Northern San Juan
SnowpackTroy Nordquist Chris Bilbrey · Dec 21, 2023 · Northern San Juan
Weather
A beautiful day with mostly clear skies and calm winds. Temperatures were unseasonably warm.
Snowpack
The snowpack below treeline is shallow and very cohesionless. Moderate Compression Test (CT) scores with dirty failures (Break, Sudden Collapse) demonstrate a stubborn slab, weak layer combination. Extended Column Test (ECT) also failed to propagate a failure across the column highlighting the lack of an intact, cohesive slab at lower elevations. Additionally, this was verified when our snowmobiles nearly punched to the ground while riding off packed surfaces. As we moved higher in elevation, over-the-snow travel improved. We credited this as a function of a more supportive, harder mid-snowpack below. Stability tests at treeline also identified two primary layers of concern. The upper most layer is the interface buried during the December 14 storm - a 4F layer of well-developed near-surface facets. Closer to the ground, large grain facets remain preserved. This is likely due to shallow snow depths and prolonged exposure to constructive metamorphism. Similar to below treeline, CTs and ECTs conclude a stubborn slab, weak layer combination on this day. However, with how shallowly these layer are buried, well within the trigger range of a rider or machine, expect them to be a problem with a moderate to significant new snow load. Triggering an avalanche in one of these upper snowpack weaknesses could initiate a step-down scenario to basal layers, and produce a larger, more dangerous avalanche. Despite the generally shallow snow cover, plenty of west through north to east-facing slopes harbor continuous snow cover for avalanches to span across terrain features. Southerly-facing slopes are thin near and above treeline, and mostly burned out below treeline.