Northern San Juan
AvalancheAnn Mellick Chris Bilbrey · Mar 14, 2022 · Northern San Juan
Weather
Sunny and warm. In the morning, winds were moving/ drifting the new snow at all elevations onto easterly and southern slopes. Drifting migrated to above 13,000 feet by the afternoon, and wind speeds decreased. The sun made a strong appearance today, and the snow surface got moist and sticky by mid-afternoon.
Snowpack
We found 8 to 12 inches of new snow around 11,000 feet and higher heavily dependent on aspect and slope location. Drifting along ridgelines and in isolated leeward concavities was around 18 inches deep and slabby. Hand pits identified a weak interface (mixture of Stellars and Plates) at the bottom of the new snow on northerly-facing slopes. This interface is most likely a short-lived concern. Below the new snow on a southeast aspect, we found an approx. 1 inch "not fully refrozen" crust with 4F facets below. The upper limit of the crust was moist while the lower portion frozen and intact. This crust/ facet combo could be a looming concern from additional warmth helping the crust soften. Compression Test results in our snowpit on an east-northeast aspect above treeline yielded sudden planar failures below the most recent snowfall (CT6SP), and harder failure (CT23) at a hardness change down 30 cm's. We did not dig down to deeper PWLs at this pit location. Despite drifting of the new snow at upper elevations, fresh slabs remained soft and we did not observe cracking or collapsing on our travels. Any part of the slope that saw direct sunlight, regardless of cardinal direction, was setting up quickly from warm temperatures and the strong March sun.
Avalanche
We investigated the human-triggered slide from March 13, 2022 in Lower Commodore Basin. This was a hard slab avalanche that broke 3 to 5 feet deep at the 2/21 interface. A prominent dust layer is visible directly above the bed surface in the crown face. The weak layer/ interface the avalanche failed in varied across the crown face. We found weak, large grain facets below the slab in steep, rocky wind-protected areas. Further away from the rocks and along both flanks, the bed surface was firm and planar consisting of a wind-pressed facet layer. The persistent weak layer/ interface the avalanche failed in formed during a 7-week dry weather period in January through mid-February. The slope-scale variability of the failure plane in this avalanche is consistent with how variable the old snow surface was prior to snowfall on February 22, 2022. The slope angle along the crown face and below ranged from 37 to 44 degrees in steepness. The stauchwall was visible and the slope angle there ranged from 31 and 34 degrees in steepness. The slide ran approx. 800 vertical feet downhill through a gully feature. The debris came to rest on a flat bench at treeline.