Bollettino valanghe

Two Thumbs

New Zealand Avalanche Advisory
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Pericolo valanghe

High Alpine · above 1,800 m
2 · Moderato
Alpine · 1,200 m–1,800 m
2 · Moderato
Sub Alpine · below 1,200 m
1 · Debole
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In sintesi

There is good skiing to be found - BUT it is where you will also find the windslab. Frozen surfaces still pose a significant risk.

The safest lines will be in wind filled gullies below 30 degrees!

Problemi valanghivi

  • Wind Slabs

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    1

    Across the range we have received 5-20cm of new snow with moderate wind speeds, which will more likely form wind slabs. At elevations above 1700m the wind kept transporting snow throughout the day. We also have observed a natural wind slab avalanche.

    Also be aware that on the Eastern half, some of the old snow might still be around to add to some deeper pockets of wind slab. The Two Thumbs also have a lot of ridge features that run from SW to NE and are prone to cross-loading, especially with wind shifting from Northwest to Southerlies. The gullies behind those features are the most likely places for wind slabs. However, all our observations tell us that the areas of wind slab, despite being reactive are very small and patchy - so be on the lookout for larger pockets, as you might trigger it with just one person walking / skinning / skiing on it.

    Slab avalanches will be more of a problem the further North you venture and the higher you go in the North.

Forecast Confidence

Moderate

  • The snowpack structure is well understood.
  • We are uncertain due to a highly variable snowpack.
  • We are uncertain due to the variability of wind effect on the snowpack.

Recent Avalanche Activity

Naturally triggered size 1-2 !!! wind slab avalanche below Peak 1972m, North of Round Hill ski field, on East side of Camp Stream. We assume it was triggered by cornice fall on an E/SE aspect at approximately 1800m. It likely ran on 30-31 July.

No other observations from several ski touring groups around the range.

If you see any activity on the Two Thumb Range, please make a public observation. It really helps us to produce accurate forecasts.

Current Snowpack Conditions

The snow depth in the range South of Stag Saddle is 40-60cm with some deeper pockets of 100-120cm in sheltered areas. With the rain on Thursday 30 July terrain below 1200-1300m has lost a significant amount of snow, but this got topped up by new snow to low levels on Friday evening and night. In the South 5-10 cm fell - and more in the North. This snow was immediately redistributed by moderately Southerly winds and accumulated in gullies on the Eastern half or very sheltered bowls.

The hurricane-force winds prior to Friday, from the Northwest have stripped (sublimated = snow turns directly into water vapor and "disappears" into the air) much of the old dry snow. That has exposed many of the old surface crusts. Windward ridges are stripped back to rocks. The new cold snow can be found in the sheltered higher bowls - more in the centre of the range and in the North. It has accumulated mainly on the Eastern half. Elsewhere, expect many surfaces to be stripped back to mostly supportive icy crusts. Under the crust layers the snowpack is well consolidated.

We now have evidence of some sugary weak layers developing around the crusts below the new windslabs. At present, this does not seem a problem - at least in the South of the range. The loading on those weak layers is not enough - however that might be different in the North of the region.

The snowpack North of Stag Saddle is not well understood, with no direct observations. We know from rain gauges in the valleys that above about 2000m deep snow will now have accumulated in sheltered areas.

Mountain Weather

Partly cloudy, but for most of the Two Thumbs range it is likely to be sunny. Winds will return, though. Above 1800m expect Southwesterly wind of 50 km/h, rising to Westerly gale 80 km/h about the Mackenzie Basin in the afternoon.

The free air freezing level will be at 1000m, lifting to 1500m in the morning and to 2000m in the evening. With the winds increasing the temperatures on the snow surface and felt temperatures will most likely remain rather cold.

Sliding Danger

We have received rain to high elevations, then the temperatures dropped with much wind and that refroze most surfaces. There are many areas with significant sliding danger, in particular in exposed terrain (=windward, steep & where you find signs of frozen rain under any new surface snow). Also consider that the winds have exposed many areas back to rocks. Check your run-out areas carefully.

Ski crampons will be required! If you like to venture into more exposed terrain tomorrow, then bring an ice axe and boot crampons .

New Zealand Avalanche Advisory (NZ Mountain Safety Council)

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