Southern San Juan
AvalancheChris Bilbrey · Dec 28, 2023 · Southern San Juan
Weather
Abundant sunshine, temperatures upper teens to twenties, and moderate north wind.
Snowpack
We found a shallow, weak snowpack near and below treeline consisting of faceted snow grains of various shape and size. Snow depths were generally less than about 2 feet deep and boot penetration was to the ground with no resistance. The lack of an intact slab supports observed results in snowpack tests - non-propagating Extended Column Test (ECT) with hard force, and dirty shears in Compression Test (CT). If and when a significant new snow load is added on top, the avalanche danger will quickly ramp upward at lower elevations - especially northerly-facing slopes. As we approached the physical treeline, we could feel things change below our feet. Probing identified a more intact upper slab over weaker snow, and surface conditions got more firm. Cracking and collapsing was fairly predictable in wind-drifted terrain features with some cracks shooting outward around 50 feet. Snowpack tests in our west and northeast snowpits produced propagating results in ECTs that failed with 20 and 21 taps respectively. These occurred in a 20 cm layer of 3 to 5 mm facets/ depth hoar at the ground. The failure plane was cleaner and snappier in our northeast snowpit as the overlying slab was harder and slightly thicker. This seems to fit the loading/ scouring pattern observed on alpine slopes across our travels. North to east-facing slopes have deeper, more broad snow coverage and where an avalanche could break wider. West and northwest-facing slopes have seen more brute force winds, and problematic spots appear confined to cross-loaded terrain features. The recent strong northerly winds built hard slabs on the surface 3 to 4 inches thick. Sitting below are weaker snow grains. As we moved closer to shallower areas on alpine slopes or places more protected from the wind, these hard slabs were much thinner and breakable. Sunny slopes have a new surface crust that varied in thickness depending on slope angle and aspect.
Avalanche
Several older crowns that likely ran during the 12/23 to 12/24 storm cycle, and smaller more recent Wind Slab avalanches that likely ran during the strong northerly wind event that followed.