Weak layer in snowpack is potentially dangerous

An example of a weak layer in the snowpack. Image: NZ Mountain Safety Council

Mountain Safety Council - MSC - are reminding people that there’s a potentially dangerous ‘persistent weak layer’ in the snowpack present throughout many areas of the country, in particular Queenstown, Wanaka, Aoraki/Mt Cook, Fiordland, Nelson Lakes and Tongariro.

This persistent weak layer, first identified several weeks ago, could extend to snowpacks in other regions.

The MSC is advising anyone heading into the backcountry to exercise caution.

Snowpack video demonstrating a weak layer from early August :

The nature of the problem lies deeper than the more recent fresh snow and can become reactive under certain conditions, says IFMGA guide and co-ordinating forecaster Jamie Robertson.

“Persistent slabs have the ability to be triggered by smaller avalanches or multiple parties travelling over the same place. The kind of avalanches triggered by persistent slabs can be a lot bigger than expected,” says Jamie.

“This is a complex problem and requires very careful management. We advise people to stick to low-angle terrain or aspects that don’t have persistent layers.

“It’s imperative that people check all the details of the avalanche advisory, not just the headline danger level. There’s critical information within these forecasts that help to guide people to safe aspects and elevations.”

With relatively good weather predicted for this weekend the avalanche danger characteristics should be a key conversation for any groups that are intent on heading beyond avalanche-controlled areas.

For the month of August, the MSC is aware of more than 35 reported avalanches that have been accidentally triggered by skiers, snowboarders and climbers. Of these, 12 avalanches caught the person and resulted in some degree of burial in the avalanche debris. In all but one case the avalanches occurred on East to South-East facing slopes at 1700m and above.

Jamie said that the recent avalanches are indicative of the kinds of large avalanches possible at the moment.

“The problem relates to more than just the new snow so management is more about avoiding the slopes that might have the persistent layers, you can’t trick the snowpack.”

“You need to know the full details of the forecast and make conservative travel decisions while you’re out there.”

However, the council are keen to point out that this doesn’t mean other aspects or elevations are automatically safe. The advice from the Mountain Safety Council is to follow the guidance in the NZAA forecasts and to read them thoroughly.

More information is available at avalanche.net.nz or at the mountain safety council’s Facebook page dedicated to the snow and alpine community.


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