The measurements at Kappl weather station bear witness to the highly variable conditions over recent days. In the thawing-point curves the shift from good weather phase (thaw point far distant from air temperature) to bad weather phase (thaw point close to air temperature) is visible.
As a cold front moved in starting on 10.03, the
snowfall level began to descend in the evening, eventually reaching the valley
floor. In the western regions, in Karwendel, Tux and Zillertal Alps, there was
generally 30-50 cm of fresh snow registered, as much as 70 cm from place to
place. Strong-velocity winds transported the fresh snow massively in some areas.
Since Monday morning (11.03) there has been 30-50 cm of fresh snow registered in Tirol, more from place to place.
The precipitation, as forecast by ZAMG Weather Service, was highly convective. We know such situations from summer, when thunderclouds move in and heavy precipitation results. Convective precipitation favors the formation of graupel, which is exactly what happened, a layer of it was generated inside the fresh snow. Graupel is a potential weak layer for any snowdrifts which get deposited on top of it, especially when the graupel layer is thick (1 cm).
Graupel was observed in many places during the recent bout of precipitation, as here in Sölden. Graupel constitutes a persistent weak layer, but will no longer be a problem after a few days. (photo: 10.03.2019).
Other potential weak layers for slab avalanches are found primarily on very steep, shady slopes close to the surface. Most likely candidates are the faceted snow crystals beneath thin melt-freeze crusts at altitudes of 2000-2400 m. In addition, covered powder snow, particularly at high altitudes, is triggerable for a brief spell and in all aspects.
Potential weak layers are currently limited to uppermost layers. (1) and (4) are embedded layers of graupel; (2) and (3) are faceted snow crystals.
Due to quite high temperatures in some places over recent weeks, melt-freeze crusts were generated. Beneath them are frequently faceted crystals, generated by the high temperature gradients. In isolated cases, fractures can be provoked in this layer, as illustrated above as a partial fracture on the Aifner Spitze, NW, 2130 m, 34 degrees. (© LWD Tirol). Generally, this layer could no longer be triggered.
Today’s improved weather conditions
(12.03) including higher temperatures and solar radiation will lead to
heightened avalanche activity. We expect numerous loose-snow avalanches in
extremely steep, sunny terrain. We also expect heightened trigger sensitivity
of the fresh snowdrift accumulations, frequently behind edges in the terrain on
very steep slopes, starting at the timberline.
The snow quality is still good, at
least to start with (powdery). During the course of the day, the snow on sunny
slopes at very least will rapidly become sticky.
The snow quality fluctuates between powdery...and not quite so powdery. (photo: 10.03.2019).
