Avalanche Bulletin Wednesday, 01.05.2013, at 07:30
Final scheduled avalanche report of the season
The avalanche danger on 1 May is contingent on altitude: above approximately 1800m the danger level is moderate, below 1800m it is low, since the slopes have become exceedingly bare of snow. The major hazard is for backcountry skiers and freeriders to trigger wet, loose snow avalanches, most frequently above approximately 2300m in extremely steep terrain where the snowpack surface is wet. Caution is urged towards cornices, which also fracture and trigger avalanches. Slab avalanches can be released only in rare cases, particularly on steep, W to N to E facing slopes above approximately 2400m, usually through large additional loading, e.g. from a wet, loose snow avalanche. Full depth snowslides are possible in isolated cases on steep, grassy slopes. For that reason, avoid areas where glide cracks appear in the snowpack surface. Conditions above approximately 2800m, where winds have cooled the snowpack and exercised a positive influence, are more favourable.
At low and intermediate altitudes, the slopes are rapidly becoming bare of snow. A cohesive, area-wide snowpack is now seldom. The snow cover is at least superficially moist to over 3000m, becoming thoroughly wet down to the ground with descending altitude. Below about 2300m the old snowpack is settled and well bonded, in the process of transforming to firn snow. Weakly bonded layers threatening slab avalanches are rare, at best where snow is shallow and a mixture of melt-freeze snow and old depth hoar is evident near the ground.
Alpine Weather Forecast (ZAMG-Weather Service Innsbruck)
Weather: The Alps remain in the path of a warm southwesterly airstream bringing foehn tendencies to North Tirol. Over the weekend, a low pressure zone will reach us from the southwest and bring precipitation. Temperatures will recede somewhat. Mountain weather today: heavily overcast, a few rain showers in western regions, snowfall level at 3000m. The sun will come out later on in North and East Tirol, but it will remain windy. Towards evening in the Lechtal Alps and south of the Main Alpine Ridge, a tendency towards thunderstorms. The night will be dry but cloudy, clear skies seldom. Temperature at 2000m: plus 10 degrees; at 3000m: plus 1 degree. Moderate to strong velocity southerly winds, in the Tux Alps winds will blow at storm strength.
Short Term Development
Forecast: Conditions will improve if the nights are clearer and cooler than last night, they also improve with ascending altitude. Heed the daytime warming cycle which creates fluctuating avalanche danger each day. Good opportunities for backcountry tours can also be found on high altitude ski runs where winter operations have terminated. Thank you for the numerous reports on conditions throughout this last winter 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 (gm) 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 (gm) 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.