Persistently treacherous – persistent weak layer

 To start off: 

The persistent weak layer which has persisted for a long period is still of significance for avalanche releases in isolated cases. This was amply demonstrated last week by several avalanche triggerings in which persons were involved.

• Wednesday, 15.02.2023: negative avalanche Gstreinjöchl, Ridnaun Massif
• Thursday, 16.02.2023: negative avalanche Pfaffenbichl, western Tux Alps
• Saturday, 18.02.2023: two negative avalanches Hohe Warte (Schmirn), northern Zillertal Alps
• Saturday, 18.02.2023: 1 person injured, Hohe Warte (Schmirn), northern Zillertal Alps
• Sunday, 19.02.2023: negative avalanche Gammerspitze, northern Zillertal Alps
• Sunday, 19.02.2023: 1 person injured, Geraer Hütte, northern Zillertal Alps
• Tuesday, 21.02.2023: negative avalanche Hohe Warte (Schmirn), northern Zillertal Alps

What stands out is the conglomeration of avalanche releases reported in the northern Zillertal Alps, four of them near the Hohe Warte. The avalanches were loosed at 2200 - 2600m altitude, mostly on north-facing slopes. East-facing and west-facing slopes are equally endangered. The avalanches were all unleashed in transition zones from shallow to deep snow in very steep to extremely steep terrain.

Danger zones are...where the best skiing is

What is so treacherous in the present situation is the fact that the avalanche prone locations are precisely where the best skiing is to be found. In wind-protected zones at this altitiude and in this aspect there is frequently “knobby powder” to be found, in other words, beautifully skiable, expansively metamorphosed, loose snow. Below that, there still is a “slab” in the relevant places from the last round of precipitation at the beginning of February. Thus, you don’t break through to the ground, as is often the case.

Treacherous, because there are only few danger zones...and they’re not visible to the naked eye

Throughout Tirol, similar such steep slopes in the relevant altitudes and aspect are currently being skied on, without avalanches being triggered. That underscores the low frequency of danger zones in general. Nevertheless - and this is the crucial point - the weak layers lurking inside the snow cover can still be triggered by winter sports enthusiasts in unfavourable circumstances. The steeper the slope and the less-tracked it is this winter, the more frequently this occurs. Caution: terrain which was tracked only after the bout of precipitation at the beginning of February is also endangered. (Tracks don’t mean that this is safer. With a bit of bad luck, you touch a hot spot.)

Avalanches Hohe Warte: violet: beginning of February; magenta: avalanches on 18.02 (the longest one was where someone was injured, ending up in the lowermost avalanche zone) red: avalanche on 21.02.2023; 2500m, NE (photo: 21.02.2023)

Avalanches Gampesspitz (left) and Gammerspitze (right) each involving persons, no injuries, 2500m, N (photo: 21.02.2023)

Avalanche below Geraer Hütte: 2200m, West (photo: 21.02.2023)

Instructive profile for still existing problem zones: near-surface slab just sufficiently pronounced (90-70cm); beneath that, the still prone-to-triggering weak layers. Hohe Warte, 2460m, NE, 35°
(for additional profiles, as always see www.lawis.at