Southern San Juan
AvalancheChris Bilbrey · Mar 30, 2023 · Southern San Juan
Weather
Mixed-bag flavor of weather - clouds, wind, snow, and sun. Skies transitioned from overcast to obscured to broken and moments of filtered sunshine. Strong southwest winds with periods of intense, whiteout gusts. Snowfall struggled to get going during the morning, but settled in around 1130 - mostly S1 with minimal accumulations by midday.
Snowpack
The snow surface below treeline experienced a decent re-freeze overnight. The crusted surface ranged a 1/4 to 1/2 inch thick, was breakable but supportive to skis. Boot penetration ranged 8 to 12 inches on east to south-facing slopes. Wednesday's wetting front was around 10 cm below the surface. Otherwise, the snow below the crust was dry and difficult to make into a snowball. Even northerly-facing slopes below 11,000 feet developed a micro-thin crust from above freezing ambient temperatures the day prior. As we climbed higher in elevation, the most notable storyline was the wind. The wind did a more efficient job of stripping and stiffening the snow surface versus forming reactive slabs. Strong winds mixed down to about 11,000 feet touching almost all open areas - only slopes extremely sheltered from the wind were spared. Swirling, erratic winds contributed to random loading patterns that varied depending on micro-terrain features. Small, drifted test slopes were stubborn to ski pressure and we could hardly make an impression in the snow surface in spots. Our snowpit on a north-facing slope around 11,600 feet yielded 3 to 5 cm of wind-packed rounds on the surface above 4F hard snowfall from earlier this week. This stiff over soft surface layering did not produce notable results in our pit. The 3/20 faceted interface sits around 60 cm below the surface and grains were a mixture of angular and rounding facets, as well decomposing fragments. It appears the lower-density snow that fell at the start of the last big storm has allowed the old snow surface to heal slower than if a more dense layer was directly above. Compression Tests produced resistant planar failures with 22 taps in multiple test, but Extended Column Test did not produce propagating failures in the same weak layer. The overlying slab has likely gained strength faster than the faceted grains below - a classic stubborn Persistent Slab avalanche behavior. There is another in-between storm rounding facet interface about 75 cm below the surface, but it was unreactive in snowpack tests.
Avalanche
We found one large avalanche that likely ran during the 3/22-23 natural cycle on a northeast-facing slope. The other released on a more westerly-facing slope and assume it ran during the latest warm up - similar to other soft/ moist slab activity we've seen across the San Juan's.