Juneau Area Avalanche Advisory
2005-12-04
Mt. Troy
by Bill Glude, SAAC Observer
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Yesterday's welcome new snow provided a settled accumulation of about 10 cm. Unfortunately, in many places that new snow was on areas blown bare last week or areas with only about 10 cm of weak, sugary intermediate faceted grains and some surface hoar beneath, so there is still no base to lift travelers above the rocks and bushes.

In other spots, particularly from 500 to 700m, water from the recently rain-saturated soil has seeped out under the snow, where it has frozen into extremely hard and slippery ice some 3 to 10 cm thick. The ice makes for very difficult travel in all wet spots. We used our ice axes and wished for crampons.

There are a few small snow patches remaining from before the November thaws above 700m, but those have frozen into a very fast knife hard layer, so despite excellent surface snow quality, it is very difficult to keep speed low enough to dodge the rocks and thin spots.

Our tests showed very weak strength, low stress, moderate energy, and weak structure. The main sign of instability was shooting cracks, but these were confined to the most wind loaded areas. With more stress from new or windloaded snow and the higher-energy fracture propagation of new or windloaded snow, this will be a very weak snowpack. But for now it is not quite there, despite weak test values.

Caution is in order when we do get more snow though, as everyone will be hungry for fresh powder and the combination of eager riders with unstable layers sliding on fast icy bed surfaces potentially carrying people into still-protruding rocks and stumps could be quite dangerous.

Until then, take your ice axe and crampons, enjoy the fine views, take care when above anything you would not want to slide into or over, and watch out for any windloaded spots.

Field Notes
Photos
The greatest hazard to travelers right now is aufeis, or overflow ice, that is forming under the snow as groundwater seeps out of soil saturated from the recent record rains and freezes into a hard, rough, and slippery layer underfoot. We have seen similar ice in the wetter muskegs before, but never such widespread, hard, and thick ice. We were glad we brought ice axes and were wishing for crampons at times. The snow over the ice is very loose and sugary intermediate faceted grains overlain by the new snow from yesterday, which is now becoming sugary and faceted too. These are classic weak layers and there is virtually no bonding of the faceted grains to the ice so instability is likely when they are loaded by new snow.
Settlement cones around plant stems reveal that the snow has settled some 3 cm from its original surface level, where it is still sticking to the stems.
Snow quality is excellent on the higher slopes, but there is really no base except in the smooth spots where some snow survived the November thaws. That base is fast and knife hard, making it impossible to maneuver quickly enough when rocks and thin spots loom. It was gorgeous above the fog, though.

Block tests of the snow on these upper slopes gave very weak tap compression test (CT3 Q2, average fracture on the third tap from the wrist) and AK Block test results (AK2 Q2, average fracture on approach), indicating low strength. Structure was weak, with four of five possible lemons, indicators of weakness. But loading was low and the snow did not have quite enough energy to drive a fracture. We got some localized shooting cracks, but none beyond 1m and those only where the snow was windloaded. Slab tests gave moderately reactive #3 results.
Winter evening light as the shadows of the peaks crept over downtown Juneau was a welcome treat after all the recent cloudiness.
The humidity and temperature were so close to making fog all day that even slight flow over the Dan Moller Pass was enough to form fog as the air rose and cooled, then evaporate it as the air fell down the other side of the pass. The shape of the fogbank reveals the airflow.
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