Wind slab and loose wet avalanche concerns will develop associated with the approaching storm system. Wind and precipitation will increase over the next 24 hours. Wind slabs will form rapidly once the new snow starts to get moved around. Recommend delaying any backcountry alpine travel until conditions improve.
Wind slabs are likely to form mostly on south aspects. Snowfall and accumulation are forecasted over the next 24 hours. The new snow will immediately be transported by the strong to moderate winds. Sheltered terrain features such as rocky catchments near ridgelines are the locations where wind slab deposits will develop. Any loaded start zone lacking support will be an area of concern and should be treated with caution.
A loose wet avalanche problem will develop as rain starts to break down surface structure at the lower elevations. Current surface structure is good, thanks to the robust rime crusts, which will delay any surface breakdown. Steep slopes will become prone to triggering if freezing levels become elevated over a prolonged period with rain.
No avalanche activity has been reported or observed over the past 24 hours.
Currently, the main snowpack feature in the northwest is a robust, widespread rime crust that is impenetrable at some locations and breakable at others. In the south, this rime crust also exists but only on exposed slopes that have been wind-stripped. Very little precipitation has affected the snowpack over the past 24 hours. A hard wind slab with a density inversion formed during the last storm event. These deposits have been gaining strength and heading in the right direction. In the northwestern areas near Whakapapa, near-surface facet development has been noted. At Turoa, a buried graupel layer has been identified, which has also been noted near Whakapapa. Snow cover exists on Mt Ngauruhoe and Mt Tongariro. These northern mountains are likely to have wind slabs also. If doing the Tongariro Crossing, you will be traveling over snow and ice with start zones at threshold. Recommend alpine gear and take avalanche precautions. 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. Recent snowfalls have improved snow cover overall and lowered the threshold elevation in the northern and eastern areas (about 1600 metres elevation).
Sunday the 9th of August: rain, heavy at times with snow above 1700 metres, easing to a few showers in the afternoon. The winds are strong to moderate northerly, tending northwest over the next 24 hours. The freezing level is about 1800 metres in elevation but will rise throughout Sunday to 2100 metres. The ridge of high pressure moves east of the North Island today, while the front on the northern and western South Island moves slowly eastwards across central and northern New Zealand tomorrow. A low forms east of the South Island tomorrow.
Crusts exist, and at some locations they are very slick and icy. 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.
Low
New Zealand Avalanche Advisory (NZ Mountain Safety Council)
