Southern San Juan

Avalanche

Chris Bilbrey · Mar 4, 2023 · Southern San Juan

Weather

Nice sunny day to start with light winds and moderate gusts. Wind speeds increased throughout the day as high clouds moved overhead. Strong winds were starting to erode the crust on windward slopes and we observed an increase in snow transport. Effective wind loading observed on leeward slopes/ start zones.

Snowpack

The previous days warmup contributed to a stark difference in snow surfaces along the south half of the compass compared to the north. Sunny slopes had a stout, breakable crust that ranged from 3 to 5 cm with soft, dry snow below. The crust was harder and more robust below treeline tapering to about a 1 cm zipper crust near treeline - the crust was less pronounced on east-facing slopes near treeline. Stronger winds were eroding the crust and snow transport increased in response. Despite prominent southwest winds during recent storms, there is a plethora of snow to be laid-back onto easterly slopes, especially once the crust breaks down across fetches. Probing across northerly-facing slopes near and below treeline, we found fairly homogenous upper snowpack layers. There was a very discernable weaker layer that ranged from 35 to 45 cm below the surface and consist of small faceted grains and some of those grains were beginning to round. This layer likely formed on the surface around February 25 and was easier to identify in snowpits on northerly-facing slopes. This layer consistently produced resistant planar results with moderate force. We also observed easy force progressive collapse failures in Compression Test and hand pits in the top 20 cm of recent snowfall. It was easy to initiate loose snow sluffs in the lower-density surface snow on steeper slopes. The Valentine's Day interface ranged from 115 to 125 cm below the surface, was fairly easy to identify while probing, and was consistently 1F- hardness. We did not observe failures in this layer in snowpack tests. The wind had more of a pressing effect to the snow near treeline and we found a slightly inverted snow surface in the afternoon in wide open areas while pole planting and breaking trail. The only surface cracks we observed were in deep, wind-thickened drifts on steep sidewalls of gullies.

Avalanche

The greater landscape was absent of crown lines from the latest round of snowfall. We got a better set of eyes on the rider-triggered slide in Rusty's that fully buried a snowmobiler on February 25. We triggered several loose snow sluffs below treeline on northerly-facing slopes pushing 35 degrees and steeper - one grew large enough to potentially catch an unaware rider off guard.