The next westerly storm is pushing in on Wednesday afternoon. Avalanche hazard will rise to Moderate as the likelihood of loose wet avalanches increases below the rain-snow line, and wind slabs build above it.
Freezing level is forecast to rise to 2300m on Wednesday, with cloud and 5-10mm of rain forecast below 2000m. This is recipe for greenhousing and rain on snow, which is likely to trigger small loose wet avalanches below the rain-snow line.
These are more likely on shady slopes that are still holding loose dry snow. Look out for rollerballing or wet, sticky snow as these are clues that the snowpack is losing strength.
Roughly 10cm of new snow is forecast on Wednesday afternoon in the north of the forecast area, tapering to 5cm around Ohau Ski Area. Northerly winds through the day could build wind slab on sheltered, south facing slopes above around 2000m.
These slabs are expected to be small, but watch out for terrain traps like gullies, cliffs, or rocky areas where even a small avalanche could have nasty consequences.
More snow and cooling temperatures are expected on Wednesday evening. If this change arrives early, wind slabs could be larger and found at lower elevations. If it's snowing and blowing where you are, watch out for slab development.
Moderate
One small wind slab avalanche was observed at Ohau ski area on August 10th on a northeast aspect at around 1950m.
Several small natural and ski cut loose wet avalanches also ran on the 9th at Ohau ski area below 1650m on northeast through to southeast facing slopes.
An estimated 20cm of new snow fell in the northern part of the forecast region over the weekend, with roughly 15cm falling near Ohau ski area. This new snow is probably still dry above around 2100m on shady aspects, but it will be settling quickly. On sunny slopes, or below around 2000m in the shade, the snow became moist during Monday and Tuesday and will be capped with a melt-freeze crust on Wednesday morning.
The lower pack was a mix of melt freeze crusts on sunny slopes, windscoured crusts in exposed areas, and drifts of firm, windpressed snow. With a thin snowpack in places and some cold nights, faceting has been observed in the lower pack, but so far we're not seeing any weak layers that are causing problems.
Cloudy periods. Rain developing in the morning, more persistent and heavier from the afternoon. Snow lowering to 2000 metres.
Wind at 1000 metres: Northerly 40 km/h, rising to gale 70 km/h in the morning.
Wind at 2000 metres: Northerly gale 65 km/h, rising to severe gale 90 km/h in exposed places in the morning.
Freezing level: 1800 metres, lifting to 2300 metres in the afternoon.
Expect melt-freeze crusts on Wednesday morning, and strong winds may scour exposed areas to slick crusts at high elevation. Take an ice axe and crampons for travel above the snowline.
New Zealand Avalanche Advisory (NZ Mountain Safety Council)
