With low danger throughout our forecast area, we chose to further investigate the early December crust/facet layer that our neighbors to the north saw large failures on this past week (https://avalanche.ca/blogs/persistent-slab-avalanches-in-southern-interior). After many days of hunting, we found two very large (D3) avalanches off Mt. Penrose.
Today, we felt confident skiing terrain steeper than 35 degrees at upper elevations due to the presence of a thick, supportable crust.
The two largest slides started in steep, wind loaded terrain near 7150 feet. The slide closest to the SE ridge of Penrose appears to have started from a cornice collapse, given that the crown is very close to the ridgeline and part of the cornice appeared to be missing. The other two large slides on the east face of Penrose appeared to start below cliffs. One near the ridgeline, the other a few hundred feet down the ridgeline. Based on the presence of exposed rocks below the starting zone, we suspect that failure occurred deep in the snow pack with all three slides on the east face of Penrose. The most likely failure point deep in the snow pack is the December 3 crust/facet layer that we had been finding at similar elevations and aspects.
Tracing back this week's weather, Tuesday brought more snow than forecast to our region and strong winds. Five-foot-tall windrows on the SE ridge of Penrose are evidence of the strength of recent winds. Tuesday afternoon began to warm and by Wednesday rain had fallen up to 6000 feet. We suspect that these slides happened during this window, with combination of recent loading and warming temperatures being the biggest contributing factors.
Tally from observed storm slabs. Best estimated time of occurrence is 1/12/22. Crowns averaged to be 1-2 feet tall and one propagated an estimated 1500 feet wide. There were several crowns associated with this slide in starting zones between 6300 and 6700 feet.
Wet loose observed. Estimated time of occurrance.
