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

The next storm is hitting the range with rain followed by snow and some very strong winds. If you are heading out tomorrow, plan to stay in the sheltered wide valleys at lower elevations and keep it low(er) angle.

Avalanche problems

  • Wind Slabs

    • All elevations
    Likelihood
    Likely
    Size
    1.5

    New snow will fall over the next 2 days - we just do not know exactly at what elevation. Initially it could be rain to the tops, in particular in the South of the region. With the forecast strong NW winds, new wind slabs could form relatively quickly 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.

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

  • Loose Wet

    • All elevations
    Likelihood
    Possible
    Size
    1.5

    Tomorrow, all forecast predict some rain at the start of the storm cycle. That might cause releases of some loose wet avalanches. We do not expect those avalanches to be large, as the available snow to produce avalanches only exists in pockets.

    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 with the forecast potentially heavy rain showers.

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

Rain, heavy at times about the Divide, spreading elsewhere from afternoon. Snow lowering to 1200 metres. The rain and snow amounts for tomorrow are forecast to taper from the North to the South. There is high uncertainty of the precipitation amounts. Some models predict 20cm around Stag Saddle and 5-10cm at Mount Dobson.

Winds are forecast to be gale-force northwesterly 80 km/h, rising to severe gale 120 km/h for a time late morning to early evening.

The free air freezing level starts out at 2200m in the South, but 1800m closer to the Divide. During the day it will be lowering to 1700m in the South (that is approx the car park of the Mount Dobson ski field), but 1400m about the Divide in the evening.

Sliding Danger

Firm surfaces exist, so be prepared for a possible sliding danger and assess each slope. However, the incoming storm will soften the surfaces, either with some rain at lower elevation and in the morning, then later with some new snow.

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

Moderate

  • We have a good understanding of the snowpack structure and confidence in the weather forecast.
  • We are uncertain if precipitation will fall as rain or snow.
  • We are uncertain due to the variability of wind effect on the snowpack.

New Zealand Avalanche Advisory (NZ Mountain Safety Council)

Regions covered