Nuking in the Swan

Location Name: 
Forecaster Observation - Spring Slide Mountain, Swan Range
Observation date: 
Monday, January 7, 2019 - 18:15
Is this an Avalanche Observation: 
Yes
Observation made by: Forecaster

Tabs

Quick Observation

Investigated the site of the avalanche accident on Spring Slide Mountain on January 5, 2019.   Will update the accident observation tomorrow with details.  Today, we found dangerous avalanche conditions with numerous natural and human triggered storm slabs.

  • 12" to 16" of new snow at mid and upper elevations.   Very heavy snowfall rates this afternoon, ~2"/hr.  Strong winds with intense snow transport at ridgetop.
  • We observed a large (D2) storm slab run naturally this afternoon and saw the crown of a smaller storm slab from earlier in the day.  We also intentionally skier triggered several slabs up to 15" thick, D1 to D1.5.  
  • We noted shooting cracks ranging from 1' to 10' long at mid and upper elevations.  Slabs that were stiffer from wind affect (both leeward and windward) were more reactive and produced larger cracks than in sheltered slopes.  We observed one large collapse with cracks radiating 50 feet on a shallow, windward aspect. 
  • Pits showed easy, propagating results within the storm snow, 1' deep, and no results on deeper layers.  
  • Given plenty of obvious signs of instability, we kept to mellow slope angles on our descent until about 5500'.  Below that elevation, slabs were generally too incohesive to be reactive, and we practiced sluff management in steep but small pitches.  

 

Snowpack, Avalanche, Weather Images: 
Snowpack, Avalanche, Weather Videos: 
Travel Details
Region: 
Swan Range - South (south of Swan Lake)
Route Description: 

Spring Slide Mtn via North Fork of Lost Creek

Activity: 
Skiing
Snowmobiling
Snowpack Details
Snowpack and Weather Details: 
Hide Terrain
Elevation of observation: 
5000-6500 ft
Above 6500 ft
Aspect(s) of observation: 
N
NE
SE
S
NW
Red Flags: 
Shooting cracks
Collapsing / whumpfing noises
More than a foot of new snow or heavy snowfall rates (>1"/hr)
Blowing snow
Persistent Weak Layers: 
Facets or Faceted Crust
Buried
New Snow in the past 24 hours: 
16.00in.
Total Snow Depth: 
200 cm
More comments about the snowpack and weather: 
Snow profile on NW aspect at 6700': HS 200 cm. ECTP2 down 30 cm on mid-storm rimed stellar dendrites. 4F hard, 1mm facets (Dec 9) down 70 to 130 cm, depending on drifting, below a 1F slab. ECTX results on this layer and PST 98/100 END.
Blowing Snow: 
Intense
Wind Speed: 
Strong (Whole trees in motion)
Wind Direction: 
Southwest
Air temperature: 
Below Freezing
Snow line: 
Valley floor
Highest Precipitation Rate: 
Very Heavy Snowfall (S10)
Avalanche Details
Avalanche Details: 
Date and Time of Avalanche: 
Monday, January 7, 2019 - 15:15
Avalanche Type: 
Soft Slab
Failure Plane/Weak Layer: 
Within storm snow
Hide Trigger
Trigger: 
natural
Hide Terrain
Start Zone Slope Angle: 
38
Aspect: 
Northwest
Starting Elevation: 
above-treeline
Hide Size
Destructive Size: 
D2 Could bury, injure, or kill a person.
Relative Size: 
R2 Small
Crown Height: 
1 ft
Avalanche Length (Vertical Run): 
400ft.
Avalanche Location: 
Date and Time of Avalanche: 
Monday, January 7, 2019 - 13:45
Avalanche Type: 
Soft Slab
Failure Plane/Weak Layer: 
Within storm snow
Hide Trigger
Trigger: 
skier
Trigger Modifier: 
Intentionally Triggered
Hide Terrain
Aspect: 
Northwest
Starting Elevation: 
above-treeline
Hide Size
Destructive Size: 
D1 Relatively harmless to people.
Relative Size: 
R1 Very Small
Crown Height: 
1 ft
Avalanche Length (Vertical Run): 
200ft.
Avalanche Width (Average width): 
20ft.
Avalanche Location: