Avalanche Bulletin Thursday, 01.05.2014, at 19:30
Caution! Increasing avalanches in high and high-alpine regions
Avalanche activity has risen markedly: at high altitudes and in high alpine regions the danger level is frequently considerable (3). Below about 1800 m, as well as in northwestern regions, the danger level is frequently low. In case of intense, diffuse solar radiation or rainfall (which makes the advancing wetness of the snowpack drastically increase) even a "tense" level 3 could be reached up to high altitudes. Increasingly frequent avalanches during the last 3 days corroborate this heightened alarm level. Generally, wet avalanches have released immediately following the snowfall on extremely steep slopes, to some extent medium-sized, in isolated cases even large-sized slab avalanches, particularly on W to N to E facing slopes above approximately 2500 m. Large additional loading is generally necessary to trigger large-sized slab avalanches, e.g. the impulse of a loose avalanche or small, superficial slab releases.
The snowpack is deteriorating massively and rapidly. However, at high altitude and in high alpine regions, particularly along the Main Alpine Ridge and south thereof, there is still a great deal of snow which is becoming increasingly wet down to the ground by variable April weather conditions. Faceted layers near the ground which until now were quite stable are swiftly becoming prone to triggering. In addition, graupel grains have been deposited repeatedly over the last few days which have formed a seriously weak layer on the uppermost surface of the snowpack, especially above approximately 2500 m.
Alpine Weather Forecast (ZAMG-Weather Service Innsbruck)
Dense cloud and moist air masses have reached Tirol. Particularly in the Tirolean uplands and in the Vinschgau, Friday was heavily overcast accompanied by repeated rain showers throughout the day, the snowfall level hovering around 2000 m. Between Brenner and Innsbruck and eastwards thereof, as well as in East Tirol, it will still be dry during the morning, accompanied by some bright spells and a bit of sunshine. In the afternoon, rain showers will reach there as well, along with a few thunderstorms. On Friday night, the snowfall level will drop down to 1500 m in North Tirol. Lowest temperatures will be 5 to 9 degrees; highest 10 to 15 degrees; in Lienz, 18 degrees; in Bolzano, 17 degrees.
Short Term Development
Generally unfavourable avalanche situation at high altitudes and in high alpine regions to begin with. In case of major changes, updates will be published.
[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) 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) 9 - graupel blanketed with snow
In avalanche instruction courses, weak layers inside the snowpack are often compared with ball bearings. Yet this image is suitable only for graupel: a ball-shaped form of precipitation which is deposited in thunderstorm-like showers particularly in springtime. It is easy to grasp that snowdrift which collects on top of it is usually inadequately bonded with it, and the avalanche danger thus escalates. Graupel is often spread over small areas and is very difficult to spot, even by experts, without looking inside the snowpack. It is a thoroughly treacherous situation, which fortunately causes problems only for short periods of time.
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.