Juneau Area Avalanche Advisory
2005-11-15
Mt. Stewart
by Bill Glude, SAAC Observer
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The big storm we were all anticipating last night and today only dropped 9 cm of slightly moist new snow in the mountains, so we had no avalanche cycle despite the slick underlying faceted melt-freeze crust overlain by weak, sugary faceted grains and even weaker surface hoar that formed during the cold period from November 11 to 13. Early in the day though, before the snow became wetter, drifted areas were quite sensitive. But once the surface layers warmed and wetted a little, they became less brittle and showed little sensitivity, though sliding ski edges easily revealed the poor bond to the melt-freeze layer.

Today we went to Mt. Stewart, where the Cutback AK block, AK Block, and Rutschblock all yielded moderately strong #5 (second harder jump) shears at Q3 (irregular, slow shears) on 37° on the faceted melt-freeze crust. The weak bond should have slid at a lower value, but our skis sank through the surface layers without putting much force into them, the slope angle was a bit flat, and the snow was wetting and settling when we did the tests. The structure rated 4 lemons (weak), the stress was moderately low, the energy was moderately high in the morning but rapidly becoming low, and the signs of instability were showing a similar steady decrease through the day.

Without more snow, this snowpack is unlikely to produce large avalanches. Rapid loading with new snow or rain may still change that, but a prolonged thaw like that forecast for the last part of the week should not have much additional effect.

Field Notes
Photos
Drifts of new and windloaded snow over the surface hoar and facets, like this one, were quite tender early in the day while the snow was dry. But with only 9 cm of new snow from the overnight storm, only the drifted areas were loaded enough to be sensitive. By late in the day the snow was becoming wet and was settling rapidly. This wetter snow was not very reactive at all.
This glide avalanche appeared to have slid during the November 8 to 10 warm weather. It only showed dimly through fog and blowing snow, but we were able to classifly it as SS-NU-R1D2-G. It was about 40 m wide and 0.8 m deep.
We tried the first snow grain photos with our new super closeup macro lens and flash unit today. These are the faceted melt-freeze grains we have been studying for years but have been unable to photograph until now. Poor bonding of overlying snow to grains like these has killed at three people and caused a number of near-misses during the last ten years in the Juneau area, yet the process is not documented anywhere in the avalanche literature.

These faceted melt-freeze grains were not perfect for recording, as they were already beginning to round in today's mild weather, but the characteristic sharp 120° angled corners are still visible on the large, otherwise rounded, fish egg shaped, melted and refrozen grains.

We do not yet have the stand or tripod we need to hold the camera steady so we get better depth of field at this large magnification. Look for better photos as we get the field setup dialed in for this lens.

A huge thanks to the Senator Kim Elton, Representative Beth Kerttula, and Representative Bruce Weyhrauch, who secured the legislative capital grant that allowed us to buy the equipment to take these photos and provide better information on a previously undocumented snow type that is a known killer in our region.

The large fresh snowflake at top center is a barely visible stellar crystal inside a thick coating of frozen rime droplets. Snowflakes rime when they encounter supercooled water droplets in the air on their journey to the ground. Supercooled droplets are below freezing but not frozen because they have no impurity, speck of dust, or other seed to start growing a crystal on. They freeze instantly as individual droplets when they contact a surface or falling flake. Riming is very common in our high latitude maritime climate.
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