Professional Observation

Observation Details

Signs of Unstable Snow

Key Points

We went to one of the first places we had reports of avalanches failing on a weak layer buried on 3/19 to establish a trend and see whether the problem is getting better. We also wanted to see today’s wind slab problem from one of the windiest parts of the forecast area.

Good News: We got no direct feedback from the persistent weak layer. When we originally received reports from this area on 3/20, they were of shooting cracks and remotely-triggered avalanches. Today we got no such evidence for the 3/19 weak layer.

Bad News: The layer gave us propagating test results on a northwest aspect near treeline. Plus, in this area, the weak grains were not just small and hard to identify near-surface facets. There were fragmented grains (DF), near-surface facets (FCsf), but also surface hoar crystals in the failure plane of our tests, which is a notorious weak layer. We probed around and found this interface down 70-85 cm below the surface which means that an avalanche breaking on the layer could be large enough to bury or kill someone. We found the layer on west, northwest, north, northeast, and east aspects near and below treeline. Colder than normal temps could slow the bonding process for these weak grains.

Good News: The wind slabs we found were small.

Bad News: They were reactive and we got shooting cracks and triggered very small slabs on leeward test slopes. We triggered one 12 inches thick that broke 40 feet wide and ran 75 feet (D1) on a 42-degree north-facing slope near treeline. The slabs were soft (4F) and broke on non-persistent grians (2mm DF) in the upper snowpack.

Any slopes that have gotten recent sun had a 1-inch thick melt-freeze crust with trace amounts of new snow on top. Low-angled, northerly slopes had deep boot and ski penetration, and skied well.