Tongariro

Wind slabs exist on a wide range of aspects. Recent evidence has shown these wind slabs are reactive. Recommend a conservative mindset if you do venture into the alpine backcountry over the next 24 hours. A cautious approach to route finding is advised. The weather is improving with fine conditions and easing winds forecasted.

Multiple avalanches were triggered by Whakapapa and Turoa ski patrols during slope testing and assessment today, the 1st of August. These were on a wide range of aspects and elevations up to size 2. The notable event was a natural size 2 avalanche at the Mungawhero headwall, Turoa (southeast aspect at 1750metres).

Snowfall and wind have affected the snowpack over the past 48 hours. On average, about 30cm to 40 cm of new snow has accumulated at an elevation of 2000 metres over the last couple of days. Intense gale winds in the southern areas have affected distribution, with stripping occurring on exposed slopes. Wind-transported pockets over 100 cm deep can be expected in upper-elevation start zones. A reactive density change has been observed within the new snow deposits at Whakapapa. A brittle rime crust has formed in southwest areas below 2300 metres. The older, deeper snowpack has been shown to be consolidated and supportive recently. Snow cover exists on Mt Ngauruhoe and Mt Tongariro. If doing the Tongariro crossing, you will be traveling over snow and ice. Alpine gear and precautions need to be considered. The south-west areas have enough snow cover in start zones above 1500 metres for avalanches to run freely (threshold). In the northern areas, the threshold is at a slightly higher elevation, but this is also changing fast. This recent snowfall has improved snow cover overall and lowered the threshold elevation in the northern areas and east (about 1600 metres elevation).

Tomorrow, Sunday the 2nd of August, will be fine, with evening valley fog. The winds will be from the south, strong but easing, becoming moderate over the next 24 hours. The freezing level is about 1100 metres elevation, lifting to 1800 metres in the morning. A low passes over the top of the North Island today, while a ridge builds over the central parts of New Zealand. In the south of the South Island, a weak front passes over later today. The ridge retreats to the North Island tomorrow, while westerlies build over the South Island.

Despite the new snow, slick rime crusts will remain. Gale winds have stripped ridgelines and windward terrain features, exposing hard icy crusts. Numerous hazards, such as exposed rocks and cliff bands, remain. Carrying an ice axe and crampons is recommended in the backcountry. It is essential to have strong alpine skills and competence when travelling over slick surfaces with high consequences.

Moderate

• We are uncertain due to a limited number of field observations.
• We are uncertain due to the variability of wind effect on the snowpack.