Thursday, January 17, 2019 - 12:00am
We investigated a natural avalanche that was discovered yesterday evening by a public observer in the trees below the "Perch" in the Sherwins adjacent to Mammoth Lakes. The avalanche happened at some point during the storm of Jan 16-17. The highest point of the avalanche started at 9200ft and encompassed NW-N-NE aspects, on 31-35 degree slopes below treeline within areas of moderately spaced trees to open gladed terrain with large old growth trees throughout. A majority of the avalanche crown had initiated on convex roll overs within the slope and connected from tree to tree and granite outcroppings.
The avalanche was quite large with the crown measuring from 3-4ft in height. The total width of the crown extended nearly 1500ft across the slope and the avalanche ran about 500 vertical feet down through mature timber (R4-D2.5-N-3/4'crown-1500'width-500'vert). The majority of the avalanche ran on the basal facet layer 55cm off the ground (or 135cm below the surface) in this specific locality. Within the central area of this wide avalanche there was a section of slab that appeared to be just the new storm snow, but both the steeper NW side (skiers right) and the more windloaded NE side (skiers left) of the slide all slid down to the facets. This event was a confirmation of the persistent slab issue we have found throughout the forecast zone. The storm cycle of this week had a significant amount of water content, which equals significant weight, thus causing the newly deposited storm slab and residing old snow persistent slab to slide on the basal facet layer.