Generations of Wind Slabs

Location Name: 
Forecaster Observation - Central Swan Range
Observation date: 
Saturday, February 15, 2020 - 14:45
Is this an Avalanche Observation: 
Yes
Observation made by: Forecaster

Location

Tabs

Quick Observation

We wanted to see the central Swan Range before this evening's storm rolls in. We found the wind slab problem covered in today's avalanche forecast, and watched fresh slabs beginning to form.

  • We found wind slabs from the past few days below large cornices in the lee of most alpine ridgelines. They were stiff and took hard stomping or undercutting to get them to slide. We found hard drifts up to 18" thick which tapered gradually down the slope.
  • We saw moderate southwest winds building soft new slabs on top of the older drifts with what little snow was available. These newer slabs were only a few inches thick and isolated, but were more sensitive and cracked around us on test slopes. If the wind and snow forecast comes through tonight, I expect these slabs to grow by tomorrow.
  • We found the early February crusts down under 1 to 2 feet of soft, but settled snow. We did experience one collapse in the soft snow between the 2/5 and 2/1 crusts where they were closer to the surface. We didn't see any shooting cracks resulting from the collapse, but it's worth keeping in mind that an additional load may cause the upper crust to break.
  • We were comfortable getting into steep, wind sheltered terrain. Only the top 2 inches of new snow sluffed from our turns and didn't run very far. I expect more loose dry snow to make larger sluffs if the storm verifies tonight. 
Snowpack, Avalanche, Weather Images: 
Travel Details
Region: 
Swan Range - West Side (Flathead Valley access)
Route Description: 

To 6700'

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: 
NE
E
SE
S
SW
W
NW
Red Flags: 
Shooting cracks
Collapsing / whumpfing noises
Blowing snow
Persistent Weak Layers: 
Surface Hoar
On the surface
New Snow in the past 24 hours: 
2.00in.
More comments about the snowpack and weather: 
The 2/13 sun crust is about 1" thick on SW-S-SE aspects and lower and middle elvations. We found a thin layer of graupel and rimed surface hoar on top on sheltered, westerly, mid elevation slopes. Higher up the 2/13 crust was covered with about an inch of drifted snow capped by a thin wind crust and rime. Other pre-storm surfaces consisted of F-4F DFs and Grpl in sheltered areas and a thin rime crust over a few inches of DFs on wind scoured slopes. Lee of ridgelines we found wind deposited rounds ranging from 4F (new, up to 4" thick) or old (1F, up to 18" thick). The snow between the 2/5 and 2/1 crusts was a mix of needles, DFs, graupel, and NSFs. It was F hard. No results in hard pits in sheltered areas.
Blowing Snow: 
Light
Wind Speed: 
Moderate (Small trees sway)
Wind Direction: 
Southwest
Air temperature: 
Below Freezing
Snow line: 
2000'
Sky Cover: 
Obscured by fog, etc (X)
Highest Precipitation Rate: 
Very Light Snowfall (S-1)
Avalanche Details
Avalanche Details: 
Date and Time of Avalanche: 
Thursday, February 13, 2020 - 20:15
Number of avalanches: 
1
Avalanche Type: 
Soft Slab
Failure Plane/Weak Layer: 
New/old snow interface
More information or comments about the avalanche: 

I expect this was the result of intense wind loading on the eveing of the 13th.

Hide Trigger
Trigger: 
natural
Trigger Modifier: 
Cornice Fall Triggered
Hide Terrain
Start Zone Slope Angle: 
35
Aspect: 
Northeast
Starting Elevation: 
above-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): 
50ft.
Avalanche Width (Average width): 
20ft.
Avalanche Location: