Remote and Natural Triggers - Widespread Instability

Location Name: 
Billy Bowl
Region: 
Selkirk Mountains
Date and time of avalanche (best estimate if unknown): 
Wed, 02/03/2021 - 13:45
Location Map: 
United States
48° 29' 14.1324" N, 116° 42' 3.6936" W
US


Red Flags: 
Whumphing noises, shooting cracks, or collapsing
Recent loading by new snow, wind, or rain
Obvious avalanche path

Observation made by: Public
Avalanche Observations
Avalanche Type: 
Dry
Slab
Slope: 
40degrees
Trigger type: 
Skier
Crown Height: 
2 ft
Aspect: 
North
Weak Layer: 
Storm Snow
Avalanche Width: 
800ft.
Terrain: 
Above Treeline
Elevation: 
6 000ft.
Bed Surface: 
Old Snow
Avalanche Length: 
1 500ft.
Number of similar avalanches: 
1
More detailed information about the avalanche: 

Our group was skinning up a south facing slope in Lost Creek with the intention of skiing back down the same slope. The first group member to approach the ridge line that drops into Soldier Creek (Billy Bowl) was about 20 feet away from the cornice when they heard a "boom" and then felt vibration under their feet. When we approached the cornice we observed they caused a remote cornice trigger that resulted in a dry slab avalanche of all the recent storm snow (~18") down to the prominent rain base. It appeared to have released with much energy and ran to the bottom of the runout.

 

This slide remotely triggered a secondary avalanche about 300 feet away that was about 300 feet wide and shared path and runout with the primary avalanche. It is possible that the runout of the primary triggered the secondary from below. Judging by the point release in the photo I believe that the secondary was actually triggered by the initial cornice fall and lateral failure of the PWL beneath the storm slab. The area between the two avalanches is still considered unstable based on the MfCr bed surface beneath the storm slab. In the photo you will also notice a significant amount of hangfire above the secondary avalanche.

On a different isolated and small north facing slope I witnessed a snowmobiler go up and down once and then leave the area. I witnessed the slope slide about 5 minutes later, so it's hard to say whether it was natural or machine triggered.

Earlier in the day at 5000' elevation a snowmobile triggered a crack that shot across a creek drainage. The crack propagated through a 5cm MfCr that was about 10 cm below storm snow.

My conclusion is that the smallest of trigger can have large consequences during this avalanche cycle, especially on loaded slopes.

-Jake

Avalanche Photos: 
Weather Observations
Blowing Snow: 
No
Cloud Cover: 
50% of the sky covered by clouds
Air temperature: 
Below Freezing
Wind Speed: 
Calm
Precipitation: 
None
Air temperature trend: 
Cooling
Wind Direction: 
Southwest
Accumulation rate: 
More detailed information about the weather: