New snow and high winds will rapidly increase the avalanche danger.
If you decide to go into the mountains on Sunday, head for sheltered areas away from avalanche terrain.
Fresh windslabs will form on terrain that faces south, southeast and east.
Windslabs will be larger and more dangerous at the northern end of the forecast area where much heavier snow is expected.
Poor visibility will make it very hard to identify areas of concern and safely manage terrain.
Staying away from avalanche terrain is recommended.
Storm slab is also possible where new snow accumulates on previous snow surfaces particularly old crusts.
No recent avalanche activity has been reported. However, observations have been limited due to the weather.
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
Expect about 20cm of new snow over Saturday night and into Sunday morning. This tapers to only about 10cm in the far south of the range. The new snow will be distributed by severe gale NW and W winds.
Ridge-lines and slopes exposed to the south-west winds, are heavily scoured and stripped back to slick crusts and scree on upper ridges.
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.
This storm will deliver considerably more snow to the north end of the range. Around 100cm is forecast for the area around the Thumbs.
Prior to this storm, 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.
Photos are of the snowpack prior to the current storm cycle.
Showers with snow lowering to 500 metres, easing in the evening.
Wind at 1000 metres
Northwesterly severe gale 120 km/h, easing to westerly gale 80 km/h in the afternoon.
Wind at 2000 metres
Northwesterly severe gale 140 km/h, easing to severe westerly gale 100 km/h in the afternoon.
Freezing level
2100 metres, lowering to 900 metres.
New snow should decrease sliding hazard in the north. Further south the hazard may persist. Plenty of runouts have bare rock or scree. 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)
