Two Thumbs

Watch high winds again tomorrow. Stay in sheltered basins and avoid frozen surfaces, which still pose a significant sliding risk.

The wind drifted snow has now bonded well to the old surfaces.

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. 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 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. Up to 20cm of new cold wind-drifted 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. It often is capped with a soft rime crust, but in the South that rime crust is also frozen solid in many paces.

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.

Partly cloudy with a few showers, mainly about the Divide. Snow lowering to 1000 metres at night. Most models show very few showers on Monday morning with no real new snow accumulation. Wind are forecasted to be Westerlies severe gale 100 km/h, easing to 45 km/h in the afternoon!

Free air freezing level is at 2000m through the night, then gradually lowering to 1300 metres from the south through the day.

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 (not recommended with the high winds), 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.