AdvisorySAAC04

02-10 Mt Stewart

We went up high to see how the weak layers from last weekend's cold and windy period were changing, and how the strong SE wind and heavy snow from the night of Feb 9 - 10 had affected stability. We found a number of large slab avalanches from the overnight storm, all appparently releasing on the facets and windloaded snow from last weekend's Feb 4 - 5 cold temperatures and NE wind transport.

We spent some time attempting to break off a large enough piece of cornice to trigger one large slope, but could not get a big enough piece to break off. Also, the slope we worked on had already slid before the most recent storm, so the weak layer may have been gone.

Out block test values all indicated strength, but we gave more weight to the extensive recent avalanche activity and noted especially that these were the largest slides of the season here. The consequences of triggering slides so large would be very serious. We chose to ride only gentler slopes and small steep rolls.

Snow conditions were yielding but fairly firm beneath, at some 200 kg/m3, with a dusting of lighter 150 kg/m3 snow on top. We found that it rode quite well, accepting turns if they weren't too sharp.

The snowpack is now deep. The last storm brought heavy snowfall and drifting. Our pit site had over 4 m of snow. There were many large drifts from the storm, and they indicated a more easterly wind direction than the usual SE storm wind, about ESE on the average.

Irregularities are filling in and the snow level has been below the Eaglecrest base elevation for long enough that even the lower elevations are becoming smooth. Brush is becoming covered and only brief thaws on the lower slopes have affected the snow quality.

The forecast indicates that we are coming into a period of partly cloudy to sunny weather, with cool enough temperatures to keep good snow. The snowpack should settle and strengthen rapidly in these conditions, though it has had tremendous loading in the last week and may still need some time.

Deep weaknesses remain and may become active again when heavy loading or thawing come.

Small but deep hard slab from new snow and heavy windloading overnight Feb 9 - 10 on the E edge of the bowl E of Bunny Tow Pass, D006. Slab thickness estimated at 0.5 - 1.5 m, width 25 m, 820 -> 780 m, classified HS-NO-R1D2-I.

Large soft slab from new snow and heavy windloading overnight Feb 9 - 10 in the upper part of D007 Mt Stewart N Bowl, estimated 0.5 - 1.5 m deep, 500 m wide, 1100 -> 830 m, classified SS-NC-R3D3-I. The probable trigger was a small chunk of cornice breaking off near the center of the crown face, though it could have been triggered by the heavy windloading.

Mike Janes pushes off a chunk of cornice while on belay. We sawed and broke off cornices in an attempt to trigger the slope below without much success. Most of the pieces we were able to break off were about this size.

The largest cornice chunk we broke off triggered this small and relatively shallow soft slab. Given the large slabs during the night, we hoped to trigger something more. But the area where we broke off the largest cornice pieces had already produced a cornice - triggered slab five days ago, cleaning out most of the weak layer, and the pieces we broke off were not as big as is optimal for such testing.

Both our AK Block and Rutschblock broke only after multiple hard jumps (#6) on 38 - 40° at Q2 (average shears). The weak layer was facets and windloaded snow from the cool weather and NE winds on February 4 - 5.

This probably the same weak layer as the slabs we observed, they were deeper than our 40 cm because they were on slopes with more windloading. None of the slabs appeared to be fracturing to the December 23 - 24 faceted melt freeze crust and its associated facets.

Despite the strong block test values, we gave more decisionmaking weight to the recent very large slides and chose to ride only gentle slopes and small steep rolls.