Persistent weak layer in some places – Slight daytime danger cycle for avalanche risks due to solar radiation and rising temperatures

Last week’s biggest threat, the persistent weak layer, remains with us above 2000m. In the interim, frequency of avalanche prone locations where slabs can be triggered has diminished significantly. Avalanches are mostly triggerable on very steep slopes in transition zones from shallow to deep snow. Therefore: restraint and caution in untracked, very steep terrain is imperative.

To this risk must now be added the increasing degree of snowpack wetness during the course of the day. On very steep to extremely steep sunny slopes, the snowpack can forfeit its firmness thereby. But if backcountry tourers plan their time carefully, the reward at intermediate altitudes will be great firn snow.

Forfeiting firmness of snowpack due to solar radiation and higher temperatures

Splendid winter weather including higher temperatures and intensive solar radiation has its impact: at very least the near-surface part of the snowpack becomes thoroughly wet. Where the snow is shallow, wetness penetrates much deeper. Water seepage in the snowpack generates a loss of overall firmness. That means the melt-freeze crust which formed during the night (and was often capable of bearing loads) gets softened up. 

Three scenarios are conceivable:

• On extremely steep slopes, moist or wet loose-snow avalanches trigger.
• These loose-snow avalanches burden the snowpack on their plummet path so much that slab avalanches are triggered.
• Water seeps down to the faceted layer which caused the persistent-weak-layer problem (beneath a thin melt-freeze crust). Thereby the weak layer loses its firmness. The likelihood of a slab avalanche triggering thereby increases. This scenario is especially applicable to extremely steep S/SE to S to S/SW facing slopes where the snow is shallow at intermediate altitudes.

Die aktuelle Situation

Avalanche Kellerjoch on 12.02.2023

Yesterday (12.02.2023) an avalanche near the Kellerjoch in the direction of Naunzalm was reported. Since it was not certain if persons were buried in snow, a search operation was initiated that was hampered by subsequent avalanches. In the end, the all-clear signal could be given, no one was buried in the snow.

The analysis of this avalanche proved to be highly interesting, since we had no reason to expect a naturally triggered slab avalanche based on available data. However, as is so often the case, the explanation was simple…it lies in the devilish details. On Saturday, cloudbanks which moved in during the afternoon caused an intense moistening of the snowpack. We assumed that at least the latter part of the night would have clear skies, but that was definitely not what happened in the Lower Inn Valley. In the accident zone, nocturnal skies were heavily overcast with only tiny gaps. Thus, the prerequisites for longwave outgoing radiation and re-firming up of the superficially moistened snowpack were lacking. Solar radiation and higher daytime temperatures then generated a thoroughly wet snowpack, much wetter than anticipated. (By comparison, today on 13.02.2023 we have a better starting situation due to very clear nighttime skies and highly effective outgoing radiation.)

Apart from that: wherever a melt-freeze crust which is capable of bearing loads has formed during the night, you need to pay close attention to potential risks of taking a fall in steep terrain…otherwise there is awesome firn snow to be enjoyed (with good time planning).