Juneau Area Avalanche Advisory
2006-03-30
Mt. Stewart
by Bill Glude, SAAC Observer
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We returned again today to the same slopes on Mt. Stewart that we tested two, four, and five days ago. Today's test values on the deeper 92cm layer were similar to those from two days ago, with most releasing on the second harder jump (5) at a slightly irregular but fast Quality 2. The light surface layer of new snow sheared on approach (2) but at an irregular Quality 3.

We did two sets of AK Blocks sized to each tester for our ongoing AK Block research project, for a total of six large blocks today. Those blocks showed good correlation. All the AK Blocks, inclding the cutback, went on the same values today and the larger Rutschblock went on one step higher. This is what would be expected if the block size is more important than the presence or absence of the cut back.

Today we photographed the large depth hoar crystals we found near the rock ledges on our last field trip with our macro lens.

The snow on shaded upper slopes was smooth and light today, but lower and sun-exposed slopes were bottomless slush. When afternoon shadows started the deep slush refreezing, an almost unskiable fast but breakable crust developed. Take care to not get caught in this stuff, it's legbreaking snow!

The weak layers we have been testing have gained strength, but they remain weak where the facets over crust will not resist thawing and shear loads on steeper slopes. Caution is still in order for backcountry travelers on steep slopes that are thawing.

Large avalanches reaching developed areas are unlikely.

Watch for aprupt thaw, rain, or new snow. Any of these are likely to reactivate the deeper weak layers.
Field Notes
Photos
There were several fresh slab avalanches this morning on the slopes above the Lower Cross Country Loop at Eaglecrest. All were triggered by sluffs off the trees as the morning sun warmed the new snow on their branches, and all ran on the same facets-over-crust than has been the main weak layer in our block tests over the last week.

These small slabs show that the weak layer is still active if the sun warms the surface layers enough to cause excessive creep stress and something triggers it. All today's slabs were on starting zones steeper than 40°.
Signs of spring abound this week. This caterpillar was crawling over the snow, hooters were hooting steadily all day, and varied thrush songs rang through the forest.
The new snow was beautiful dry unrimed stellar crystals that sluffed easily on slopes over 40°, as Tyler Gress demonstrates here as he works his way carefully down onto a test block. The surface snow density from last night and this morning was a very light 50 kilograms per cubic meter.
Tyler Gress shears an AK Block at average shear quality on his second harder jump on a 43° slope (AK5Q2). All our AK blocks, including the cutback AK Block and two pairs sized for each of our weights, fractured identically. The Rutschblock went on multiple hard jumps (RB6Q2). The fluffy surface snow slid off each block on approach.
We found well-developed depth hoar and cavity hoar in the thinner spots and voids near the rock ledges on our last two field trips. Today we brought our super macro lens, flash, and tripod to photograph the big (1-8mm) faceted grains.

The sharp hexagonal corners, the facets that give faceted grains their name, are clearly visible. Striations, stepped lines of crystal growth parallel to the facets, are also visible. The facets and striations are no longer crisp and chiseled-looking after a week of mild weather. In the absence of the strong temperature gradient that formed them, they retain their delicate structure but they begin to round.

Depth hoar is not a feature of the typical maritime snow climate, but is common here every year in rocky areas at higher altitudes, and is quite widespread this year due to the thin snowpack and cold spells. Ours is a high latitude maritime snow climate, with cold dry weather as well as mild wet weather. We see nearly every snow type here every year, far more variety than in most other snow climates.
Dry surface sluffs and sparkling stellar crystals carpet the upper slopes.
The snow on shaded slopes up high was excellent today, though we descended through hideous half-frozen breakable crust over bottomless slush on the lower slopes in late afternoon.
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