Juneau Area Avalanche Advisory
2005-12-29
Mt. Stewart
by Bill Glude, SAAC Observer
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We went up Mt. Stewart again today to see how the new snow from last night was bonding. The overnight and morning snow level was high at 700m but we were fortunate to have only a few light showers as we climbed. It was calm to 800m and quite windy above, reaching a steady SE 15 m/sec with gusts over 20 m/sec (30-45 knots) on the summit. The summit temperature was -1°C.

On the upper slopes, we found some 10cm of fresh snow over 5-10cm of settled moist rounds from two days ago. The new snow was also changing rapidly to moist rounds and was topped with a breakable 1cm rime crust that grabbed ski edges on the descent.

There were few signs of instability today, with only a little new snow and SE windloading present but much less loading than the wind speed would suggest because the rime crust protected the loose snow under it from the wind.

We did a number of slope and traveling tests as we climbed. There were no results on slope tests, switchback, or parallel tracks tests. But hand shear tests found two weak layers. Some fractured on the icy melt-freeze crust at 135cm in our pit profile, and some fractured just below last night's snow on a light rime crust from two days ago. A quick No-excuse Block on a fairly gentle 32° slope fractured on the 135 cm crust at average shear quality on the first hard jump, a #4Q2 result.

Our test pit results were similar, all three large blocks fracturing easily on the 135cm crust with fast and clean Quality 1 shears on a 41° slope. The AK Block released on an ankle flex (AK3Q1), the Cutback AK Block went on approach (CAK2Q1), and the Rutschblock went on an ankle flex (RB3Q1). Yet despite low strength and clean, fast shears, there were no natural avalanches and we were unable to trigger slides or create shooting cracks.

The answer was in the slab tests, which scored a consistently low-energy #5, irregular crumbling near the area we loaded. The snow simply did not propagate fracture well, it was low energy even though shear quality indicated otherwise.

We scored the strength -, stress +, energy +, and structure ~, where + indicates a stronger snowpack, - weaker, and ~ neutral. The neutral structure, low stress, and low energy prevented release.

With wind and rain in the forecast through the next few days with temperatures in the range for the rain to fall as snow on the higher slopes, either the 135cm layer or the surface rime crust could be likely fracture planes for an avalanche cycle if loading is rapid and sufficient. New and windloaded snow is likely to be much more sensitive than what we saw today. Large avalanches reaching low elevations are still unlikely, but caution is in order for backcountry travelers it there is much new snow or windload.

Field Notes
Photos
A breakable 1cm rime crust on top of the 10cm of fresh snow from last night coated blueberry bushes and the snow surface, making it difficult to turn skis.
Matt Beedle does a slab test, loading a section of snow we have undermined with a ski pole. This test scores a low energy #5, crumbling away where his hands have loaded the slab without fracturing over a larger area. The slab test is a very useful traveling test that gives an indication of how well the slab propagates fracture, a measure of its energy.
Matt cuts a quick 30 x 30cm hand shear block with his ski pole. Loaded by pulling downslope with both hands after it is cut, this is another traveling test that can be done without even removing your pack or skis. Results varied today, sometimes shearing on the icy melt-freeze crust at 135cm in the pit profile and sometimes shearing on the light rime layer at 150cm. The difference seemed to be roughness of the 135cm crust, which showed wide variation from quite rough to very icy when it was exposed on the surface last week.
Matt does a No-excuse Block, another quick traveling test. The block is dug out with your hands the width of the contact length of your skis or snowboard, sides dug as far as you can reach, then loaded like an AK Block or Rutschblock. This block on about a 32° slope fractured with average shear quality on the 135cm crust on a first hard jump, a #4Q2.
It was very foggy all morning, but just as we arrived on the summit the wind picked up to SE15-20+ m/sec (30-45 knots) and the fog broke, creating unearthly light and giving brief glimpses of mountains, valleys, and the sea. The fog lifted more as we descended out of the wind into the North Bowl.
We did a three-block test set at 900m in the North Bowl on a 41° slope. We dug shallow blocks, as the deeper layers are still quite solid. All fractured on the 135cm icy melt-freeze crust, and all were fast and clean Quality 1 shears. Our AK Block fractured on an ankle flex (AK3Q1), the Cutback AK Block pictured here fractured on approach while Matt was still some distance above it (CAK2Q1), and the Rutschblock fractured on an ankle flex (RB3Q1).

Despite the low shear values and high energy shears, the slab test showed why no avalanches were triggered and there were no shooting cracks. The slab was not storing energy for fracture at its lateral boundaries at today's loading level.
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