Juneau Area Avalanche Advisory
2005-12-11
Mt. Stewart
by Bill Glude, SAAC Observer
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Travel conditions have improved this week, with better footing now that the ice layer below 500m has melted into a few small patches and the snow covering the aufeis layer on the ground up higher has become wet enough to bond to the ice. There is now about 50 cm of settled snow at 800 m, enough to make a decent base, though it is still thin.

We found several slab avalanches that released when the December 9 rain first hit. All the slabs were the new snow from last weekend and early in the week, running on facets over the hard melt-freeze crust in areas where some snow survived the November thaws. All were relative size 1 and destructive size 2, about 50 cm deep and 80 - 200 m wide.

Interestingly, the rain of the 9th was heavy enough to completely water-saturate the old facets that were the weak layer. Water was concentrating above the November crust, and we measured our all-time high seasonal snowpack density of 710 kg/m3, a full 200 kg/m3 higher than our previous record high density.Even the November crust was very high density, at 520 kg/m3 exceeding our previous record by 10 kg/m3. Yes, folks, it DID rain pretty hard this week!

The weak layer still tested as weak in tap compression tests today, at 11 to 13 taps (from the elbow) and Quality 2 (average clean shear) on 40°, but it was weak because of the water saturation rather than the formerly faceted grains that released the slab avalanches on the 9th. We saw no signs of instability today. Loading was moderately low, and the slab has relaxed and is not storing much energy now to propagate fracture with.

Our fracture profile was in a rollover area that was probably some of the deepest snow on the mountain, just below some rock outcrops. The 135cm of snow there is not representative, snowcover averages about 50 cm where there is not snow that survived the November thaws and about 70-80 cm where there is.
Field Notes
Photos
The aufeis that made for treacherous footing under dry snow last weekend has now melted into isolated patches below 500m. Above that elevation it is covered by a layer of wet snow that gives much better traction, though travel on it still requires care. Here the overlying snow is saturated with water due to the impervious ice layer beneath.
We found several slab avalanches that released when the December 9 rain first hit the new snow from the previous two days and that from the previous weekend, all of which was on a weak sugary layer of intermediate faceted grains and surface hoar on a knife hard melt-freeze bed surface from the November thaw and subsequent freeze. We did a fracture line profile on this slab in the North Bowl of Mt. Stewart D007 at 900m. The bed surface can be seen to include some small areas of slippery aufeis as well as the frozen crust. Though the weak layer was water-saturated today, it was still dry when the slabs were triggered. None of the slabs were in areas that were not underlain by the hard melt-freeze crust from the November thaws.
This is the slide we did the fracture profile on in the North Bowl of Mt. Stewart. The runout angle was 27° and the slope angle at the crown face was 40°. There was a similar but larger slab in the upper bowl. Though it came from the area at the top of the photo, it was not visible from this angle.
There is now 50cm of relatively firm base at 800m, but snowcover is still thin. Travel conditions were good, with slushy snow at the 600m elevation of this photo and grabby breakable crust above 750m.
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