Southern San Juan
AvalancheIan Altman · Jan 22, 2019 · Southern San Juan
Weather
Clear, with moderate ridge top winds. Maybe 25-30 degrees. A warming trend this day was evident by days end; with solar aspects changing and crusting rapidly.
Snowpack
Transitional snowpack with no obvious signs of instability. No cracking or wolfing. A firm layer of storm snow about 30 inches thick blanketing everything that had not slid out during the storm cycle.
Avalanche
We observed a collection of debris piles from several R4D3+, naturally triggered avalanches. These slides appeared to have run down each major gully feature along the Star Peak massif. Most of these slide paths or gullies are by-and-large east facing, but their sides are steeper and lean to the north ans south. As we climbed higher onto the riblets separating each gully, we could see crown lines ranging in depth from 2-4 feet. These seemed to have failed on a crust/facet combo just above the October/November depth hoar layers. These long-running crowns lined the northeast sides of each sub-ridge from 9K all the way to their summits, at 11.5K. Most of these crown lines were in the trees and had cracked on every slope nearing 32-35 degrees in steepness. The fascinating part of this natural event can be seen in attached photos- where the debris piles showed evidence of multiple avalanches coming down simultaneously or in close succession. Although no one will ever know for sure, our group surmounted that the upper, more east facing loading zones came down first after the strong westerly winds overloaded the leeward sides of each peak. Although we did not see these upper crowns up close, we could see them from a distance and they were some 100 feet below the ridgetops. We theorized that these initial avalanches then triggered the steeper northeast sides of each gully; filling in the gullies with fresh debris as the upper slides then rumbled down through them. We observed trenches some 20-30 feet deep, and as we walked up the paths the debris piles resembled a glacier- with medial and terminal moraines. The end result was that each major path had run full track, leaving piles of snow at their bottoms that were some 40-50 feet high!