of the season.
[Author: Patrick Nairz]
[Translated by Jeffrey McCabe]
Current danger pattern(s)
Experience has shown that even over the course of highly varied winters, nearly identical potential-avalanche scenarios repeatedly arise as recurring danger patterns and are responsible for the greater part of avalanche accidents. An analysis of these patterns was published in the practical handbook "Avalanche - Recognizing the 10 Decisive Danger Patterns" by Rudi Mair and Patrick Nairz in November 2010.
Danger pattern (dp) 10 - springtime scenario
A particular challenge for backcountry skiers and boarders (and for avalanche analysts as well) arises in springtime. Rarely do situations considered "safe" and those considered "unsafe" occur so close together in time. And never is the spectrum of danger levels in a daily cycle as divergent as in spring. On the one hand, the avalanche danger is easy to assess in conditions of stable firn snow; on the other, there are never as many large avalanches registered in the course of a winter as during critical springtime situations.
Apart from the snow layering, a complex interaction of air temperature, humidity, solar radiation and wind exerts an enormous impact. For skiers and snowboarders, clocktime-discipline and flexibility in planning backcountry tours take on preeminent importance.
Danger pattern (dp) 1 - deep persistent weak layer
Following the first snowfall of a given winter, full depth snowslides, i.e. avalanches which slide down across steep, smooth slopes, are the major problem. After the second heavy snowfall, increasing numbers of slab avalanches then occur. These are the typical skier avalanches and are responsible for at least 95% of all avalanche fatalities. The reason the second snowfall is so important is that in the interim between the initial snow base and the second snowfall a distinctly weak layer often forms which can be easily triggered by skiers and snowboarders. Situations of this type generally occur at high altitudes (>2000 m) and in high alpine regions (>3000 m) on steep, shady slopes. This deep persistent weak layer can cause problems during the whole winter.
Danger pattern (dp) 2 - sliding snow
Snowslides are usually unleashed down towards the valley across steep, smooth slopes. Before they are released glide cracks form, i.e. easily visible fissures in the snowpack, often several meters deep. Quite opposed to an age-old belief which is still difficult to dispel, such glide cracks are now known to be not favourable signs, but on the contrary, thoroughly unfavourable harbingers of full depth snowslides. A glide crack points to the possibility of a full depth snowslide, though gives no indication about whether a snow mass will actually be triggered as a full depth snowslide and, if so, when. Full depth snowslides are among the most difficult types of avalanche to predict, in terms of their time of triggering, because they can be released literally at any time of day or night even in generally stable snow conditions, on the coldest day of winter or the warmest. Furthermore, full depth snowslides are not unleashed by additional loading.