Juneau Area Avalanche Advisory
2006-04-13
Mt. Stewart
by Bill Glude, SAAC Observer
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We returned to Mt. Stewart and found the bond to the icy melt-freeze crust from our last period of warm weather still weak and now loaded with 40 - 50 cm of new heavy snow, with more snow falling and rising winds. These conditions may tip the balance into an avalanche cycle if there is enough rapid loading, or it may wait for rain to elevation or a rapid thaw.

This instability should strengthen fairly rapidly at our current mild temperatures, but the melt-freeze crust is very hard and smooth and has resisted bonding so far.

The thick, reactive slab with a very weak bond makes human-triggered avalanches quite likely, especially in windloaded areas, or with more rapid loading from new snow. If loading is rapid enough, a natural avalanche cycle may begin. Heavy rain to elevation or rapid thaw could also tip the balance.

Large avalanches reaching developed areas are still unlikely unless there is another 50 cm of snow in the starting zones, but backcountry travelers should avoid steeps that have not been wind-scoured and stay clear of runout zones until the new snowfall slacks off and the snowpack has time to settle and bond.

It is snowing at the bottom of the Eaglecrest ski hill and the base is steadily thickening. When packed it will be good snow. Above 600m, the snow is a little moist but of good fluffy quality, so get out and support our community ski area on its last weekend of operations this year. Hiking out of bounds is unwise until the snowpack strengthens.
Field Notes
Photos
Drifts today were moderately reactive. They would crack when kicked at, but did not have much energy. The wind was light most of the day and the drifting seemed to have occurred overnight, giving the drifts time to relax. The wind picked up late in the day so drifting may be more pronounced overnight.
The fog was thick above 700m today, forcing caution in travel choices. Snow fell steadily at 1-2cm per hour and was slightly moist but fluffy and smooth above 600m.
Our test site on a 40° slope at 900m in the North Bowl of Mt. Stewart. Two AK Blocks and one Cutback AK Block fractured 40 - 50 cm deep on approach as fast and clean Quality 1 shears on the icy melt-freeze crust at 96 cm from the last warm spell (AK2Q1, CAK2Q1). The others all fractured on the first gentle jump (CAK4Q1, RB4Q1). These weak results on a thick slab are cause for concern, and confirmation that our avalanche season is not yet over.

This weak layer of rounds over an icy crust should strengthen quickly in the mild temperatures we are having, but it has not yet done so. Rapid loading from continued snowfall and wind may tip the balance into an avalanche cycle. A rapid thaw or rain to elevation would do the same.
The thick slab tested out as high energy, fracturing quickly and taking all or nearly all of the undermined area, Slab Test 1 to 2.
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