Stay off suspect slopes warning

The likelihood of more avalanches is high with skiers and boarders warned to stay off suspect slopes.

Weather that usually leads to localised areas of unstable snow has this year produced persistent, widespread instability. It is predicted to last for weeks unless a huge snowfall occurs.

"Right now, the only thing that's going to stabilise it in a hurry is if we get a whole bunch of new snow and everything avalanches off and rips out those instabilities," snow expert Gordon Smith said yesterday.

Back-country heli-skier Llynden Reithmuller died in a 100-tonne slip in a remote valley near Methven from which another buried skier was resuscitated.

On Sunday, Queenstown snowboarder Ryan Manu Campbell, 31, died after being hit by an avalanche beyond the boundary of the Coronet Peak ski area. On July 23, two Remarkables Ski Area ski patrollers were caught in an avalanche they had set off with explosives to release an unstable slope. They were not injured.

Yesterday, the Mountain Safety Council listed the avalanche danger as high in the back-country mountains of Wanaka, Queenstown, Aoraki/Mt Cook National Park, Arthur's Pass and Nelson Lakes; and moderate on Mt Ruapehu.

Avalanches often happen naturally, especially during and just after storms. They require snow, a slope and a sliding surface. Usually the sliding surface is an unstable or weakly-bonded layer of snow, but the whole snowpack, right down to ground level, can release in some circumstances.

"A typical winter in the Southern Alps starts with a small amount of snowfall through May and June, accompanied by very cold air temperatures," said Mr Smith, the council's acting avalanche programme manager. "The snow that stays on the ground is usually shallow and the cold temperatures destabilise the snow structure. Often July is a cold and clear month and this year followed that trend.

"So with an extended period of cold clear weather the snowpack has become very weak. Recent new snow and accompanying winds in August have overloaded a lot of the weaknesses deeper in the snowpack. These types of weaknesses can persist for many weeks and can remain sensitive enough to release under the weight of one person."

Most winters produced some of this type of instability, Mr Smith said. But this year, it was widespread through a range of altitudes and throughout most of the Southern Alps.

The only things likely to improve stabililty, other than a huge snowfall, were time - "probably a couple of weeks, at least" - and increasing air temperatures. The complex factors that can produce a weak, sliding layer included "surface hoar" crystals, similar to frost on grass, a crust formed by rain - buried by later snowfalls. Also, large temperature differences through the snowpack can cause snow crystals to reform, producing a sliding layer where one didn't previously exist. Because of these risks, the council encourages backcountry snow-travellers to attend an avalanche course; carry a transceiver, snow-shovel and collapsible probe; practise their use; and heed the warnings.