Two Thumbs

The wind has stripped and moved snow. In the process new windslabs might have developed, but mostly the sliding hazard will have increased.

The best snow is probably stashed at mid elevations in lower angle bowls and gullies

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.

The snow depth in the range South of Stag Saddle is 40-70cm with some deeper pockets of 100-120cm in sheltered areas. On 31 July to 1 Aug 5-10 cm new snow fell in the South - and more in the North. Continuous moderate (1+2/8) than gale (2PM + 2/8) Southerly & Southwesterly have stripped the Western half back to surface crust and deposited snow into gullies or sheltered bowls on the Eastern half.

Ridges and windward slopes are stripped (sublimated = snow turns directly into water vapor and "disappears" into the air) of dry snow back to strengthening surface crusts or even rocks. Up to 20cm of cold wind-drifted snow can be found in the sheltered higher bowls - more in the centre of the range and in the North. At mid elevations expect some areas still capped with a soft rime crust, but in the South that rime crust is frozen solid in many paces and will have gotten stronger with strong winds today 3 Aug).

Elsewhere, expect many surfaces to be stripped back to mostly supportive icy crusts. Under the crust layers the snowpack is well consolidated.

Only in the South (around Mt Dobson), we could find sugary weak layers developing around the crusts below the new windslabs. At present, this does not seem a problem. The loading on those weak layers is not enough. Further North the crusts are less severe and the snow above and below seems mostly well bonded with the crusts.

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.

Mostly cloudy. Occasional snow showers, clearing south of the Mackenzie Basin in the afternoon. Winds are still southwesterly around 50km/h, but will be stronger at 200m: severe gale 90 km/h easing to gale 70 km/h in the afternoon. Towards the evening the weind will ease back to 30 km/h south of the Mackenzie District. However, those wind predictions are not consistent across forecast models and could be less strong. Weather stations at Round Hill, Dobson and Fox Peak have recorded consistent drop in wind speeds since the morning of Monday, 3 Aug.

The free air freezing level is forecast to be at 300 metres, lifting to 500 metres for a time in the afternoon. This is a significant cooling from relatively high temperatures on Monday 3 Aug (top of Dobson T-bar recorded +1 degrees).

We have received rain to high elevations, then the temperatures dropped with much wind that refroze and stripped many surfaces. There are many areas with significant sliding danger, in particular in exposed terrain. 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 (not recommended as long as the winds are strong), then bring an ice axe and boot crampons.

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.