Juneau Area Avalanche Advisory
2005-11-27
Mt. Stewart
by Bill Glude, SAAC Observer
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The big thaw of November 17 to 24 dropped an alltime record of over 23 cm (9") of precipitation at the Juneau airport in a week, and over 36 cm (14") downtown. Temperatures soared to +10°C (+50°F) and southeast winds reached over 30 m/sec (60 knots) in town and hit 59 m/sec (114 knots) at the Sheep Mountain weather station on a ridge right behind Juneau. These were classic "Pineapple Express" tropical storms riding a southern loop in the jetstream straight from tropical latitudes to Southeast Alaska, where they came in as a continuous band of moisture-laden warm clouds that went on for days, rather than the usual spiral-shaped cyclonic frontal storm that passes in a few hours or over one night or day.

The warm wind took almost all the snow from the mountains around Juneau, leaving only a few well-soaked patches in lee areas where they had some protection. As the weather cooled over the last few days, snow showers have dropped 10 to 15 cm of fresh moist snow that has become sugary faceted grains under temperature gradients (differences in temperature within the layers) caused by the clearing weather and resultant cooling of the snow surface.

In most areas, the only snow is the new sugar. It is a potential weak layer when loaded by wind-transported or new snow, but in most areas the snowcover is too thin for it to cause more than mini-slabs even if it does load heavily. In the areas where the base of now-freezing and faceted melt-freeze remains under the sugar, instability when loaded is more likely, but those areas are still small enough to only pose a danger to the unwary backcountry traveler.

We did no block tests today as there is very little snow, the solid base layers are obviously going nowhere, and the sugary facets on top will not produce any instability until they are loaded.

We will not do fieldwork for a few days unless there is more snow, but will be out in any case with the UAS Level I course next Sunday.
Field Notes
Photos
The snowcover is so thin we can only remember one November with so little. Even the upper North Bowl of Mt. Stewart had only a few patches of snow remaining from before the big thaws of November 17 to 24. There is 10 to 15 cm of new faceted snow from the snow showers of the last few days. The ground is beginning to freeze now, but even at this 800 m elevation it was semi-frozen and below 600 m it is still mostly unfrozen and water-saturated. We had to search for a patch of snow with enough base to do a worthwhile snow profile.
We are still picking out a tripod and macro focusing screw setup for the new snow photography lens, but tried a few more handheld shots today. This is the top of the layer from the big thaw, now refrozen into faceted melt-freeze grains. Note the hexagonal facets or sharp corners on these big and generally rounded melt-freeze grains. Compare them with the early facets and ordinary unfaceted melt-freeze below.
These are early faceted grains from the snow that fell in the last few days. It fell in mild temperatures and rapidly became small rounded grains, losing its original crystal form, then turned into faceted grains in the last few cooler days. Note the abundant sharp hexagonal corners on generally small grains, markedly different from the melt-freeze grains above and below. Weak bonds between faceted grains cause the sugary, weak texture.
These are thawed moist melt-freeze grains, lacking the sharp hexagonal corners of faceted melt-freeze. This is the coarseness and degree or rounding present in ordinary melt-freeze grains. If grains like these freeze under a strong temperature gradient, they develop into faceted melt-freeze grains like those in the first photo above. If there is not a strong temperature gradient, they freeze to look much the same as these grains. Our field observations show that their bonds are much stronger than if they became faceted. Faceted melt-freeze grains are a common persistent weak layer in our region that often causes instability that is very difficult to evaluate.
The snowpack on most of our mountains is still very thin, but the peaks of the Juneau Icefield looked quite snowy in the distance. They are higher than the nearer mountains, and precipitation there is commonly three or more times what it is closer to town, so the snowy appearance may be more than just an illusion caused by the smooth underlying surface of firn snow from last winter and a coating of rime on the rocks.
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