AdvisorySAAC04

04-23-05 Mt Stewart

We found good corn snow today, a snowpack wetted out to at least the March crust at about 1m below the surface, and some large recent wet slab and glide avalanches.

The wet slabs are being triggered by cornice breaks and are fracturing initially on a subtle density and grain change weakness some 20 - 30 cm below the surface, probably the March 31 graupel layer, then stepping down to the March crust at 1 - 2m depth. Beware, these slabs are large! Stay off slopes under cornices in this warm weather.

We witnessed a large glide avalanche on the slopes above the lower cross country loop in the late afternoon. Glide cracks are becoming very active, and the warm weather is causing some releases. Stay off the glide plates and out from under glide cracks.

There was no human-triggered sluffing today in our tests on slopes to 40°, but some mid-sized natural point releases were running in the afternoon.

Our block tests results included Cutback and regular AK Blocks and a Rutschblock, all tested by a 70 Kg skier, and an AK Block for an 85 Kg skier. All were on 41°. All released after multiple hard jumps, #6 on the scale. The Rutschblock did not release at all, a #7 on the scale.

These results indicate strong bonds that are somewhat difficult to skier trigger. Ths slabs we saw were all released by large cornice triggers or were glide avalanches. Human triggered slabs are unlikely in these conditions, but still may occur on higher slopes that are just wetting out now. Wet sluffs were not very active, but some are occurring on sunny slopes, so humans may still trigger them on steeper sunny slopes.

Cornice-triggered wet slab avalanche on D006, the bowl just E of and above Bunny Tow Pass, Mt Stewart. We were out of town this week so do not know the exact dates these slides occurred, but they are probably from within the last few sunny days. We classified this slide as relative and destructive size 3 (R3D3). We estimated it at 0.3 - 1.8 m deep, 150 m wide and 600 m long, falling from 980 to 640 m.

It broke on a weak layer about 20-30 cm deep that is no longer identifiable as it melts but which we believe is likely to be the March 31 graupel layer, then stepped out to the 1 - 2 m deep melt freeze crust from the big March thaw and freeze. These layers, especially the March melt freeze, are likely to cause a widespread cornice-triggered wet slab cycle this spring.

Bill Glude stands on the debris of the slide above Bunny Tow Pass. For those who have practiced avalanche search and rescue only on an open field, this is what the real thing looks like - chunks, blocks and snow boulders you must clamber over and search between. Imagine trying to run a good probe line search pattern through this debris!

The crown face of the cornice-triggered slab above Bunny Tow Pass. Note the broken cornice and the step on the lookers' right side from the shallow (probably March 31 graupel) layer down to the March crust.

We found two large recent slabs in D007 the North Bowl of Mt Stewart. The far slab is a cornice-triggered slab similar to the one above Bunny Tow Pass. The smaller, closer slide is not directly cornice-triggered, but it may have released on the shallower March 31 graupel layer as a sympathetic release to the larger slide.

Melora Gaber releases the probable March 31 graupel layer on a Cutback AK Block after a series of very hard jumps (#6) on 41° at an average #2 shear Quality (CAK6 Q2). Katie Campbell stands by with the dogs. This block did not break at the cutback. We had the same #6 results on the non-cutback AK Block sized for her and on an AK Block sized for and loaded by a 15 Kg heavier tester. The Rutschblock loaded by her did not fracture after repeated hard jumps (RB7).

These strong test results are in agreement with our observation of no recent slab avalanche releases without large triggers or glide cracks.

We saw this large glide avalanche release on D001 Upper Fish Creek, above the lower cross country loop, in late afternoon. We estimated it at 1.5 m deep, 200 m wide, and 500 m long, falling from 840 to 500 m. There are numerous active glide cracks throughout the area this spring, and other similar releases are likely as the weather warms.