Avalanche forecast

Two Thumbs

New Zealand Avalanche Advisory
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Avalanche danger

High Alpine · above 1,800 m
2 · Moderate
Alpine · 1,200 m–1,800 m
2 · Moderate
Sub Alpine · below 1,200 m
1 · Low
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Highlights

New snow will fall on the range overnight and tomorrow and it comes with strong winds from various directions. The safe option for tomorrow is to enjoy any new snow on low angle slopes at lower elevations and away from overhead hazard.

Avalanche problems

  • Wind Slabs

    • All elevations
    Likelihood
    Likely
    Size
    1.5

    New snow will fall from this evening until Saturday morning . Initially it was rain to 1900-200m on Thursday, but the snow is now lowering and will fall to low elevation at some point during the night or tomorrow. With the forecast of strong NW winds followed by a shift to the South, new wind slabs could form on almost all aspect once the snow accumulates.

    The gale force winds at higher elevations could mean that new wind slab might form more at lower elevations and in areas that are a bit more sheltered. The tops are already stripped back to rocks in many areas.

    We assume that new wind slabs will be reactive and a single skier could trigger them.

  • Loose Wet

    • All elevations
    Likelihood
    Possible
    Size
    1.5

    Rain rain has fallen to higher elevations today. That might cause releases of some loose wet avalanches initially over night at elevations above 1800m, but those might get covered with new snow until the morning. The rain may be more persistent below that, but temperatures are on a downwards trend. Some loose wet avalanches are possible. We do not expect those avalanches to be large, as the new snow falls on soaked snowpack

    Putting the loose wet avalanche problem on all aspects of the compass rose does NOT indicate a widespread problem, it only highlights that the dangerous pockets could be on any aspect.

    The danger for Lw avalanches will decrease with falling temperatures

  • Storm Slabs

    • All elevations
    Likelihood
    Possible
    Size
    1

    As new snow accumulates small storm slab avalanches might develop. If temperatures fall more quickly this problem might rather present itself as sluffs or loose dry avalanches.

Recent Avalanche Activity

We have no reports of any natural or human triggered avalanches in the last 2 days. Wednesday 29 July was actually less windy than forecast. Very little snow transport has been reported from guides and ski fields in the range and therefore no new windslab has developed . Although, some greenhousing was reported, it did not break the surface crusts and overall snow surface temperatures stayed relatively cold for the entire day.

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

Thu 30 July: The strong winds have started late morning and precipitation only started at Rex Simpson and Round Hill in the afternoon, while the Northern Two Thumbs were in the cloud and have received between 10-30mm of rain - and we estimate 10-30 cm of snow above approximately 1800-1900m, but more likely 2000m, from this morning. In the South we have received less rain. Freezing levels are currently lowering, as is the rain-snow line to around 1300-1400m later this evening.

Fri 31 July: Periods of rain, heavy at times about the Divide until evening. Snow to 800m, lowering to 400m in the afternoon.Winds will be northwesterlies severe gale 105 km/h, turning southerly 95 km/h in the afternoon. The free air freezing level will be at 1400m, lowering to 800m in the evening.

Sliding Danger

The incoming storm will soften the surfaces, with some rain at lower elevation and in the morning, then later with some new snow. However, if exposed areas or rain soaked snow freeze overnight, patches with significant sliding hazard might develop. Be aware!

Ski crampons should be in the pack! We do not think ice axe and crampons are essential with the weather forecast - you may want to avoid exposed ridges and summits tomorrow.

Forecast Confidence

Low

  • The snowpack structure is well understood.
  • We are uncertain about forecast snowfall amounts.
  • 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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