First of all, welcome to the new weekly column at PowderGuide! In SnowFlurry, we won't just be poking around in the snow: the priority will be on currently observable processes in snowpack development and their impact on avalanche risk - illustrated with snow profiles and their interpretation. We will often use modeling and extreme simplifications or hyperbolic language to get to the bottom of the matter - in order to focus more on the essentials. There will also be a sporadic look at other departments of Winter and Brettlrutschen. The Schneestöberer also reserves the right to include provocative or cynical opinions and welcomes lively discussions. At the end of each SnowFlurry we will find a mnemonic - simply to memorize. That's it for the general blah-blah. Today we look at the avalanches in the first breaths of the season.
The initial situation
In October, a total of five people lost their lives in two avalanche events in the Zillertal Alps. The Tyrolean daily newspaper ran the headline "Avalanche danger in Tyrol: situation is not positive". The headline "What's behind the avalanches in October?" beautifully illustrates an unfortunately recurring opinion that often links avalanches in German-speaking countries to the months of December to April. Both accidents are basically due to a chain of unfortunate circumstances. Particularly in the case of the one on the Olperer, hardly anyone would have expected that a weak layer could have formed here without prior snowpack investigations, so that a break in it could propagate so far - nevertheless, avalanches are possible at this time of year. Added to this was the fresh drift snow and the extreme steepness of the northern slope. This brings us to the avalanche problems that are primarily problematic in autumn:
New snow & drift snow
"Feel & think like an avalanche" is still one of the best premises for assessing avalanche danger with existing background knowledge, even without a situation report or bulletin. Although it is still (or: already) green in the valleys, this has little or nothing to do with the condition of the snow and its affinity for mass migration at higher altitudes. Those who live at over 1500m (like the snowstormer) find it a little easier to "put themselves in the shoes of snow and avalanches" because they are closer to the action.
Nevertheless, you should ignore the spring fever or autumn depression of the valley dweller as much as possible and use webcams, weather stations, a good weather report and of course the WeatherBlog to get an accurate picture of the situation in advance and combine the available data with your broad-based avalanche knowledge before the first shark-pow-shark session of the still young winter. To do this, we first think about fresh snow and drifting snow: How much has it snowed? Where did the wind come from? How strong was it blowing? How long did it blow for? And above all: how much time and at what temperature has passed since the last drifting snow was able to form?
Snow, like many other materials, is more brittle the colder it is - simply put: it breaks more easily. This means that the colder fresh drift snow is, the easier it is to break. If you only look at the pure drift snow problem without reference to other interactions, the following applies: the more time passes after the formation of drift snow, the less likely it is that the snow grains will "grow together" (sinter). This process is faster at higher temperatures.
This is why we learn that cold preserves the danger, while warmth contributes to faster settling and a reduction in danger. The extreme cases range from drifting snow that can only be disturbed for a few hours to drifting snow that can be disturbed for tens of days. The time window often (but by no means always!) ranges from one to three or four days. In the LWD-Tirol blog you can read about the accident on the Olperer: "The only lesson to be learned from this accident is that the danger of hard snow slabs should be considered for at least a few days after storm events, regardless of the time of year!"
