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
1 · Debole
Sub Alpine · below 1,200 m
1 · Debole
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In sintesi

The wind is shifting again to the NW and might form new wind slabs tomorrow. If showers start earlier and fall as rain, loose wet avalanches might be possible.

It still is a good day to get out, but be prepared to turn around early!

Problemi valanghivi

  • Wind Slabs

    • Tutte le quote
    Probabilità
    Likely
    Dimensione
    1

    There is still plenty of dry loose snow available for transport in the centre of the range, especially on the E half. With moderate and later strong NW winds, new wind slabs could form relatively quickly in the morning. They might build on rather slick crusts. However, the old surface is extremely patchy with a high variability, so we expect the wind slabs to develop in small patches and pockets.

    In contrast to today, the slabs might, however, be a lot more reactive and a single skier could trigger it.

  • Loose Wet

    • Tutte le quote
    Probabilità
    Unlikely
    Dimensione
    1

    Tomorrow, warmer temperatures will continue. The sun may only be out in the morning, but with clouds moving in a green-housing effect could warm the snow surface enough to trigger some small loose wet avalanches, in particularly where unconsolidated snow sits on a solid crust. Keep an eye out for signs of the snow heating - like roller-balling.

    The rose does NOT indicate a widespread problem, it only highlights that the isolated pockets could be on any aspect with greenhousing and potential rain showers!

Recent Avalanche Activity

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.

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. 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.

Mountain 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.

Sliding Danger

The winds that have continued over the last few days will have stripped many areas back to the old crust. Strong solar radiation has created some melting and re-freezing of surfaces as well. Some very firm patches exist in the range.

Come prepared for a possible high sliding danger and assess each slope. Expect lots of rocks in the runouts.

Ski crampons should be in the pack! Ice axe and crampons are recommended if your party plans to climb to the tops - especially in the South of the range.

Forecast Confidence

High

  • We have a good understanding of the snowpack structure and confidence in the weather forecast.
  • We are uncertain due to a limited number of field observations.
  • We are uncertain due to the variability of wind effect on the snowpack.

New Zealand Avalanche Advisory (NZ Mountain Safety Council)

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