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Missoula Avalanche

Professional Observation

Basic Information

Observation Details

Observation Date:
December 22, 2021
Submitted:
December 22, 2021
Observer:
WCMAC - Geoff Fast and Hannah Fast
Zone or Region:
Southern Bitterroots
Location:
Lost Trail Backcountry

Signs of Unstable Snow

Recent Avalanches? 
Yes
Cracking? 
None Experienced
Collapsing? 
None Experienced

Snow Stability

Stability Rating: 
Fair
Confidence in Rating: 
High
Stability Trend: 
Improving

Bottom Line

- Facets buried deep (~100cm down) on shaded northerly aspects are rounding after additional snowfall and time, but the possibility to trigger a slab on this layer remains.
- Wind slabs likely in alpine above 7500 feet.
- New snow from 12/19 is bonding well to older snow, and upper snowpack has few concerns.

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Advanced Information

Weather Summary

Cloud Cover:
Mostly Sunny
Temperature:
-1 C / 30 F
Wind:
Moderate , SW

Todays observations 12/22 - Mostly sunny skies, strong SW wind on ridgelines; Nearly reached freezing (0 C)
Previous wind scouring of S and W slopes and depositing on N and E slopes.
Past 5 days have seen temps in 20's, with slight rise above freezing previous day, and during avalanche activity.

Avalanche Observations

 #  Date Location Size Type Bed Sfc Depth Trigger Comments Photo
1 Past 24 hours Lost Trail Backcountry - Wolverine Point
NE 8000
D2.5 SS O-Old Snow 100cm AC-Cornice Drop Likely during warming previous day. Crack, showing signs of cornice separating from mountain. Portion of cornice likely failed and started D2.5 avalanche in heavily wind loaded location.

Snowpack Observations

Objective to target faceted layer around 100cms deep. Facets appear to be rounding well deep in the snowpack. The layer did not react in compression or extended column tests. Propagation Saw Test Test showed propagation on layer is likely with saw propagation reaching 50cms into 100cm block. PST50/100 end down 100cms on 211205 layer.

Avalanche Problems

Problem Location Distribution Sensitivity Size Comments
Wind Slab
Isolated
Specific
Widespread
Unreactive
Stubborn
Reactive
Touchy
D1
D1.5
D2
D2.5
D3
D3.5
D4
D4.5
D5
Persistent Slab
Isolated
Specific
Widespread
Unreactive
Stubborn
Reactive
Touchy
D1
D1.5
D2
D2.5
D3
D3.5
D4
D4.5
D5
Layer Depth/Date: 100cm deep / 12-22-2021
PST50/100 End down 100cm on 211205 layer

- Wind slabs on leeward sides of ridgelines above 7500 feet.
- Watch persistent slab on northerly slopes if rapid warming or heavy load arrives.

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