A special weather forecast today, because the weather for both the short and the long term will be mild and sunny for the Alps. A huge belt of high pressure areas is taking over the Alps, keeping the fronts at distance and bringing sunny and warm weather the next couple of days. PowderAlert #21 was the last PowderAlert for now. Next week will be characterized by sun, sun and….sun.
PowderAlert #21 was a good one. It locally brought a lot of freshies and colder temperatures. That resulted in nice reports. Like the one below from the Queyras. But as from today, spring will take over.
High pressure areas result in sunny days with mild temperatures during the day and cold and clear nights in this time of the season. The sun and the temperature will be pretty important factors to determine the avalanche danger. Especially in the resorts where you can still find a weak layer (deep) in the snow cover, this could cause some problems.
The snow cover doesn’t really care about the changes of the air temperature. This is because one characteristic of the snow cover is the perfect isolation. Only when the snow cover has to deal with one of the three points below, the snow cover deteriorates and tensions will form.
direct radiation of the sun
a combination of relatively warm air and clouds. The clouds conduct the warmer, more humid air into the snow cover
We don’t have to deal with the second and the third element, so only the direct radiation of the sun has some influence this week. Rain and clouds influence every exposition of the slope. This isn’t the case with the sun. The path of the sun is a nice constant. She’ll show up in the east, moves south and calls it a day in the west. (Steep) Northern slopes are unreachable and are in the shadows all day. It’s much colder (especially mid-winter) on northern slopes and the snow stays pretty well conserved. And that’s the same for the weak layers.
Because the sun is much lower in winter, compared with spring, autumn or summer, this has some consequences for the snow cover. The consequence mid-winter is that the sun can never (directly) radiate on northern expositions. The sun goes around them and not in them, that is, if she’s high enough to reach those northern slopes anyway.
In short: a slope perpendicular to the sun will have to deal with a powerful direct radiation, then a slope where the angle is smaller. That’s the reason why steep southern faces in mid-winter are much more influenced by the sun than not so steep southern slopes. And it also explains why the snow in the trees is much better conserved, because the sun can’t radiate into the snow cover directly (most trees have branches). If later in the season the sun is higher this difference is smaller and flat slopes also have to deal more and more with a more direct radiation of the sun.
The result is that the sun will warm up the snow cover and the faster creep of the upper layers causes tensions in the snow cover. Because thick layers of Schwimmschnee arose particularly at the start of the season in parts of the Alps (especially in the northern and western Alps) the sharp rise in temperature over the next week is a major problem. There is a chance that entire slopes can come down. Read your local avalanche bulletin well the next couple of days and remember where you can ride (and where not). The danger is difficult to recognize, but the past has shown that the first week of spring always makes for nasty avalanches. You are warned.