Avalanche Bulletin Friday, 07.03.2014, at 07:30
Widespread moderate danger, beware fresh, high alpine ridgeline snowdrift
Avalanche danger in Tirol's backcountry touring regions is predominantly moderate, to some extent considerable along the Main Alpine Ridge. The major hazards stem from fresh snowdrift accumulations which are quite prone to triggering and can in some places be released even by minimum additional loading. Avalanche prone locations are found in high alpine ridgeline terrain more than anywhere else, particularly in W to N to E facing aspects. Over the course of the day the snowpack will lose some of its firmness due to solar radiation. Particularly on sunny slopes below about 2500m, naturally triggered superficial loose avalanches can be expected. In East Tirol where snowfall has been heaviest, gliding avalanches are still a threat.
High altitude easterly to northeasterly winds blew at moderate strength yesterday, but in high alpine ridgeline terrain were frequently above transport velocity, thus giving rise to fresh, generally small-sized snowdrift accumulations. Since these drifted masses were often depositied atop loosely packed new fallen snow, they are relatively easy to trigger. The old snowpack has consolidated well, it is stable, area-wide weak layers are lacking. As of late morning today, the snowpack will forfeit much of its firmness, due to daytime warming and solar radiation.
Alpine Weather Forecast (ZAMG-Weather Service Innsbruck)
Weather: Tirol lies in the path of a powerful high pressure front centered over Russia, amidst extremely dry air masses. Over the next few days, this high will strengthen still further. Mountain weather today: Superb mountain weather conditions will prevail, maximum sunshine, good visibility, the zero-degree level just over 2000m. High altitude clouds will move into eastern regions this evening. Northeasterly winds will be brisk. Temperature at 2000m: +1 degree; at 3000m: -5 degrees. Light to moderate easterly to northeasterly winds this morning, becoming brisker this afternoon.
Short Term Development
Avalanche peril moderate, a daytime rise in danger level
[Author: Rudi Mair]
[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) 6 - cold, loose, new fallen snow and wind
"Wind is the architect of avalanches" this classic adage of Wilhelm Paulcke from the 1930s still has unaltered validity today. Wind influences both falling snow and already deposited snow, it is one of the major formative factors in potential avalanches. If the snow is loosely packed and dry, wind always leads to its transport, thus increasing the danger of avalanches. The colder the transported snow is, the more sensitively it reacts to additional loading, since it becomes even more brittle. This danger pattern is distinct from GM.5 in that the cold, loosely packed snow has not formed over a prolonged cold period, but in a very short time. That means, it either snowed just previous, in low temperatures without wind, and subsequently the wind begins to blow; or it begins to snow without wind, and the wind increases in velocity during the snowfall. This pattern is easy to recognize as a rule.
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) 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.