Playing possum

Location Name: 
Forecaster Observation - Swan Range (west side access)
Observation date: 
Thursday, December 12, 2019 - 17:45
Is this an Avalanche Observation: 
Yes
Observation made by: Forecaster

Location

Tabs

Quick Observation

Toured into mid elevation terrain on the west side of the Swan Range this afternoon to monitor storm totals and wind transport. Did not find much of either, and persistent weak layers buried earlier this winter look to be playing possum. 

  • Some brief graupel but no sustained snow during the afernoon. 2-6" storm snow between 4700 and 6400 feet. Rimed and dense but dry. Not slabby.
  • Bout of gusty winds late morning, with variable directions, but light after that. Didn't hear stronger winds above us on ridge crest.
  • Triggered 2 avalanches with cornice drops on east-northeast facing, mid elevation slope. Broke on old/ new interface; no propagation wider than debris. More like sluffs than slabs.
  • Cornices on lee (NE side) of mid elevation ridge hard but sensitive to a person's weight. Broke up to 20 feet from trigger. Slabs and drifts on lee side 1-2 feet deep. Did not extend more than ~50 feet below ridge. 4F hard with a few harder spots.Not reactive to a person's weight where we got on them. Surprisingly little recent wind scouring on exposed  ridge itself.
  • Felt no collapses while traipsing through open, mid elevation forest looking for likely surface hoar factories.
  • Found 11/23 surface hoar buried 55cm down in profile at 6075' on northerly slope. Propagated in large coumn test with hard force (ECTP28).
  • 11/19 facets/ crust obvious in that profile and another at 6300' (easterly) but no sudden failures or propagation in small or large column tests.
  • No propagation in other layers. 

Based on cornice drops, tests, and lack of wind drifted snow, we felt comfortable skiing a planar, 35 degree, leeward slope. 

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

December, 12 2019: Reactive Cornice

Travel Details
Region: 
Swan Range - West Side (Flathead Valley access)
Snowpack Details
Snowpack and Weather Details: 
Hide Terrain
Elevation of observation: 
5000-6500 ft
Aspect(s) of observation: 
N
NE
E
W
Persistent Weak Layers: 
Surface Hoar
Facets or Faceted Crust
Buried
New Snow in the past 24 hours: 
6.00in.
Total Snow Depth: 
40 in
More comments about the snowpack and weather: 
Cloud deck lifted from ~5000 feet to ridgecrest by late afternoon. A bout of moderate winds midday, but other wise light winds, even on exposed ridge. Temps hovered near freezing about 6000 feet. Found 11/23 surface hoar buried 55 cm below surface at 6075', northerly slope. Below 1F hard wind slab. Debris from wind mixed in with SH when pulled block off. SH 1-3 mm in size; layer <1 cm thick. Test results: CTH23RP, ECTP28, PST 72/115 SF. 11/19 rain crust an obvious pane of glass less than 1 cm thick in 6075' profile (buried 65 cm below surface) and another at 6300' E facing (40 cm below surface). P hard at least. 4F small facets above it. Didn't test in 6075' profile; CTX, ECTN25. Also found thin, soft crust buried 20 cm below surface that didn't produce sudden failures or propagation. Still seems like a possible sliding surface or slab stiffener.
Blowing Snow: 
None
Wind Speed: 
Light (Twigs in motion)
Wind Direction: 
Southwest
Air temperature: 
Above Freezing
Snow line: 
4700
Sky Cover: 
Mostly Cloudy (BRK)
Highest Precipitation Rate: 
Very Light Snowfall (S-1)
Avalanche Details
Avalanche Details: 
Date and Time of Avalanche: 
Thursday, December 12, 2019 - 14:00
Number of avalanches: 
2
Avalanche Type: 
Cornice
Failure Plane/Weak Layer: 
New/old snow interface
More information or comments about the avalanche: 

Triggered by dropping recently-formed cornices along SE ridge of pt. 6437 above Camp Misery. Cornices hard, sensitive to a person's weight, breaking up to about 20 feet from trigger. Debris flushed slope below. No propagation past debris; looked to involve recent snow only. 

Hide Trigger
Trigger: 
skier
Trigger Modifier: 
Intentionally Triggered
Hide Terrain
Start Zone Slope Angle: 
35
Aspect: 
Northeast
Starting Elevation: 
near-treeline
Hide Size
Destructive Size: 
D1 Relatively harmless to people.
Relative Size: 
R2 Small
Crown Height: 
Less than 1 ft
Avalanche Length (Vertical Run): 
100ft.
Avalanche Width (Average width): 
25ft.
Hide People Involved
Number of people caught: 
0
Avalanche Location: