Two Thumbs

Avalanche problems

• 

  Wind slab

  Wind slabs are formed when loose snow in near-surface layers (new snow or old snow) is transported and deposited by wind.

  Avoid wind-drifted deposits in steep terrain.

  Affected aspects

  EastSoutheastSouthSouthwest
• 

  Wet snow

  The avalanche problem is related to a weakening of the snowpack due to the presence of liquid water. Water infiltrates the snowpack due to melt or rain.

  Good timing and trip planning are important. Consider avalanche runout zones.

  Affected aspects

  NortheastEastSoutheastSouthSouthwestWestNorthwestNorth

Stability

The snow depth in the range South of Stag Saddle is 40-60cm with some deeper pockets of 100-120cm in sheltered areas. The Two Thumbs still have good snow cover to elevations around 1100m. The dry snow that fell throughout the range on Friday and Saturday has been redistributed by strong then moderate southwest and west winds at higher elevations. Windward ridges are stripped with loaded areas up to 50cm deep. Even at lower elevations, katabatic (cold air falling down the mountain) winds caused by the cold overnight temperatures, have moved the surface snow around. However, in the sheltered higher bowls in the centre of the range on the Eastern half, 5-15cm of dry none-wind affected snow is still present down to 1600m. The snowpack below the surface is capped by a crust of several layers that is highly variable over the terrain. In places it is thin and breakable with skis and in other places it forms large areas of solid concrete-like patches. Under the crust layers the snowpack is well consolidated. Guides and ski fields have reported some sugary weak layers developing around the crusts. At present, this does not seem a problem in the South of the range. The snowpack North of Stag Saddle is not well understood, with no direct observations. It is possible that above about 2300m very deep snow has accumulated.

Snow quality

No natural avalanches were observed from the top of the Richmond Range today (28 Jul) on a "state of snowpack tour" A natural windslab was observed on Pk 2211, NE of Erebus. E aspect, Sz 1.5-2. likely ran during the storm on Friday or Saturday. If you see any activity on the Two Thumb Range, please make a public observation. It really helps us to produce accurate forecasts.

Weather

Mainly fine, with severe morning frosts. About the Divide: showers developing south of Arthur's Pass in the morning, sleety about the tops. In the Southern Two Thumbs we expect the showers not to start until evening of late afternoon. The morning starts with sun, but winds will increase already. The wind will be a Northwesterly 45 km/h, rising to gale 65 km/h. Towards the evening the winds might even gust to 80km/h.The free air freezing level is at 2200m for most of the day, then lowering to 1800 metres about the Divide before dawn.