Near-miss, Halfmoon Slide

Location Name: 
Forecaster Observation, Halfmoon Slide
Observation date: 
Saturday, February 16, 2019 - 19:15
Is this an Avalanche Observation: 
Yes
Observation made by: Forecaster

Tabs

Quick Observation

This morning, a skier reported triggering a large slide that occurred on Feb. 15. Thank you for the prompt report! We skied to the site to learn more.

  • This was a large, soft slab avalanche (could bury, injure, or kill a person) triggered remotely from above. It broke on old snow approximately 15-22" below the snow surface (SS-ASr-R3D2-O).
  • It likely released on weak snow just above a rain crust that formed Feb. 2. 
  • The convex start zone faces south and has slope angles of 37-45 degrees. 
  • The skier was not caught or carried in the debris, and was uninjured.

The near-miss is a reminder that it's still possible to trigger large, dangerous avalanches on steep slopes.

Snowpack, Avalanche, Weather Images: 
Snowpack, Avalanche, Weather Videos: 

20190216 Near miss, Halfmoon Slide

Travel Details
Region: 
Whitefish Range - Southern (south of Coal Creek)
Activity: 
Skiing
Snowpack Details
Snowpack and Weather Details: 
Hide Terrain
Elevation of observation: 
5000-6500 ft
Aspect(s) of observation: 
S
Persistent Weak Layers: 
Facets or Faceted Crust
Buried
New Snow in the past 24 hours: 
4.00in.
Total Snow Depth: 
225 cm
More comments about the snowpack and weather: 
Test profile on adjacent undisturbed slope (skier's left of slide) at base of rock outcrops like on slope that released. approx 125 E of slide. HS 225 cm, due South, 45*. Elev. 6140 ft, roughly same as highest part of crown. ECTN16, CTM 13 SP on rimed crust 42 cm below surface. Likely the 2/2 crust. While this would have been closer to the surface the day of the slide, this layer the only weak layer in the pit apart from a very hard crust 112 cm above ground (115 cm below surface). Debris ran full length of slope below start zone and a ways into trees/ gully below.
Blowing Snow: 
None
Wind Speed: 
Light (Twigs in motion)
Wind Direction: 
South
Air temperature: 
Below Freezing
Sky Cover: 
Obscured by fog, etc (X)
Highest Precipitation Rate: 
Light Snowfall (S1)
Avalanche Details
Avalanche Details: 
Date and Time of Avalanche: 
Friday, February 15, 2019 - 15:15
Avalanche Type: 
Soft Slab
Failure Plane/Weak Layer: 
Old snow
More information or comments about the avalanche: 

Rider reported getting off route and attempting to traverse back to his intended line (skier's left to right). He was crossing a steep, narrow chute when he experienced a whumpfing collapse. He saw the larger slope below him avalanche, with a powder cloud hitting the trees below. He was not caught. He skied down the bed surface to get out of trouble. 

Hide Trigger
Trigger: 
skier
Trigger Modifier: 
Remotely Triggered
Hide Terrain
Start Zone Slope Angle: 
45
Aspect: 
South
Starting Elevation: 
6100
Hide Size
Destructive Size: 
D2 Could bury, injure, or kill a person.
Relative Size: 
R3 Medium
Crown Height: 
2 ft
Avalanche Length (Vertical Run): 
300ft.
Avalanche Width (Average width): 
175ft.
Hide People Involved
Number of people caught: 
0
Number of partial burials: 
0
Number of full burials: 
0
Avalanche Location: