Vail & Summit County
AvalancheAndrew McWilliams · Mar 29, 2023 · Vail & Summit County
Weather
Above-freezing temperatures and scattered clouds became broken to nearly overcast by late afternoon. The wind was moderate from the southwest.
Snowpack
Near and below treeline the top 5-10cm of the snowpack was moist with the above-freezing temperatures and as I descended a crust was beginning to form. Below this surficial layer, the snow remained dry. At the ridgeline, the moderate winds were keeping the surface snow, but the ample sunshine yesterday had already formed a crust on the surface on southerly aspects. Southwest winds were moving some snow, but two days of sun and warm temperatures seemed to limit the amount of snow available for transport. The snow that was moving was forming thin, dense slabs high in start zones. My snowpit on a north-facing slope near treeline had a depth of 275cm and was likely in a more wind-drifted area than I anticipated. Extended Column Tests (ECT) had moderate to hard non-propagating results below a layer of wind-packed decomposing fragments within the upper 20cm of the snowpack and about 40cm below the surface below a decomposing crust layer. This melt-freeze crust is resting on grains that show significant rounding. However, it appeared that this layer has been alternating between rounding and faceting with the recent temperature swings and still showed some sharp edges under magnification. I called it rounds, but the depth of this layer fits with the persistent weak layer we've been tracking about one to three feet deep across Summit County. As I approached the corniced ridgeline to look at south and east-facing slopes and look for signs of warming, I remotely triggered a small Wind Slab avalanche that broke about 10-30cm deep. As this avalanche slid, it entrained more snow on the steep slope below and became larger. The debris pile at the bottom appeared wet, but I didn't feel comfortable descending the slope to look more closely. The avalanche failed on a layer of faceted grains above a melt-freeze crust. A snowpit near the crown got a non-propagating result in an ECT after one tap from the elbow on this layer. The slab of the slide was composed of decomposing fragments that ranged from fist to one-finger hard on the Hand Hardness Scale. The snowpack in this location was shallow at 116cm deep. Below the fracture plane, the snowpack was composed of faceted grains and depth hoar to the ground. My ECT test got a propagating result after three taps from the shoulder at the depth hoar interface.
Avalanche
I remotely triggered a small wind slab avalanche on a southeast-facing slope near treeline on Bald Mountain that entrained moist snow as it made its way to the valley bottom.