Very Large Remotely triggered slide

Location Name: 
Avalanche Near-Miss: Devils Hump
Observation date: 
Sunday, February 21, 2021 - 11:15

Is this an Avalanche Observation: 
Yes
Observation made by: Public

Tabs

Quick Observation

A group of 3 snowboarders remotely triggered a very large avalanche on west-facing slopes on Devils Hump. They were regrouping after the first pitch of their 2nd run (caution symbol on attached map image). Their account:

Once regrouped i noticed the powder cloud from the north flank unstitching the slope several hundred feet adjacent to us. Moments later, we saw the debris/powder cloud crash into the bottom of the basin.

We were in a precarious position and needed to act quick as the precip intensity had picked up in the last 5 minutes before descending and was coming down 1-2 inches per hour (most of day was s1 prior to this pulse). We chose to traverse hard to the left where we knew the slope angle was less than directly below us. At this point we were thinking that we witnessed a large natural. (2:25pm)

While traversing we discovered that the avalanche that we saw far to our right had propagated directly below us as well. We ended up at the corner of the crown and southern flank. (Photo attached). We were unsure whether or not to descend the debris at this point due to hangfire so we made the choice to continue traversing until we felt fall line was mellow enough to ski safely. We did not observe any additional cracking or signs of instability while traversing.

I briefly inspected the crown while photographing and felt that it was very similar to the stiff slab that we observed at our pit site (pit site is P on the map). Once back home i noticed that the crown was practically at the same elevation as well. We dug this pit before our first run.

Green line is one of my partners track from the day that he was recording. The skin track is furthest south. Our first run is the line that was later engulfed by the flank of the slide. And the our final run is the one that wraps around the slide to the skiers left of the flank.

Note: The party is not certain the slide was remotely triggered. They did not feel a collapse, the slope below them did release.

 

Snowpack, Avalanche, Weather Images: 
Travel Details
Region: 
Flathead Range/Glacier National Park - John F. Stevens Canyon
Activity: 
Snowboarding
Snowpack Details
Snowpack and Weather Details: 
Hide Terrain
Elevation of observation: 
Above 6500 ft
Aspect(s) of observation: 
W
Red Flags: 
Avalanches from the past 2 days
More than a foot of new snow or heavy snowfall rates (>1"/hr)
Persistent Weak Layers: 
Facets or Faceted Crust
Highest Precipitation Rate: 
Heavy Snowfall (S5)
Avalanche Details
Avalanche Details: 
Date and Time of Avalanche: 
Saturday, February 20, 2021 - 14:15
Number of avalanches: 
1
Avalanche Type: 
Hard Slab
Failure Plane/Weak Layer: 
Old snow
More information or comments about the avalanche: 

See quick obs tab

Hide Trigger
Trigger: 
snowboarder
Trigger Modifier: 
Remotely Triggered
Hide Terrain
Start Zone Slope Angle: 
33
Aspect: 
West
Starting Elevation: 
7250
Hide Size
Destructive Size: 
D3.5
Relative Size: 
R4 Large
Crown Height: 
3 ft
Avalanche Length (Vertical Run): 
1650ft.
Avalanche Width (Average width): 
750ft.
Hide People Involved
Number of people caught: 
0
Avalanche Location: