The surface hoar monsters lurk in dark and quiet terrain

Location Name: 
Forecaster Observation- Swan Range
Observation date: 
Tuesday, February 25, 2020 - 15:15
Is this an Avalanche Observation: 
Yes
Observation made by: Forecaster

Tabs

Quick Observation

We chased the biggest storm totals to the Swan Range and found dangerous avalanche conditions resulting from buried surface hoar below the 16" to 24" of storm snow.

  • On wind protected, shady aspects at mid elevations, we produced numerous shooting cracks, collapses, slab releases on test slopes, and saw recent natural avalanches. These were consistently failing on the 2/23 surface hoar layer, 16" to 24" deep. 
  • Test slopes in previously wind affected terrain or previously sun cooked aspects did not produce results; the surface hoar was not present in hand pits at these locations. 
  • We also triggered and observed small natural sluffs that ran today.
  • The sun came out this afternoon and was moistening the snow at lower elevations as we were leaving  at 2:30 pm. There were some rollerballs and pinwheels.
  • Given the numerous red flags, we kept our terrain choices simple: short, inconsequential pitches and slope angles less than 35 degrees.  

 

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

February 25, 2020. Buried surface hoar in the Swan

Travel Details
Region: 
Swan Range - West Side (Flathead Valley access)
Route Description: 

To 7100'

Activity: 
Skiing
Snowmobiling
Snowpack Details
Snowpack and Weather Details: 
Hide Terrain
Elevation of observation: 
3500-5000 ft
5000-6500 ft
Above 6500 ft
Aspect(s) of observation: 
N
NE
SW
Red Flags: 
Shooting cracks
Collapsing / whumpfing noises
More than a foot of new snow or heavy snowfall rates (>1"/hr)
Rollerballs / pinwheels
Persistent Weak Layers: 
Surface Hoar
Buried
New Snow in the past 24 hours: 
12.00in.
More comments about the snowpack and weather: 
6-10” of new snow at low elevations, became moist on sunny aspects this afternoon. This snow was on a melt-freeze crust on all aspects where we did hand pits. No SH found. 16-24” of new snow at mid and upper elevations. This was on SH at mid elevation, northerlies and on sun crust on southerlies. Near and on the leeward side of ridges, it was on wind packed grains. The snow was still dry on all aspects by mid afternoon, but the sun came out strong as we were leaving. @6400', NE aspect in Noisy Basin, ECTP 2, SC on 2/23 SH, down 57 cm under a F to 4F- slab.
Blowing Snow: 
Light
Wind Speed: 
Moderate (Small trees sway)
Wind Direction: 
Southwest
Air temperature: 
Below Freezing
Sky Cover: 
Decreasing Clouds
Highest Precipitation Rate: 
Very Light Snowfall (S-1)
Avalanche Details
Avalanche Details: 
Date and Time of Avalanche: 
Monday, February 24, 2020 - 18:45
Number of avalanches: 
2
Avalanche Type: 
Soft Slab
Failure Plane/Weak Layer: 
New/old snow interface
More information or comments about the avalanche: 

Small road cuts

Hide Trigger
Trigger: 
natural
Hide Terrain
Aspect: 
Northeast
Starting Elevation: 
near-treeline
Hide Size
Destructive Size: 
D1 Relatively harmless to people.
Relative Size: 
R2 Small
Crown Height: 
1 ft
Avalanche Length (Vertical Run): 
15ft.
Avalanche Width (Average width): 
30ft.
Avalanche Location: 
Date and Time of Avalanche: 
Tuesday, February 25, 2020 - 13:45
Number of avalanches: 
1
Avalanche Type: 
Loose Dry/ Sluff
Failure Plane/Weak Layer: 
Within storm snow
Hide Trigger
Trigger: 
natural
Hide Terrain
Aspect: 
West
Starting Elevation: 
near-treeline
Hide Size
Destructive Size: 
D1.5
Relative Size: 
R1 Very Small
Avalanche Location: 
Date and Time of Avalanche: 
Tuesday, February 25, 2020 - 13:00
Number of avalanches: 
2
Avalanche Type: 
Loose Dry/ Sluff
Failure Plane/Weak Layer: 
Within storm snow
More information or comments about the avalanche: 
Hide Trigger
Trigger: 
skier
Trigger Modifier: 
Intentionally Triggered
Hide Terrain
Aspect: 
Northeast
Starting Elevation: 
near-treeline
Hide Size
Destructive Size: 
D1 Relatively harmless to people.
Relative Size: 
R1 Very Small
Crown Height: 
Less than 1 ft
Avalanche Length (Vertical Run): 
100ft.
Avalanche Location: