BTL Persistent Slab Avalanche

CB Avalanche Center2016-17 Observations

Location: Crested Butte Area
Date of Observation: 02/22/2017
Name: Arden Feldman
Subject: BTL Persistent Slab Avalanche
Aspect: North, North West
Elevation: 9800

Avalanches: Checked out a natural D2/R3 persistent slab avalanche on a NNW aspect BTL that occurred before the most recent snow this past weekend. The slide was about 300 feet wide and occurred on a 33 degree slope. It had impressive propagation, propagating around tree islands. The slab was 55 cm thick with 10-15 cm of new snow on top. The slab was 4f+ hardness and had a non planar melt freeze crust in the middle of it. CTH 23 BRK on small grained facets/DF’s beneath the melt freeze crust. No failure occurred on the avalanche’s weak layer. The weak layer was most likely surface hoar (1/19 interface) but in the location of the compression test it had either already collapsed or decomposed. There were remains of a significant warming event that caused large rollerballs/small wet loose avalanche activity to come down from the steeper terrain above the avalanche – it seemed possible that this warming event and subsequent wet loose avalanche activity triggered the persistent slab avalanche.
Weather: Warm with broken clouds and moderate winds. Light graupel at times.
Snowpack: See avalanche description. Height of snow was 200 cm.