There is still reactivity in the snowpack, after new snow and wind transport.
Small avalanches can have big consequences.
Assess loaded features and runouts carefully.
Strong south to south-west winds have formed reactive wind slabs at ridge top and in cross-loaded terrain features.
Look out for smooth, rounded pillows of wind-drifted snow, changes in snow texture and hollow-feeling slabs over firmer old snow.
With sun on new snow, slick old surfaces, and rising freezing levels, small loose wet avalanches are possible in steep terrain during the heat of the day.
Watch for roller-balling and signs that the snow is losing cohesion. Check your runout — cliffs, gullies and exposed rocks can have big consequences.
18th Aug: Multiple size 1 wind slabs were ski cut at Roundhill. Small (< size 1) loose snow avalanches ran naturally on very steep terrain at Fox Peak. Both were on north-east aspects above 2000m.
If you see any activity on the Two Thumb Range, please make a public observation. It helps us to produce accurate forecasts.
Southern end of range — south of Stag Saddle
Up to 20cm of cold southerly snow fell with little to no wind in the south and east of the range over the weekend. This tapered to 5cm in the west with stronger winds, and isolated wind drifts up to 30-50cm deep. In places this is capped by a thin rime crust that formed in south-east cloud. The crust is breaking down with cold clear nights.
Tuesday brought strong south to south-west winds, which formed small pockets of reactive wind slab lee to exposed ridges.
Ridge-lines and slopes exposed to the south-west winds, are heavily scoured and stripped back to slick crusts and scree on upper ridges — these are likely only thinly covered by the new snow. In areas where the new snow was wind affected, it has been scoured back to the old ice crust.
Multiple crusts exist in the old snow on all elevations and aspects. Facets (sugary crystals) have begun to form around these crusts at upper elevations, but have not posed problems. At mid-lower elevations, the old snow is well settled and consists of mainly melt forms or round crystals.
Snow depth varies from 40-75cm, with deeper accumulations of 100-120cm in sheltered areas.
Northern end of range — north of Stag Saddle
The snowpack North of Stag Saddle is not well understood. We only have one observation from Monday 10th August from the area around Captains Peak and Mt Toby, reporting only slightly more snow than around Camp Stream or the Richmond Range, with very little old avalanche debris observed. A similar snowpack to the South was reported.
Partly cloudy with morning frosts, clearing to fine in the afternoon. Frosts returning at night.
WIND AT 1000 METRES
Southwesterly 35 km/h, dying out in the afternoon.
WIND AT 2000 METRES
Southerly gale 70 km/h, but 50 km/h about the Mackenzie Basin, easing to southwesterly 35 km/h everywhere in the afternoon.
FREEZING LEVEL
1400 metres, lifting to 2100 metres in the afternoon.
Prior to the new snow over the weekend, the old snow was firm and icy in many places. Plenty of runouts had bare rock or scree — these will now only be thinly covered. Slopes exposed to wind, have been stripped back to the crust. A slip or fall on steep terrain could have serious consequences.
Assess runout zones before committing to steep slopes, and carry equipment appropriate for firm alpine conditions, such as crampons and an ice axe. Conservative route selection remains the most effective way to manage these hazards.
Moderate
New Zealand Avalanche Advisory (NZ Mountain Safety Council)
